A Convex Mirror Always Shows a Small Upright Image
Apply the laws of reflection to a plane mirror, learn the terms for a spherical mirror and the relation between radius and focal length, draw ray diagrams for concave and convex mirrors, and match each mirror to its practical use.
Why does a curved mirror change the size of what you see?
Look at yourself in a flat mirror and the reflection is always life-size, however close or far you stand. Look at the back of a steel spoon and your face is small and upright; turn the spoon over and look at the inside, and your face is large — or upside down, if you hold it far enough away.
All three surfaces obey exactly the same laws of reflection. What differs is the direction of the normal at each point. On a flat mirror every normal points the same way, so a parallel beam stays parallel. On a curved mirror the normals fan out or converge, so a parallel beam is brought together or spread apart — and that is what changes the size, the position and even the orientation of the image.
Two kinds of spherical mirror do the two opposite things:
- A concave mirror curves inward, like the inside of a spoon, and converges light
- A convex mirror curves outward, like the back of a spoon, and diverges light
Once you can predict where the rays go, you can predict the image without ever setting up the mirror — and that prediction is done with a ray diagram, using only four rules.
This page covers the first part of the CBSE Class 10 Science chapter on light: the laws of reflection and the plane-mirror image, the terms and relations for a spherical mirror, ray diagrams for both types, and the practical uses that follow.
All three surfaces obey exactly the same laws of reflection. What differs is the direction of the normal at each point. On a flat mirror every normal points the same way, so a parallel beam stays parallel. On a curved mirror the normals fan out or converge, so a parallel beam is brought together or spread apart — and that is what changes the size, the position and even the orientation of the image.
Two kinds of spherical mirror do the two opposite things:
- A concave mirror curves inward, like the inside of a spoon, and converges light
- A convex mirror curves outward, like the back of a spoon, and diverges light
Once you can predict where the rays go, you can predict the image without ever setting up the mirror — and that prediction is done with a ray diagram, using only four rules.
This page covers the first part of the CBSE Class 10 Science chapter on light: the laws of reflection and the plane-mirror image, the terms and relations for a spherical mirror, ray diagrams for both types, and the practical uses that follow.
What do the laws of reflection say, and what image does a plane mirror give?
Two laws, and they hold for every reflecting surface — flat or curved, polished metal or still water.
- The angle of incidence equals the angle of reflection, both measured from the normal — the line perpendicular to the surface at the point where the ray strikes
- The incident ray, the reflected ray and the normal all lie in the same plane
Both angles are measured from the normal and never from the surface, and that is where most diagram marks are lost. A ray striking a mirror at to the surface has an angle of incidence of , because the normal is perpendicular to the surface.
The image in a plane mirror has four properties, all of which follow from the two laws:
- Virtual — it cannot be caught on a screen, because the rays only appear to come from behind the mirror
- Erect — the right way up
- The same size as the object
- As far behind the mirror as the object is in front, and laterally inverted — left and right are exchanged
Worked example 1. A boy stands m in front of a plane mirror. How far is he from his image, and what happens if he walks m towards the mirror?
The image is m behind the mirror, so the distance between him and his image is
After walking m forward he is m from the mirror, so his image is m behind it, and the separation becomes
So the image approaches him at twice his own speed — he closed the gap by m while moving only m. That doubling is a favourite question, and the reason is that both he and the image moved.
Worked example 2 — lateral inversion. Hold up a piece of paper with AMBULANCE written on it in front of a mirror, and the reflection reads backwards. That is why the word is painted reversed on the front of the vehicle: a driver looking in a rear-view mirror sees it the right way round.
The property that is most often misstated. Lateral inversion exchanges left and right, not top and bottom. Your reflection's head is still at the top. **A question asking why the letters look reversed wants lateral inversion, and one asking whether the image is upside down wants no, it is erect.**
- The angle of incidence equals the angle of reflection, both measured from the normal — the line perpendicular to the surface at the point where the ray strikes
- The incident ray, the reflected ray and the normal all lie in the same plane
Both angles are measured from the normal and never from the surface, and that is where most diagram marks are lost. A ray striking a mirror at to the surface has an angle of incidence of , because the normal is perpendicular to the surface.
