Move a Candle Slowly Towards a Concave Mirror and Watch the Image Flip
Learn all six image cases for a concave mirror and draw each ray diagram, see why a convex mirror behaves the same way every time, and understand why shaving mirrors, headlights and rear-view mirrors are shaped as they are.
At what point does a concave mirror image turn upright?
Hold a concave mirror at arm's length and look at your face in it. You see yourself upside down and small.
Now bring it slowly closer. The image grows, stays inverted, and at one particular moment it blurs away entirely — and then reappears the right way up and magnified.
The moment of the flip is when your face passes the principal focus. Outside the focus a concave mirror forms a real, inverted image; inside it, a virtual, erect, magnified one.
That single crossing organises the whole of this page. A concave mirror does six different things depending on where the object sits relative to F and C, and knowing which case you are in tells you the answer before any drawing.
A convex mirror has no such drama. Wherever the object is, near or far, the image is small, erect and virtual — always. That reliability is precisely why it is bolted to the side of every vehicle.
This page covers the third part of the ICSE Class 9 Physics chapter on the reflection of light — the six concave cases, the convex case, and why each mirror is chosen for the job it does.
Now bring it slowly closer. The image grows, stays inverted, and at one particular moment it blurs away entirely — and then reappears the right way up and magnified.
The moment of the flip is when your face passes the principal focus. Outside the focus a concave mirror forms a real, inverted image; inside it, a virtual, erect, magnified one.
That single crossing organises the whole of this page. A concave mirror does six different things depending on where the object sits relative to F and C, and knowing which case you are in tells you the answer before any drawing.
A convex mirror has no such drama. Wherever the object is, near or far, the image is small, erect and virtual — always. That reliability is precisely why it is bolted to the side of every vehicle.
This page covers the third part of the ICSE Class 9 Physics chapter on the reflection of light — the six concave cases, the convex case, and why each mirror is chosen for the job it does.
What are the six cases for a concave mirror?
Mark P, F and C on the principal axis, note which region the object is in, and the answer follows. Use rule 1 (parallel ray through F) and rule 3 (ray through C returns on itself) from the previous part of this chapter.
Take a concave mirror of focal length cm, so cm. Then F is at cm and C is at cm from the pole.
Case 1 — object at infinity. Rays arrive parallel to the axis, so by rule 1 they all converge at F.
- Image at F, that is at cm
- Highly diminished, effectively a point
- Real and inverted
This is how a concave mirror makes a bright hot spot from sunlight.
Case 2 — object beyond C, say at cm.
- Image between F and C, so between cm and cm
- Diminished
- Real and inverted
Case 3 — object at C, at cm.
- Image at C, also at cm
- Same size as the object, so the magnification is exactly
- Real and inverted
An object cm tall placed at C gives a cm image, inverted, at the same place.
Case 4 — object between C and F, say at cm.
- Image beyond C, so further than cm
- Magnified
- Real and inverted
Case 5 — object at F, at cm.
- The reflected rays come out parallel to each other, so they never meet
- Image at infinity, highly magnified
- Real and inverted in principle, and in practice no image is formed on any screen you could place
Case 6 — object between F and the pole, say at cm.
- The reflected rays diverge, so they are produced backward
- Image behind the mirror
- Magnified
- Virtual and erect
Only case 6 gives a virtual image, and it is the only case where the image is erect. That single row is the one the shaving mirror and the dentist's mirror use.
The pattern worth noticing. As the object moves in from infinity towards F, the image moves out from F towards infinity, growing all the way. Object and image pass each other at C, where both are at cm and the sizes are equal. So C is the crossover point for size, and F is the crossover point for real against virtual — two different landmarks doing two different jobs.
A real image can be caught on a screen, and cases 1 to 4 can all be demonstrated that way. Case 6 cannot, however carefully the screen is placed, because the light never goes behind the mirror. That is the practical test of the table, and it is the distinction from the first part of this chapter applied six times over.
Take a concave mirror of focal length cm, so cm. Then F is at cm and C is at cm from the pole.
Case 1 — object at infinity. Rays arrive parallel to the axis, so by rule 1 they all converge at F.
- Image at F, that is at cm
- Highly diminished, effectively a point
- Real and inverted
This is how a concave mirror makes a bright hot spot from sunlight.
Case 2 — object beyond C, say at cm.
- Image between F and C, so between cm and cm
- Diminished
- Real and inverted
Case 3 — object at C, at cm.
