One Mirror Makes You Bigger and the Other Shrinks the Whole Room
Learn to tell a concave mirror from a convex one, compare the images all three mirror types form, apply the laws of reflection to find an angle, and explain lateral inversion.
Why does one curved mirror enlarge your face and another shrink a whole street?
Because they curve in opposite directions. A concave mirror curves inward and can magnify what is close to it; a convex mirror bulges outward and shrinks everything, fitting a wide view into a small glass.
That is why a shaving mirror and a vehicle's side mirror do such different jobs. This page covers everything in the CBSE Class 8 Science chapter's first part: types of spherical mirror, the images they form, the laws of reflection, and lateral inversion.
That is why a shaving mirror and a vehicle's side mirror do such different jobs. This page covers everything in the CBSE Class 8 Science chapter's first part: types of spherical mirror, the images they form, the laws of reflection, and lateral inversion.
How do you tell a concave mirror from a convex one?
A spherical mirror is a piece of a hollow sphere with one surface silvered. Which surface reflects decides the type.
- Concave mirror — the reflecting surface is the inner, hollow side, curving inward like the inside of a spoon. It is a converging mirror, because it brings parallel rays together.
- Convex mirror — the reflecting surface is the outer, bulging side, curving outward like the back of a spoon. It is a diverging mirror, because it spreads parallel rays apart.
A steel spoon is the perfect classroom demonstration. Look into its hollow side from close up and your face appears enlarged and upside down further away. Turn it over and look at the back, and your face appears small and upright, with much of the room visible around it.
Their uses follow directly from these behaviours:
- Concave — shaving and make-up mirrors, dentists' mirrors, torch and headlight reflectors, and solar cookers, where sunlight must be concentrated
- Convex — rear-view and side mirrors of vehicles, and security mirrors at shop corners and blind bends, where a wide field of view is needed
The trick for remembering is in the word itself: a cave goes inward, so concave curves inward. And note that the silvering is on the opposite side to the one that reflects — which is why the back of a spoon works as a convex mirror at all.
- Concave mirror — the reflecting surface is the inner, hollow side, curving inward like the inside of a spoon. It is a converging mirror, because it brings parallel rays together.
- Convex mirror — the reflecting surface is the outer, bulging side, curving outward like the back of a spoon. It is a diverging mirror, because it spreads parallel rays apart.
A steel spoon is the perfect classroom demonstration. Look into its hollow side from close up and your face appears enlarged and upside down further away. Turn it over and look at the back, and your face appears small and upright, with much of the room visible around it.
Their uses follow directly from these behaviours:
- Concave — shaving and make-up mirrors, dentists' mirrors, torch and headlight reflectors, and solar cookers, where sunlight must be concentrated
- Convex — rear-view and side mirrors of vehicles, and security mirrors at shop corners and blind bends, where a wide field of view is needed
The trick for remembering is in the word itself: a cave goes inward, so concave curves inward. And note that the silvering is on the opposite side to the one that reflects — which is why the back of a spoon works as a convex mirror at all.
What kind of image does each mirror form?
Compare them on three things: size, whether erect or inverted, and whether the image can be caught on a screen.
Plane mirror
- Size — always the same as the object
- Erect, and laterally inverted
- Cannot be obtained on a screen, so it is virtual
- Formed as far behind the mirror as the object is in front
Concave mirror
- Size — depends on the object's distance. Enlarged when the object is close; diminished when far away
- Erect when the object is very close; inverted when it is further away
- Can be obtained on a screen when inverted, so that image is real; the close-up enlarged one is virtual
Convex mirror
- Size — always diminished, smaller than the object
- Always erect
- Cannot be obtained on a screen, so always virtual
A real image is one that can be caught on a screen, because light rays actually meet there. A virtual image cannot, because the rays only appear to come from it.
So a concave mirror is the only one of the three whose image changes with distance — which is exactly why it must be held close for shaving, and why moving it back flips your face upside down.
That also settles the choice of mirror for a vehicle. A convex mirror always gives a small, erect image and a wide view, so a driver sees much more of the road behind — at the cost of everything looking further away than it really is.