The image in a plane mirror has four properties, all of which follow from the two laws:
- Virtual — it cannot be caught on a screen, because the rays only appear to come from behind the mirror
- Erect — the right way up
- The same size as the object
- As far behind the mirror as the object is in front, and laterally inverted — left and right are exchanged
Worked example 1. A boy stands m in front of a plane mirror. How far is he from his image, and what happens if he walks m towards the mirror?
The image is m behind the mirror, so the distance between him and his image is
After walking m forward he is m from the mirror, so his image is m behind it, and the separation becomes
So the image approaches him at twice his own speed — he closed the gap by m while moving only m. That doubling is a favourite question, and the reason is that both he and the image moved.
Worked example 2 — lateral inversion. Hold up a piece of paper with AMBULANCE written on it in front of a mirror, and the reflection reads backwards. That is why the word is painted reversed on the front of the vehicle: a driver looking in a rear-view mirror sees it the right way round.
The property that is most often misstated. Lateral inversion exchanges left and right, not top and bottom. Your reflection's head is still at the top. **A question asking why the letters look reversed wants lateral inversion, and one asking whether the image is upside down wants no, it is erect.**
Formula
What do pole, focus and radius of curvature mean, and how is R related to f?
Six terms describe a spherical mirror, and one simple relation connects two of them.
- Pole (P) — the centre of the reflecting surface, the point where the principal axis meets the mirror
- Centre of curvature (C) — the centre of the sphere of which the mirror is a part
- Radius of curvature (R) — the distance from P to C, the radius of that sphere
- Principal axis — the straight line through P and C, perpendicular to the mirror at the pole
- Principal focus (F) — the point on the principal axis where rays parallel to the axis meet after reflection, or appear to come from
- Focal length (f) — the distance from P to F
- Aperture — the effective width of the mirror
So the focus lies exactly midway between the pole and the centre of curvature.
Worked example 1. A concave mirror has a radius of curvature of cm. Find its focal length.
Worked example 2. A mirror has a focal length of cm. Where is its centre of curvature?
Where the focus sits tells you which mirror it is.
- For a concave mirror the reflected rays actually meet, so F and C are in front of the mirror, on the same side as the object. The focus is real
- For a convex mirror the reflected rays diverge and only appear to come from a point, so F and C are behind the mirror. The focus is virtual
Worked example 3 — a spoon. The inside of a spoon acts as a concave mirror and the outside as a convex one. If the spoon's bowl is part of a sphere of radius cm, both surfaces have cm and therefore cm — but one focus is in front of the surface and the other behind it. Same number, opposite sides.
**Why the relation holds. Take a ray parallel to the axis, striking the mirror at a point near the pole. The normal at that point passes through C, and the laws of reflection make the reflected ray cross the axis at exactly half the distance to C. The proof needs the rays to be close to the axis**, which is why mirrors with small apertures are used — a wide mirror does not bring a parallel beam to a single sharp point, and that blurring is the reason the aperture appears in the list of terms at all.
- Pole (P) — the centre of the reflecting surface, the point where the principal axis meets the mirror
- Centre of curvature (C) — the centre of the sphere of which the mirror is a part
- Radius of curvature (R) — the distance from P to C, the radius of that sphere
- Principal axis — the straight line through P and C, perpendicular to the mirror at the pole
- Principal focus (F) — the point on the principal axis where rays parallel to the axis meet after reflection, or appear to come from
- Focal length (f) — the distance from P to F
- Aperture — the effective width of the mirror
So the focus lies exactly midway between the pole and the centre of curvature.
Worked example 1. A concave mirror has a radius of curvature of cm. Find its focal length.
Worked example 2. A mirror has a focal length of cm. Where is its centre of curvature?
Where the focus sits tells you which mirror it is.