- Image at C, also at cm
- Same size as the object, so the magnification is exactly
- Real and inverted
An object cm tall placed at C gives a cm image, inverted, at the same place.
Case 4 — object between C and F, say at cm.
- Image beyond C, so further than cm
- Magnified
- Real and inverted
Case 5 — object at F, at cm.
- The reflected rays come out parallel to each other, so they never meet
- Image at infinity, highly magnified
- Real and inverted in principle, and in practice no image is formed on any screen you could place
Case 6 — object between F and the pole, say at cm.
- The reflected rays diverge, so they are produced backward
- Image behind the mirror
- Magnified
- Virtual and erect
Only case 6 gives a virtual image, and it is the only case where the image is erect. That single row is the one the shaving mirror and the dentist's mirror use.
The pattern worth noticing. As the object moves in from infinity towards F, the image moves out from F towards infinity, growing all the way. Object and image pass each other at C, where both are at cm and the sizes are equal. So C is the crossover point for size, and F is the crossover point for real against virtual — two different landmarks doing two different jobs.
A real image can be caught on a screen, and cases 1 to 4 can all be demonstrated that way. Case 6 cannot, however carefully the screen is placed, because the light never goes behind the mirror. That is the practical test of the table, and it is the distinction from the first part of this chapter applied six times over.
Why does a convex mirror always give the same kind of image?
Because a convex mirror diverges every beam that reaches it, so the reflected rays can never meet in front of the mirror and the image is always virtual.
The ray diagram. Take a convex mirror with F and C behind it. From the tip of the object:
- Draw a ray parallel to the axis. By rule 1 it reflects so that it appears to come from F, which lies behind the mirror
- Draw a ray directed towards C. By rule 3 it strikes along the normal and returns along its own path
- The two reflected rays diverge, so produce them backward with dotted lines. They meet behind the mirror, between P and F
The characteristics, for every object position.
- Image between the pole and F, always
- Diminished, always
- Virtual and erect, always
Worked example. A convex mirror has cm, so
and F lies cm behind the pole. Whatever the object distance — cm, cm or m — the image falls somewhere in that cm space behind the glass, shrinking towards F as the object moves further away.
Case at infinity. Parallel rays arriving from a very distant object appear after reflection to come from F, so the image forms at F, cm behind the pole, and is a point.
So the whole image range of a convex mirror is squeezed into the focal length. However far away the object, the image cannot be further behind the mirror than F — and that is why the image is always small.
The field of view is the compensation. Because a convex mirror diverges light, it gathers rays from a much wider cone than a plane or concave mirror of the same size and shows them all in that small image. Comparing the three for the same aperture:
Small images and a wide view are the same fact stated twice. Fitting more of the world into a mirror of fixed size requires shrinking everything in it, so a convex mirror cannot be wide-angled and life-sized. That trade is the reason rear-view mirrors make traffic look further away than it is, and the reason they are still the right choice for the job.
The ray diagram. Take a convex mirror with F and C behind it. From the tip of the object:
- Draw a ray parallel to the axis. By rule 1 it reflects so that it appears to come from F, which lies behind the mirror
- Draw a ray directed towards C. By rule 3 it strikes along the normal and returns along its own path
- The two reflected rays diverge, so produce them backward with dotted lines. They meet behind the mirror, between P and F
The characteristics, for every object position.
- Image between the pole and F, always
- Diminished, always
- Virtual and erect, always
Worked example. A convex mirror has cm, so
and F lies cm behind the pole. Whatever the object distance — cm, cm or m — the image falls somewhere in that cm space behind the glass, shrinking towards F as the object moves further away.
Case at infinity. Parallel rays arriving from a very distant object appear after reflection to come from F, so the image forms at F, cm behind the pole, and is a point.
So the whole image range of a convex mirror is squeezed into the focal length. However far away the object, the image cannot be further behind the mirror than F — and that is why the image is always small.
The field of view is the compensation. Because a convex mirror diverges light, it gathers rays from a much wider cone than a plane or concave mirror of the same size and shows them all in that small image. Comparing the three for the same aperture:
Small images and a wide view are the same fact stated twice. Fitting more of the world into a mirror of fixed size requires shrinking everything in it, so a convex mirror cannot be wide-angled and life-sized. That trade is the reason rear-view mirrors make traffic look further away than it is, and the reason they are still the right choice for the job.