Plane mirror
- Size — always the same as the object
- Erect, and laterally inverted
- Cannot be obtained on a screen, so it is virtual
- Formed as far behind the mirror as the object is in front
Concave mirror
- Size — depends on the object's distance. Enlarged when the object is close; diminished when far away
- Erect when the object is very close; inverted when it is further away
- Can be obtained on a screen when inverted, so that image is real; the close-up enlarged one is virtual
Convex mirror
- Size — always diminished, smaller than the object
- Always erect
- Cannot be obtained on a screen, so always virtual
A real image is one that can be caught on a screen, because light rays actually meet there. A virtual image cannot, because the rays only appear to come from it.
So a concave mirror is the only one of the three whose image changes with distance — which is exactly why it must be held close for shaving, and why moving it back flips your face upside down.
That also settles the choice of mirror for a vehicle. A convex mirror always gives a small, erect image and a wide view, so a driver sees much more of the road behind — at the cost of everything looking further away than it really is.
Formula
What are the laws of reflection?
There are two:
1. The angle of incidence equals the angle of reflection:
2. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
The terms, which must be labelled correctly on a ray diagram:
- Incident ray — the ray striking the mirror
- Point of incidence — where it strikes
- Normal — the line drawn at to the mirror at that point, always shown dotted
- Reflected ray — the ray leaving the mirror
- Angle of incidence () — between the incident ray and the normal
- Angle of reflection () — between the reflected ray and the normal
Worked examples.
A ray strikes a mirror at to the normal. Then , so
and the angle between the incident and reflected rays is .
Now a ray making with the mirror surface. The normal is at to the surface, so
And a ray falling along the normal has , so and it returns straight back along its own path.
Both angles are measured from the normal, never from the mirror — which is the single commonest error in this topic, and why that example gives rather than .
1. The angle of incidence equals the angle of reflection:
2. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
The terms, which must be labelled correctly on a ray diagram:
- Incident ray — the ray striking the mirror
- Point of incidence — where it strikes
- Normal — the line drawn at to the mirror at that point, always shown dotted
- Reflected ray — the ray leaving the mirror
- Angle of incidence () — between the incident ray and the normal
- Angle of reflection () — between the reflected ray and the normal
Worked examples.
A ray strikes a mirror at to the normal. Then , so
and the angle between the incident and reflected rays is .
Now a ray making with the mirror surface. The normal is at to the surface, so
And a ray falling along the normal has , so and it returns straight back along its own path.
Both angles are measured from the normal, never from the mirror — which is the single commonest error in this topic, and why that example gives rather than .
What is lateral inversion?
Lateral inversion is the interchange of the left and right sides of an object in its mirror image. Raise your right hand and the image appears to raise its left.
The classic example is the word AMBULANCE painted reversed on the front of the vehicle. A driver ahead sees it in their rear-view mirror, and the mirror reverses it back — so it reads the right way round exactly when it matters.
Other everyday cases:
- A clock seen in a mirror appears to run backwards
- Writing held up to a mirror suddenly becomes readable if it was written reversed
- The parting in your hair appears on the other side
Some capital letters survive lateral inversion, because they are symmetrical about a vertical line: A, H, I, M, O, T, U, V, W, X, Y. So a word such as MOM looks unchanged in a mirror while BOOK does not.
Letters such as B, C, D, E, F, G, J, K, L, N, P, Q, R, S, Z are all altered.
What is worth being precise about is that the mirror does not swap up and down. Your reflection's head stays up and its feet stay down, because the reversal happens along the direction facing the mirror — and we describe that as left-right only because we imagine turning ourselves around to face the same way as the image.
The classic example is the word AMBULANCE painted reversed on the front of the vehicle. A driver ahead sees it in their rear-view mirror, and the mirror reverses it back — so it reads the right way round exactly when it matters.
Other everyday cases:
- A clock seen in a mirror appears to run backwards
- Writing held up to a mirror suddenly becomes readable if it was written reversed
- The parting in your hair appears on the other side
Some capital letters survive lateral inversion, because they are symmetrical about a vertical line: A, H, I, M, O, T, U, V, W, X, Y. So a word such as MOM looks unchanged in a mirror while BOOK does not.