- For a concave mirror the reflected rays actually meet, so F and C are in front of the mirror, on the same side as the object. The focus is real
- For a convex mirror the reflected rays diverge and only appear to come from a point, so F and C are behind the mirror. The focus is virtual
Worked example 3 — a spoon. The inside of a spoon acts as a concave mirror and the outside as a convex one. If the spoon's bowl is part of a sphere of radius cm, both surfaces have cm and therefore cm — but one focus is in front of the surface and the other behind it. Same number, opposite sides.
**Why the relation holds. Take a ray parallel to the axis, striking the mirror at a point near the pole. The normal at that point passes through C, and the laws of reflection make the reflected ray cross the axis at exactly half the distance to C. The proof needs the rays to be close to the axis**, which is why mirrors with small apertures are used — a wide mirror does not bring a parallel beam to a single sharp point, and that blurring is the reason the aperture appears in the list of terms at all.
How do you draw a ray diagram for a spherical mirror?
Draw any two of four standard rays from the top of the object. Where they meet after reflection is the top of the image.
The four rules.
- A ray parallel to the principal axis is reflected through F — or, in a convex mirror, appears to come from F
- A ray passing through F is reflected parallel to the principal axis
- A ray directed towards C strikes the mirror along its own normal, so it is reflected straight back along the same path
- A ray striking the pole obliquely is reflected with an equal angle on the other side of the principal axis
Two rays are enough, because two lines fix a point. The third is a check.
A concave mirror gives six different results, depending on where the object is:
- Object at infinity — image at F, real, inverted, highly diminished (a point)
- Object beyond C — image between F and C, real, inverted, diminished
- Object at C — image at C, real, inverted, same size
- Object between C and F — image beyond C, real, inverted, enlarged
- Object at F — image at infinity, real, inverted, highly enlarged
- Object between F and P — image behind the mirror, virtual, erect, enlarged
Notice the pattern in the first four. As the object moves in from infinity towards F, the image moves out from F towards infinity, and it grows all the way. Object and image swap places at C, where they are the same size — which makes C the easiest case to remember and the reference point for the rest.
And notice that only the last case is virtual. A concave mirror gives a real, inverted image for every object outside F, and a virtual, erect one only when the object is closer than the focus. That single boundary at F divides the whole table.
A convex mirror gives just one result, whatever you do. The reflected rays always diverge, so they never meet in front of the mirror:
- Image always between P and F, behind the mirror — virtual, erect and diminished
As the object moves away, the image shrinks towards F and never disappears. That is why a convex mirror can show you a whole street in a small glass, and why it can never project an image onto a screen.
The two boundary cases worth checking. For a concave mirror with the object exactly at F, the reflected rays come out parallel and no image is formed at any finite distance — which is precisely the arrangement used in a torch, run backwards. And for an object exactly at C, the image is the same size, which is the one case where a concave mirror behaves like a plane mirror in size while still inverting. Both cases are worth drawing once, because a question naming F or C is usually pointing at them.
The four rules.
- A ray parallel to the principal axis is reflected through F — or, in a convex mirror, appears to come from F
- A ray passing through F is reflected parallel to the principal axis
- A ray directed towards C strikes the mirror along its own normal, so it is reflected straight back along the same path
- A ray striking the pole obliquely is reflected with an equal angle on the other side of the principal axis
Two rays are enough, because two lines fix a point. The third is a check.
A concave mirror gives six different results, depending on where the object is:
- Object at infinity — image at F, real, inverted, highly diminished (a point)
- Object beyond C — image between F and C, real, inverted, diminished
- Object at C — image at C, real, inverted, same size
- Object between C and F — image beyond C, real, inverted, enlarged
- Object at F — image at infinity, real, inverted, highly enlarged
- Object between F and P — image behind the mirror, virtual, erect, enlarged
Notice the pattern in the first four. As the object moves in from infinity towards F, the image moves out from F towards infinity, and it grows all the way. Object and image swap places at C, where they are the same size — which makes C the easiest case to remember and the reference point for the rest.
And notice that only the last case is virtual. A concave mirror gives a real, inverted image for every object outside F, and a virtual, erect one only when the object is closer than the focus. That single boundary at F divides the whole table.