Why is a shaving mirror concave and a headlight reflector also concave?
Both use a concave mirror, and they use two completely different cases of it. The shaving mirror uses case 6; the headlight uses case 1 run backwards.
A shaving or make-up mirror. The face is held closer than the focal length, so this is case 6: the image is virtual, erect and magnified. You see an enlarged upright view of your own face, which is exactly what is wanted for a close task.
The mirror must therefore have a fairly long focal length — long enough that a face held at a comfortable distance is still inside F. A short-focus mirror would put the face beyond F and produce an inverted image.
A dentist's mirror. The same case, smaller. A small concave mirror is held very close to a tooth, well within its focal length, giving a magnified erect view of the tooth. It also converges light onto the tooth, brightening a dark corner of the mouth — two benefits from one shape.
A headlight or torch reflector. Now the mirror is used in reverse. The bulb's filament is placed at the principal focus, and by rule 2 rays leaving F emerge parallel to the principal axis after reflection.
A parallel beam does not spread, so the light stays intense over a long distance and forms a strong beam down the road. Put the bulb anywhere other than F and the beam spreads or converges and the range collapses — which is why a torch with a loose bulb gives a poor beam.
A solar concentrator or solar cooker. This is case 1: sunlight arrives effectively parallel from a very distant source, so all of it converges at F. Putting the cooking vessel at the focus concentrates the energy collected over the mirror's whole aperture onto a small area, and the temperature there rises far above the surroundings.
A doctor's head mirror and an ophthalmoscope use the converging action in the same way, gathering light and directing it into a small opening — an ear, a throat, the pupil of an eye.
A reflecting telescope uses a large concave mirror to collect light from a faint distant object and converge it, and the larger the mirror's aperture the more light is gathered.
Every one of these is the converging property, used in one of two directions. Light coming in from far away is concentrated at F — the cooker, the telescope. Light starting at F is sent out parallel — the headlight, the torch. And the magnifier is the third use, with the object inside F, which is the only case giving an erect image.
The same mirror does all three jobs; only the distances change. A concave mirror is not a magnifier or a concentrator or a beam-maker by construction — it becomes each of them according to where the object or the source is placed relative to F. So a question asking why a mirror is concave is incomplete until it says where the object sits, and the answer must name the case.
A shaving or make-up mirror. The face is held closer than the focal length, so this is case 6: the image is virtual, erect and magnified. You see an enlarged upright view of your own face, which is exactly what is wanted for a close task.
The mirror must therefore have a fairly long focal length — long enough that a face held at a comfortable distance is still inside F. A short-focus mirror would put the face beyond F and produce an inverted image.
A dentist's mirror. The same case, smaller. A small concave mirror is held very close to a tooth, well within its focal length, giving a magnified erect view of the tooth. It also converges light onto the tooth, brightening a dark corner of the mouth — two benefits from one shape.
A headlight or torch reflector. Now the mirror is used in reverse. The bulb's filament is placed at the principal focus, and by rule 2 rays leaving F emerge parallel to the principal axis after reflection.
A parallel beam does not spread, so the light stays intense over a long distance and forms a strong beam down the road. Put the bulb anywhere other than F and the beam spreads or converges and the range collapses — which is why a torch with a loose bulb gives a poor beam.
A solar concentrator or solar cooker. This is case 1: sunlight arrives effectively parallel from a very distant source, so all of it converges at F. Putting the cooking vessel at the focus concentrates the energy collected over the mirror's whole aperture onto a small area, and the temperature there rises far above the surroundings.
A doctor's head mirror and an ophthalmoscope use the converging action in the same way, gathering light and directing it into a small opening — an ear, a throat, the pupil of an eye.
A reflecting telescope uses a large concave mirror to collect light from a faint distant object and converge it, and the larger the mirror's aperture the more light is gathered.
Every one of these is the converging property, used in one of two directions. Light coming in from far away is concentrated at F — the cooker, the telescope. Light starting at F is sent out parallel — the headlight, the torch. And the magnifier is the third use, with the object inside F, which is the only case giving an erect image.
The same mirror does all three jobs; only the distances change. A concave mirror is not a magnifier or a concentrator or a beam-maker by construction — it becomes each of them according to where the object or the source is placed relative to F. So a question asking why a mirror is concave is incomplete until it says where the object sits, and the answer must name the case.
Why is a rear-view mirror convex rather than plane?