Letters such as B, C, D, E, F, G, J, K, L, N, P, Q, R, S, Z are all altered.
What is worth being precise about is that the mirror does not swap up and down. Your reflection's head stays up and its feet stay down, because the reversal happens along the direction facing the mirror — and we describe that as left-right only because we imagine turning ourselves around to face the same way as the image.
Exam tip
Exam tip: measuring angles from the normal every time
Mirror questions lose marks in three predictable places.
Measure and from the normal, not from the mirror. Draw the normal as a dotted line at the point of incidence before marking any angle, and if the question gives an angle with the surface, subtract it from first.
Label a ray diagram fully — incident ray, reflected ray, normal, point of incidence, and — with arrows showing the direction light travels. Labels carry their own marks.
When comparing images, give all three features: size, erect or inverted, and real or virtual. And define real as can be obtained on a screen.
Remember that a convex mirror always gives a diminished erect virtual image, while a concave mirror's image depends on the distance.
And for lateral inversion, name the left-right interchange and give the AMBULANCE example.
Measure and from the normal, not from the mirror. Draw the normal as a dotted line at the point of incidence before marking any angle, and if the question gives an angle with the surface, subtract it from first.
Label a ray diagram fully — incident ray, reflected ray, normal, point of incidence, and — with arrows showing the direction light travels. Labels carry their own marks.
When comparing images, give all three features: size, erect or inverted, and real or virtual. And define real as can be obtained on a screen.
Remember that a convex mirror always gives a diminished erect virtual image, while a concave mirror's image depends on the distance.
And for lateral inversion, name the left-right interchange and give the AMBULANCE example.
Did you know
Why does a convex mirror show so much more of the road?
Because its outward bulge spreads the reflected rays apart, gathering light from a much wider angle into the same small piece of glass.
A flat mirror of the same size reflects only what lies directly behind it. The curved surface of a convex mirror faces slightly different directions at every point, so each part sends you a view from a different angle — and all of them fit into one image.
The price is that everything is shrunk and so looks further away than it is. That is exactly why vehicle mirrors give a wide, safe view of traffic while a driver must still judge distances with care.
A flat mirror of the same size reflects only what lies directly behind it. The curved surface of a convex mirror faces slightly different directions at every point, so each part sends you a view from a different angle — and all of them fit into one image.
The price is that everything is shrunk and so looks further away than it is. That is exactly why vehicle mirrors give a wide, safe view of traffic while a driver must still judge distances with care.
Key takeaways
Mirrors and reflection: quick revision
- A concave mirror reflects from its inner hollow surface and converges light; a convex mirror reflects from its outer bulging surface and diverges it.
- Plane mirror — same size, erect, laterally inverted, virtual. Convex — always diminished, erect, virtual, with a wide field of view. Concave — size and orientation depend on distance, and it can give a real image.
- A real image can be caught on a screen; a virtual one cannot.
- Concave mirrors are used for shaving, dentistry, torch reflectors and solar cookers; convex mirrors for vehicle and security mirrors.
- Laws of reflection: , and the incident ray, reflected ray and normal lie in one plane — both angles measured from the normal, so a ray at to the mirror has .
- Lateral inversion swaps left and right, which is why AMBULANCE is painted reversed, while letters such as A, H, I, M, O, T, U, V, W, X and Y look unchanged.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Plane mirror — same size, erect, laterally inverted, virtual. Convex — always diminished, erect, virtual, with a wide field of view. Concave — size and orientation depend on distance, and it can give a real image.
- A real image can be caught on a screen; a virtual one cannot.
- Concave mirrors are used for shaving, dentistry, torch reflectors and solar cookers; convex mirrors for vehicle and security mirrors.
- Laws of reflection: , and the incident ray, reflected ray and normal lie in one plane — both angles measured from the normal, so a ray at to the mirror has .
- Lateral inversion swaps left and right, which is why AMBULANCE is painted reversed, while letters such as A, H, I, M, O, T, U, V, W, X and Y look unchanged.
You will remember all of this far better after answering five questions on it than after reading it twice.