A convex mirror gives just one result, whatever you do. The reflected rays always diverge, so they never meet in front of the mirror:
- Image always between P and F, behind the mirror — virtual, erect and diminished
As the object moves away, the image shrinks towards F and never disappears. That is why a convex mirror can show you a whole street in a small glass, and why it can never project an image onto a screen.
The two boundary cases worth checking. For a concave mirror with the object exactly at F, the reflected rays come out parallel and no image is formed at any finite distance — which is precisely the arrangement used in a torch, run backwards. And for an object exactly at C, the image is the same size, which is the one case where a concave mirror behaves like a plane mirror in size while still inverting. Both cases are worth drawing once, because a question naming F or C is usually pointing at them.
How do the images compare, and what is each mirror used for?
Concave mirrors are used wherever light must be gathered or magnified; convex mirrors wherever a wide view matters more than size.
Uses of a concave mirror, each following from one row of the table.
- Torch and vehicle headlight reflectors. Put the bulb at the focus and the reflected rays come out parallel, producing a strong beam that stays narrow over a distance
- Shaving and make-up mirrors. Hold the face between F and P and the image is virtual, erect and enlarged
- A dentist's mirror, for the same reason — an enlarged erect view of a small nearby object
- Solar furnaces and solar cookers. A large concave reflector gathers sunlight and concentrates it at the focus, where the temperature becomes high enough to cook
The headlight and the shaving mirror use the same mirror in opposite directions. In one, a source at the focus sends out a parallel beam; in the other, a nearby object gives an enlarged image. Both are rows of the six-case table, which is why the table is worth learning rather than the uses.
Uses of a convex mirror.
- Rear-view mirrors on vehicles. The image is always erect, and because the mirror diverges the light it takes in a much wider field of view than a flat mirror of the same size — so the driver sees more of the road behind
- Reflectors for street lights, spreading light over a larger area
Worked comparison — the same object in three mirrors. Place a small object cm from each of a plane mirror, a concave mirror of focal length cm, and a convex mirror of focal length cm.
- Plane mirror: image cm behind, same size, erect, virtual
- Concave mirror: the object is beyond C (which is at cm), so the image is real, inverted and diminished, between F and C
- Convex mirror: image virtual, erect and diminished, between P and F
Three mirrors, three different images from one object position — and you can predict all three without touching an instrument.
The trade-off in the rear-view mirror is worth stating. Everything looks smaller and therefore farther away than it really is, which is a genuine disadvantage. The wide field of view is judged worth it, because the danger a driver most needs to see is a vehicle outside the narrow view a flat mirror would give.
One misconception to clear. A virtual image can be seen perfectly well — you look at one every morning in the bathroom mirror. What it cannot do is form on a screen, because no light actually arrives where the image appears to be. **Virtual means not formed by real intersection of rays, not impossible to see.**
Uses of a concave mirror, each following from one row of the table.
- Torch and vehicle headlight reflectors. Put the bulb at the focus and the reflected rays come out parallel, producing a strong beam that stays narrow over a distance
- Shaving and make-up mirrors. Hold the face between F and P and the image is virtual, erect and enlarged
- A dentist's mirror, for the same reason — an enlarged erect view of a small nearby object
- Solar furnaces and solar cookers. A large concave reflector gathers sunlight and concentrates it at the focus, where the temperature becomes high enough to cook
The headlight and the shaving mirror use the same mirror in opposite directions. In one, a source at the focus sends out a parallel beam; in the other, a nearby object gives an enlarged image. Both are rows of the six-case table, which is why the table is worth learning rather than the uses.
Uses of a convex mirror.
- Rear-view mirrors on vehicles. The image is always erect, and because the mirror diverges the light it takes in a much wider field of view than a flat mirror of the same size — so the driver sees more of the road behind
- Reflectors for street lights, spreading light over a larger area
Worked comparison — the same object in three mirrors. Place a small object cm from each of a plane mirror, a concave mirror of focal length cm, and a convex mirror of focal length cm.
- Plane mirror: image cm behind, same size, erect, virtual
- Concave mirror: the object is beyond C (which is at cm), so the image is real, inverted and diminished, between F and C
- Convex mirror: image virtual, erect and diminished, between P and F
Three mirrors, three different images from one object position — and you can predict all three without touching an instrument.