Because a convex mirror shows a much wider stretch of road in a mirror of the same size, and it always gives an erect image so the view is easy to read.
A rear-view or wing mirror. The image is erect, which is essential — an inverted view of the traffic behind would be unusable. It is diminished, which is the price paid. And the field of view is wide, so a driver sees a large sweep of the road behind and to the side without moving the head.
A plane mirror of the same size would give a life-sized image and a much narrower view, leaving a large blind region. A concave mirror would be worse still — a narrow view, and an inverted image for anything beyond its focus.
A road-safety mirror at a blind corner. A large convex mirror mounted at a bend or at the exit of a narrow lane lets a driver see traffic approaching from a road they cannot look down directly. The same mirror in a shop lets a shopkeeper watch several aisles at once.
A street-light reflector uses a convex surface for the reverse reason: it spreads the light from the lamp over a wide area of road rather than concentrating it in one spot.
The limitation, and it is a real one. Because the image is diminished, an approaching vehicle looks smaller and therefore further away than it really is, and it appears to be moving more slowly. A driver relying only on a convex mirror can badly misjudge the gap — which is why such mirrors carry a printed warning and why drivers are taught to turn and look before changing lane.
Comparing the three mirrors, for the same aperture.
- Plane mirror — image virtual, erect, same size, as far behind as the object is in front; field of view medium
- Concave mirror — image real or virtual depending on the object's position, magnified or diminished, inverted or erect; field of view smallest
- Convex mirror — image always virtual, always erect, always diminished, always between P and F; field of view largest
Each mirror is chosen by which property the job needs most. A driver needs coverage and accepts small images, so convex. A person shaving needs a magnified erect view of a nearby face, so concave used within F. A person checking their appearance needs a true-size view, so plane.
No mirror gives a wide field of view AND a life-sized image, and that is not a limitation of manufacturing. A fixed area of glass showing more of the world must show all of it smaller — the two demands are in direct conflict, and every mirror on this page is a choice about which one matters. So "which is the best mirror" has no answer without the job, and naming the job is the first line of a complete response.
A rear-view or wing mirror. The image is erect, which is essential — an inverted view of the traffic behind would be unusable. It is diminished, which is the price paid. And the field of view is wide, so a driver sees a large sweep of the road behind and to the side without moving the head.
A plane mirror of the same size would give a life-sized image and a much narrower view, leaving a large blind region. A concave mirror would be worse still — a narrow view, and an inverted image for anything beyond its focus.
A road-safety mirror at a blind corner. A large convex mirror mounted at a bend or at the exit of a narrow lane lets a driver see traffic approaching from a road they cannot look down directly. The same mirror in a shop lets a shopkeeper watch several aisles at once.
A street-light reflector uses a convex surface for the reverse reason: it spreads the light from the lamp over a wide area of road rather than concentrating it in one spot.
The limitation, and it is a real one. Because the image is diminished, an approaching vehicle looks smaller and therefore further away than it really is, and it appears to be moving more slowly. A driver relying only on a convex mirror can badly misjudge the gap — which is why such mirrors carry a printed warning and why drivers are taught to turn and look before changing lane.
Comparing the three mirrors, for the same aperture.
- Plane mirror — image virtual, erect, same size, as far behind as the object is in front; field of view medium
- Concave mirror — image real or virtual depending on the object's position, magnified or diminished, inverted or erect; field of view smallest
- Convex mirror — image always virtual, always erect, always diminished, always between P and F; field of view largest
Each mirror is chosen by which property the job needs most. A driver needs coverage and accepts small images, so convex. A person shaving needs a magnified erect view of a nearby face, so concave used within F. A person checking their appearance needs a true-size view, so plane.
No mirror gives a wide field of view AND a life-sized image, and that is not a limitation of manufacturing. A fixed area of glass showing more of the world must show all of it smaller — the two demands are in direct conflict, and every mirror on this page is a choice about which one matters. So "which is the best mirror" has no answer without the job, and naming the job is the first line of a complete response.
Exam tip
Exam tip: mark P, F and C first and name the case
Mark P, F and C on the axis before drawing anything, equally spaced with . For cm, F is at cm and C at cm.
Say which region the object is in — beyond C, between C and F, between F and P — and then state the image. The region is the answer.
Use rules 1 and 3: the parallel ray through F, and the ray through C returning on itself.