The trade-off in the rear-view mirror is worth stating. Everything looks smaller and therefore farther away than it really is, which is a genuine disadvantage. The wide field of view is judged worth it, because the danger a driver most needs to see is a vehicle outside the narrow view a flat mirror would give.
One misconception to clear. A virtual image can be seen perfectly well — you look at one every morning in the bathroom mirror. What it cannot do is form on a screen, because no light actually arrives where the image appears to be. **Virtual means not formed by real intersection of rays, not impossible to see.**
Exam tip
What layout keeps a ray-diagram answer complete?
Draw the principal axis first, mark P, F and C to scale, then draw two rays with arrowheads. Diagram marks are awarded for the marked points and the arrows as much as for the image.
- Mark F halfway between P and C, using . A diagram with F and C in the wrong order loses everything that follows
- Use two of the four standard rays and label where each one goes. A ray drawn without a rule behind it earns nothing
- Put arrowheads on every ray to show the direction of travel
- Use dotted lines behind the mirror for a virtual image, and say virtual in words
- State the image in three words: nature (real or virtual), position, and size relative to the object. Every question asks for those three
- Measure angles from the normal, never from the surface
- **Say laterally inverted for the plane mirror, and remember that it is left-to-right, not top-to-bottom
- Match a use to its row of the table: a headlight is a source at F, a shaving mirror is an object between F and P
The misconception to name.** Real does not mean visible and virtual does not mean invisible. A real image can be caught on a screen because light actually converges there; a virtual image cannot, because the rays only appear to diverge from it — yet both can be seen with the eye. A question asking why the image in a plane mirror cannot be obtained on a screen wants exactly that reason.
- Mark F halfway between P and C, using . A diagram with F and C in the wrong order loses everything that follows
- Use two of the four standard rays and label where each one goes. A ray drawn without a rule behind it earns nothing
- Put arrowheads on every ray to show the direction of travel
- Use dotted lines behind the mirror for a virtual image, and say virtual in words
- State the image in three words: nature (real or virtual), position, and size relative to the object. Every question asks for those three
- Measure angles from the normal, never from the surface
- **Say laterally inverted for the plane mirror, and remember that it is left-to-right, not top-to-bottom
- Match a use to its row of the table: a headlight is a source at F, a shaving mirror is an object between F and P
The misconception to name.** Real does not mean visible and virtual does not mean invisible. A real image can be caught on a screen because light actually converges there; a virtual image cannot, because the rays only appear to diverge from it — yet both can be seen with the eye. A question asking why the image in a plane mirror cannot be obtained on a screen wants exactly that reason.
Did you know
Why do rear-view mirrors make everything look farther away?
Glance at the wing mirror of a car and the vehicle behind looks small and distant. Look over your shoulder and it is uncomfortably close. Both views are honest — the mirror is simply showing you a diminished image.
That shrinking is the price of the wide view. A convex mirror diverges the light that reaches it, so rays from a much larger area of the road can all arrive at the driver's eye. A flat mirror of the same size would show a narrow strip and nothing on either side of it — and a vehicle overtaking from outside that strip would be invisible until it arrived.
So the design accepts a real disadvantage to remove a worse one:
- The image is always erect, so nothing is confusing to interpret
- The image is always virtual, so it appears behind the mirror where it is comfortable to look at
- The field of view is much wider than a plane mirror could manage
- Everything looks smaller and therefore farther, which is the cost
That is why some vehicles carry a printed warning that objects in the mirror are closer than they appear. The mirror is not faulty; the geometry of a diverging surface makes a diminished image unavoidable.
The same physics appears in the mirror above a shop counter. A single convex mirror in a corner lets the shopkeeper see the whole room from one glass, and the price is again that everyone in it looks small. Coverage and magnification pull in opposite directions, and a designer chooses which one the situation needs.
And the opposite choice is made in a torch. There, coverage is exactly what is not wanted — the light must be concentrated into a narrow beam — so a concave reflector is used with the bulb at the focus. Two mirrors, two opposite goals, and one table of six cases predicts both.