Give all three facts about an image: position, size and nature (real or virtual, erect or inverted). A question asking for the characteristics wants all three.
Only the object between F and P gives a virtual erect image in a concave mirror — every other case is real and inverted.
At C the image is the same size and also at C, so the magnification is exactly .
At F the image is at infinity, and the reflected rays come out parallel.
A convex mirror gives the same answer every time: virtual, erect, diminished, between P and F. Say for every position of the object.
Use dotted lines for any ray path behind the mirror, and for a virtual image.
For a use, name the case, not just the mirror: a shaving mirror is concave with the face inside F; a headlight has the bulb at F.
And for field of view quote the order convex > plane > concave, and say that a wide view and a life-sized image cannot both be had.
Say which region the object is in — beyond C, between C and F, between F and P — and then state the image. The region is the answer.
Use rules 1 and 3: the parallel ray through F, and the ray through C returning on itself.
Give all three facts about an image: position, size and nature (real or virtual, erect or inverted). A question asking for the characteristics wants all three.
Only the object between F and P gives a virtual erect image in a concave mirror — every other case is real and inverted.
At C the image is the same size and also at C, so the magnification is exactly .
At F the image is at infinity, and the reflected rays come out parallel.
A convex mirror gives the same answer every time: virtual, erect, diminished, between P and F. Say for every position of the object.
Use dotted lines for any ray path behind the mirror, and for a virtual image.
For a use, name the case, not just the mirror: a shaving mirror is concave with the face inside F; a headlight has the bulb at F.
And for field of view quote the order convex > plane > concave, and say that a wide view and a life-sized image cannot both be had.
Did you know
Why a mirror cannot be both wide-angled and life-sized
A driver would love a mirror that showed the whole road behind and showed every vehicle at its true size. No such mirror can be built, and the reason is arithmetic rather than engineering.
Think about what a mirror of a fixed size has to do. It must take light arriving from some cone of directions and fit the whole of that cone into its own small area. Widen the cone and everything inside it has to be squeezed smaller. There is no more glass to spread the picture across.
A plane mirror shows a cone of moderate width at life size. A convex mirror bends the rays outward, so it accepts a much wider cone — and everything in it is correspondingly shrunk. A concave mirror does the opposite, accepting a narrow cone and magnifying it.
So the three mirrors are three points on one trade, and magnification and field of view move in opposite directions:
There is a neat consequence. The warning printed on a vehicle's convex mirror — that objects are closer than they appear — is not a defect notice. It is a statement that the mirror has been chosen to maximise coverage, and that the cost of coverage is always misjudged distance.
The same trade shows up elsewhere. A wide-angle camera lens fits more of a scene onto the same sensor and so records everything smaller; a telescope's narrow view is the price of its magnification. A door's peephole is a strongly diverging lens for exactly the reason a wing mirror is convex — see the whole corridor, see everyone in it tiny.
Nobody chooses between a good mirror and a bad one. The choice is always which half of the trade the job can afford to lose.
Think about what a mirror of a fixed size has to do. It must take light arriving from some cone of directions and fit the whole of that cone into its own small area. Widen the cone and everything inside it has to be squeezed smaller. There is no more glass to spread the picture across.
A plane mirror shows a cone of moderate width at life size. A convex mirror bends the rays outward, so it accepts a much wider cone — and everything in it is correspondingly shrunk. A concave mirror does the opposite, accepting a narrow cone and magnifying it.
So the three mirrors are three points on one trade, and magnification and field of view move in opposite directions:
There is a neat consequence. The warning printed on a vehicle's convex mirror — that objects are closer than they appear — is not a defect notice. It is a statement that the mirror has been chosen to maximise coverage, and that the cost of coverage is always misjudged distance.
The same trade shows up elsewhere. A wide-angle camera lens fits more of a scene onto the same sensor and so records everything smaller; a telescope's narrow view is the price of its magnification. A door's peephole is a strongly diverging lens for exactly the reason a wing mirror is convex — see the whole corridor, see everyone in it tiny.
Nobody chooses between a good mirror and a bad one. The choice is always which half of the trade the job can afford to lose.
Exam relevance
How are mirror image cases tested in JEE Main and NEET?
Because the six cases become the sign-convention arithmetic of Class 12, and the applications are asked directly as reasoning questions.
This is the foundation for Class 12 Physics Ray Optics and Optical Instruments, examined in JEE Main and NEET. The table built on this page is reproduced there by the mirror formula and the magnification formula:
Every row of the six-case table is a sign pattern in those two equations. A real image gives negative under the Cartesian convention and negative, which is what inverted means arithmetically; a virtual image gives positive and positive, which is erect. So the cases learnt as a table become a way of checking whether a calculated answer is sensible — if the algebra says an object beyond C has a magnified image, the algebra is wrong.
The at-C case is the one worth remembering as a landmark. There and exactly, which makes it the standard check on any newly written sign convention. JEE Main questions frequently set the object at C precisely because the answer is clean.
Case 5 becomes the collimator. An object at F giving parallel rays out is how a collimated beam is produced, used in a spectrometer and in every optical instrument needing parallel light — and it is the headlight of this page in laboratory dress.
Case 1 becomes the telescope. Class 12 treats the reflecting telescope, where a large concave mirror forms a real image of a distant object at its focus, and the magnifying power depends on the focal lengths. The solar concentrator and the telescope are the same case with different purposes.
The field-of-view ordering is asked as a conceptual item in both papers, usually as a reason for the convex rear-view mirror, and the expected answer is the trade described on this page rather than a formula.
For NEET Physics, expect numericals on image position and magnification and conceptual questions on the uses; for NEET Biology, the eye's own converging optics form a real inverted image on the retina, which is case 2 of this table performed by a lens — and the fact that the image is inverted while we see the world upright is a standard question.
What the questions look like. For board work, expect draw the ray diagram and state the image for a given object position, the full six-case table, the convex mirror's characteristics for any position, and explain a named use with its reason — the shaving mirror, the dentist's mirror, the headlight, the solar cooker, the rear-view mirror. Diagrams with dotted virtual rays carry much of the credit. For JEE Main and NEET, expect mirror-formula numericals, magnification signs, and the field-of-view reasoning.
How board and competitive emphasis differ. A board paper rewards the labelled ray diagram and a reason naming the case. A competitive paper gives and with signs and asks only for and , so the table's value there is as a sanity check on the arithmetic.
The single trap that costs the most marks. Saying a concave mirror magnifies without stating where the object is. It magnifies for an object between C and F (real, inverted) and for an object between F and P (virtual, erect), and it diminishes for an object beyond C. The defence is to name the region in the first line of every answer — once between F and the pole is written down, the rest of the description follows without thought.
This is the foundation for Class 12 Physics Ray Optics and Optical Instruments, examined in JEE Main and NEET. The table built on this page is reproduced there by the mirror formula and the magnification formula:
Every row of the six-case table is a sign pattern in those two equations. A real image gives negative under the Cartesian convention and negative, which is what inverted means arithmetically; a virtual image gives positive and positive, which is erect. So the cases learnt as a table become a way of checking whether a calculated answer is sensible — if the algebra says an object beyond C has a magnified image, the algebra is wrong.
The at-C case is the one worth remembering as a landmark. There and exactly, which makes it the standard check on any newly written sign convention. JEE Main questions frequently set the object at C precisely because the answer is clean.
Case 5 becomes the collimator. An object at F giving parallel rays out is how a collimated beam is produced, used in a spectrometer and in every optical instrument needing parallel light — and it is the headlight of this page in laboratory dress.
Case 1 becomes the telescope. Class 12 treats the reflecting telescope, where a large concave mirror forms a real image of a distant object at its focus, and the magnifying power depends on the focal lengths. The solar concentrator and the telescope are the same case with different purposes.
The field-of-view ordering is asked as a conceptual item in both papers, usually as a reason for the convex rear-view mirror, and the expected answer is the trade described on this page rather than a formula.
For NEET Physics, expect numericals on image position and magnification and conceptual questions on the uses; for NEET Biology, the eye's own converging optics form a real inverted image on the retina, which is case 2 of this table performed by a lens — and the fact that the image is inverted while we see the world upright is a standard question.
What the questions look like. For board work, expect draw the ray diagram and state the image for a given object position, the full six-case table, the convex mirror's characteristics for any position, and explain a named use with its reason — the shaving mirror, the dentist's mirror, the headlight, the solar cooker, the rear-view mirror. Diagrams with dotted virtual rays carry much of the credit. For JEE Main and NEET, expect mirror-formula numericals, magnification signs, and the field-of-view reasoning.