One thing you can check yourself with a steel spoon. Hold the convex back of it at arm's length and your face is small and upright, however far you move it. Turn it over to the concave side and move it slowly away: the image is large and upright at first, blurs at one particular distance, and then flips upside down. That flip happens as your face passes the focus, and it is the boundary between the last two rows of the concave table.
That shrinking is the price of the wide view. A convex mirror diverges the light that reaches it, so rays from a much larger area of the road can all arrive at the driver's eye. A flat mirror of the same size would show a narrow strip and nothing on either side of it — and a vehicle overtaking from outside that strip would be invisible until it arrived.
So the design accepts a real disadvantage to remove a worse one:
- The image is always erect, so nothing is confusing to interpret
- The image is always virtual, so it appears behind the mirror where it is comfortable to look at
- The field of view is much wider than a plane mirror could manage
- Everything looks smaller and therefore farther, which is the cost
That is why some vehicles carry a printed warning that objects in the mirror are closer than they appear. The mirror is not faulty; the geometry of a diverging surface makes a diminished image unavoidable.
The same physics appears in the mirror above a shop counter. A single convex mirror in a corner lets the shopkeeper see the whole room from one glass, and the price is again that everyone in it looks small. Coverage and magnification pull in opposite directions, and a designer chooses which one the situation needs.
And the opposite choice is made in a torch. There, coverage is exactly what is not wanted — the light must be concentrated into a narrow beam — so a concave reflector is used with the bulb at the focus. Two mirrors, two opposite goals, and one table of six cases predicts both.
One thing you can check yourself with a steel spoon. Hold the convex back of it at arm's length and your face is small and upright, however far you move it. Turn it over to the concave side and move it slowly away: the image is large and upright at first, blurs at one particular distance, and then flips upside down. That flip happens as your face passes the focus, and it is the boundary between the last two rows of the concave table.
Exam relevance
How do mirrors feed into JEE and NEET physics?
This is foundation work for Class 11 Ray Optics and Optical Instruments, examined in both JEE Main and NEET.
Where it leads. Class 11 keeps the same laws, the same and the same four ray rules, and adds the derivation of the mirror formula, the treatment of spherical aberration, and the combination of mirrors with lenses and prisms in instruments — telescopes, microscopes and periscopes. The six-case table is assumed knowledge there, and questions rely on your being able to say instantly whether an image is real or virtual.
Where the ray rules go. They become the basis of every optical-instrument diagram in Class 11 and 12. JEE Advanced sets multi-element problems in which the image formed by one surface becomes the object for the next, and getting the nature and position right at each step is the whole solution.
Where the field-of-view idea goes. It reappears in the treatment of the eye, of the telescope's objective and of the reason large astronomical telescopes use concave mirrors rather than lenses. The trade-off between coverage and magnification is a recurring theme.
Question types to expect. At this level: state the laws, define the terms, draw the diagram, name the image and the use. In competitive papers: numericals using the mirror formula, which is the next part of this chapter, and assertion-reason items on whether a given image can be formed on a screen.
The single trap that costs marks. Measuring an angle from the mirror surface instead of from the normal. A ray at to the surface has an angle of incidence of , and every later angle in the question is then wrong. In JEE the same slip appears in refraction problems, where it changes the answer completely.
A second trap. Assuming a concave mirror always gives a real image. It gives a virtual, erect, enlarged image whenever the object is closer than the focus, and that is exactly the case a shaving-mirror question is built on. Assertion-reason items pair those two statements deliberately.
Board versus competitive emphasis. The CBSE paper marks the labelled ray diagram, the three-word description of the image and the named use; a competitive paper marks a numerical answer or a single correct statement. The transferable asset is the six-case table — learn it as a pattern rather than six separate facts, and the numericals of Part 2 become checks on something you already know.
Where it leads. Class 11 keeps the same laws, the same and the same four ray rules, and adds the derivation of the mirror formula, the treatment of spherical aberration, and the combination of mirrors with lenses and prisms in instruments — telescopes, microscopes and periscopes. The six-case table is assumed knowledge there, and questions rely on your being able to say instantly whether an image is real or virtual.