How board and competitive emphasis differ. A board paper rewards the labelled ray diagram and a reason naming the case. A competitive paper gives and with signs and asks only for and , so the table's value there is as a sanity check on the arithmetic.
The single trap that costs the most marks. Saying a concave mirror magnifies without stating where the object is. It magnifies for an object between C and F (real, inverted) and for an object between F and P (virtual, erect), and it diminishes for an object beyond C. The defence is to name the region in the first line of every answer — once between F and the pole is written down, the rest of the description follows without thought.
Key takeaways
Concave and convex mirror images and their uses: quick revision
- Mark P, F and C first, equally spaced with . For cm: F at cm, C at cm.
- Concave, object at infinity — image at F, highly diminished, real inverted.
- Beyond C (say cm) — image between F and C, diminished, real inverted.
- At C ( cm) — image at C, same size (), real inverted. A cm object gives a cm image.
- Between C and F ( cm) — image beyond C, magnified, real inverted.
- At F ( cm) — reflected rays parallel, image at infinity, highly magnified.
- Between F and P ( cm) — image behind the mirror, magnified, virtual erect — the only virtual case.
- As the object moves in from infinity to F, the image moves out from F to infinity, and they cross at C where both are at cm.
- C is the crossover for size; F is the crossover for real against virtual.
- Convex mirror, every object position — image between P and F, diminished, virtual erect. With cm, cm, so the whole image range is cm behind the glass.
- Field of view: convex > plane > concave for the same aperture.
- Shaving and make-up mirror — concave, face inside F, giving a magnified erect image; needs a long focal length.
- Dentist's mirror — concave used within F: magnified erect view and light converged onto the tooth.
- Headlight and torch — concave with the bulb at F, so the beam emerges parallel and carries far.
- Solar cooker and concentrator — concave with sunlight arriving parallel, converging all of it at F.
- Reflecting telescope and head mirror — the same converging action, collecting faint or scattered light.
- Rear-view and wing mirror — convex, for an erect image and a wide field of view; the cost is that vehicles look smaller and further away.
- Road-safety mirror at a blind corner and a shop mirror — convex, for coverage.
- Street-light reflector — convex, to spread light widely.
- Plane mirror: virtual, erect, same size, equal distances, medium field of view.
- No mirror gives a wide field of view AND a life-sized image — a fixed area showing more must show it smaller.
- "Concave magnifies" is incomplete — it diminishes for an object beyond C, so always name the region.
Find a steel spoon and move it slowly towards your eye until the image flips upright, then measure that distance — you have just located the focal length of a spoon.
- Concave, object at infinity — image at F, highly diminished, real inverted.
- Beyond C (say cm) — image between F and C, diminished, real inverted.
- At C ( cm) — image at C, same size (), real inverted. A cm object gives a cm image.
- Between C and F ( cm) — image beyond C, magnified, real inverted.
- At F ( cm) — reflected rays parallel, image at infinity, highly magnified.
- Between F and P ( cm) — image behind the mirror, magnified, virtual erect — the only virtual case.
- As the object moves in from infinity to F, the image moves out from F to infinity, and they cross at C where both are at cm.
- C is the crossover for size; F is the crossover for real against virtual.
- Convex mirror, every object position — image between P and F, diminished, virtual erect. With cm, cm, so the whole image range is cm behind the glass.
- Field of view: convex > plane > concave for the same aperture.
- Shaving and make-up mirror — concave, face inside F, giving a magnified erect image; needs a long focal length.
- Dentist's mirror — concave used within F: magnified erect view and light converged onto the tooth.
- Headlight and torch — concave with the bulb at F, so the beam emerges parallel and carries far.
- Solar cooker and concentrator — concave with sunlight arriving parallel, converging all of it at F.
- Reflecting telescope and head mirror — the same converging action, collecting faint or scattered light.
- Rear-view and wing mirror — convex, for an erect image and a wide field of view; the cost is that vehicles look smaller and further away.
- Road-safety mirror at a blind corner and a shop mirror — convex, for coverage.
- Street-light reflector — convex, to spread light widely.
- Plane mirror: virtual, erect, same size, equal distances, medium field of view.
- No mirror gives a wide field of view AND a life-sized image — a fixed area showing more must show it smaller.
- "Concave magnifies" is incomplete — it diminishes for an object beyond C, so always name the region.
Find a steel spoon and move it slowly towards your eye until the image flips upright, then measure that distance — you have just located the focal length of a spoon.