Where the ray rules go. They become the basis of every optical-instrument diagram in Class 11 and 12. JEE Advanced sets multi-element problems in which the image formed by one surface becomes the object for the next, and getting the nature and position right at each step is the whole solution.
Where the field-of-view idea goes. It reappears in the treatment of the eye, of the telescope's objective and of the reason large astronomical telescopes use concave mirrors rather than lenses. The trade-off between coverage and magnification is a recurring theme.
Question types to expect. At this level: state the laws, define the terms, draw the diagram, name the image and the use. In competitive papers: numericals using the mirror formula, which is the next part of this chapter, and assertion-reason items on whether a given image can be formed on a screen.
The single trap that costs marks. Measuring an angle from the mirror surface instead of from the normal. A ray at to the surface has an angle of incidence of , and every later angle in the question is then wrong. In JEE the same slip appears in refraction problems, where it changes the answer completely.
A second trap. Assuming a concave mirror always gives a real image. It gives a virtual, erect, enlarged image whenever the object is closer than the focus, and that is exactly the case a shaving-mirror question is built on. Assertion-reason items pair those two statements deliberately.
Board versus competitive emphasis. The CBSE paper marks the labelled ray diagram, the three-word description of the image and the named use; a competitive paper marks a numerical answer or a single correct statement. The transferable asset is the six-case table — learn it as a pattern rather than six separate facts, and the numericals of Part 2 become checks on something you already know.
Key takeaways
What should you know about mirrors before the mirror formula?
Two laws, seven terms, one relation and one table.
- Angle of incidence equals angle of reflection, both measured from the normal; and the incident ray, reflected ray and normal lie in one plane
- A plane mirror image is virtual, erect, the same size, as far behind as the object is in front, and laterally inverted — left to right, not top to bottom
- Walk towards a plane mirror and the gap closes at twice your speed, because the image moves too
- Pole, centre of curvature, radius of curvature, principal axis, principal focus, focal length and aperture — learn all seven
- **, so the focus lies midway between P and C
- Concave mirrors converge and have a real focus in front; convex mirrors diverge and have a virtual focus behind
- The four ray rules: parallel goes through F, through F comes out parallel, towards C returns along itself, at the pole reflects at an equal angle
- A concave mirror gives a real inverted image for any object beyond F, the same size at C, and a virtual erect enlarged image only between F and P
- A convex mirror always gives a virtual, erect, diminished image between P and F
- Uses: concave for headlights (source at F), shaving mirrors (object inside F) and solar cookers; convex for rear-view mirrors and street lights
- Virtual means not formed on a screen**, not invisible
The sharpest self-test is the spoon. Hold the concave side of a spoon at arm's length and bring it slowly towards your face, and name the row of the table at each stage — including the moment the image flips.
- Angle of incidence equals angle of reflection, both measured from the normal; and the incident ray, reflected ray and normal lie in one plane
- A plane mirror image is virtual, erect, the same size, as far behind as the object is in front, and laterally inverted — left to right, not top to bottom
- Walk towards a plane mirror and the gap closes at twice your speed, because the image moves too
- Pole, centre of curvature, radius of curvature, principal axis, principal focus, focal length and aperture — learn all seven
- **, so the focus lies midway between P and C
- Concave mirrors converge and have a real focus in front; convex mirrors diverge and have a virtual focus behind
- The four ray rules: parallel goes through F, through F comes out parallel, towards C returns along itself, at the pole reflects at an equal angle
- A concave mirror gives a real inverted image for any object beyond F, the same size at C, and a virtual erect enlarged image only between F and P
- A convex mirror always gives a virtual, erect, diminished image between P and F
- Uses: concave for headlights (source at F), shaving mirrors (object inside F) and solar cookers; convex for rear-view mirrors and street lights
- Virtual means not formed on a screen**, not invisible
The sharpest self-test is the spoon. Hold the concave side of a spoon at arm's length and bring it slowly towards your face, and name the row of the table at each stage — including the moment the image flips.