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Light: Mirrors and Lenses: questions with answers

Class 8 Science, Chapter 10 (NCERT). 15 questions by concept, 2 with pictures. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

A concave mirror converges and a convex mirror diverges

Question 1

Parallel beams strike a concave mirror and leave it no longer parallel. Does this break the law that the angle of reflection equals the angle of incidence?

  1. (a)Yes, but only for the beams that strike near the edge of the mirror.
  2. (b)Yes, which is why curved mirrors behave differently from flat ones.
  3. (c)No, because the usual laws apply only to plane mirrors.
  4. (d)No. Each beam follows it against the normal at its own point of the curve.
Show answer

(d) No. Each beam follows it against the normal at its own point of the curve.

Each beam meets the mirror at its own point, where the normal has its own direction. Every beam obeys the law against that normal. The reflected beams end up converging precisely because the normals differ across the curve, not because any law failed.

How spherical mirrors differ from a plane mirror

Question 2

All that is known about an image is that it is erect. Can the mirror be named from that?

  1. (a)No, because no mirror can be identified from its image.
  2. (b)Yes, it must be a plane mirror.
  3. (c)No. Plane and convex mirrors give erect images, and a concave mirror can also do so.
  4. (d)Yes, it must be a concave mirror.
Show answer

(c) No. Plane and convex mirrors give erect images, and a concave mirror can also do so.

A plane mirror gives an erect, equal-sized image. A convex mirror gives an erect, smaller image. A concave mirror can give an erect, enlarged image when the object is close.

Telling a concave mirror from a convex one

Question 3

Two spherical mirrors have rims of exactly the same shape and colour. Does that settle which is concave and which is convex?

  1. (a)No, because the rim tells us nothing about which way the surface curves.
  2. (b)Yes, because two mirrors with the same rim must also be the same size and type.
  3. (c)No, but the colour of the rim would settle it if it differed.
  4. (d)Yes, because the shape of the rim identifies the mirror.
Show answer

(a) No, because the rim tells us nothing about which way the surface curves.

What distinguishes a concave mirror from a convex one is the direction the reflecting surface curves in. The rim is only a mounting: two mirrors can carry identical rims and curve in opposite directions.

The laws hold for every kind of mirror

Question 4

Light strikes a curved mirror. What makes the reflected rays change direction at the reflecting surface?

  1. (a)The rays pass through the curved glass like light through a lens.
  2. (b)A special rule applies because the usual laws cover only plane mirrors.
  3. (c)Reflection, with each ray obeying the laws of reflection at its point of incidence.
  4. (d)The reflected rays follow curved paths through the air.
Show answer

(c) Reflection, with each ray obeying the laws of reflection at its point of incidence.

A curved mirror changes the direction of light by reflection. Each ray obeys the laws at its own point of incidence. The normal varies along the curved surface.

What spherical mirrors are

Question 5

A curved reflecting surface gives an image quite unlike a flat one's. Do the laws of reflection apply to it?

  1. (a)Yes, but only when light strikes along the normal.
  2. (b)Only when the object is far enough away.
  3. (c)No, because a curved surface is not really a mirror.
  4. (d)Yes, they hold at every point of it, just as they do on a flat surface.
Show answer

(d) Yes, they hold at every point of it, just as they do on a flat surface.

The laws of reflection hold at every point of every reflecting surface. What differs on a curved mirror is that the normal points a different way at each point, so parallel rays leave in different directions. The laws themselves are unchanged.

Where concave and convex mirrors are used

Question 6

At a blind road corner, a convex mirror is replaced by an equally sized plane mirror. From the same viewing position, what is reduced?

  1. (a)The magnification that the convex mirror provides.
  2. (b)Nothing, because a plane mirror gives an erect image just as a convex one does.
  3. (c)Nothing at all, provided the plane mirror is angled correctly.
  4. (d)The wide view, since a flat mirror of the same size takes in much less.
Show answer

(d) The wide view, since a flat mirror of the same size takes in much less.

Both mirrors can give erect images. From the same position, a convex mirror shows more of the road than a plane mirror of equal size.

Images formed by a concave mirror

Question 7

An object is moved from far away to close up in front of a concave mirror. Does its image change?

  1. (a)No, because a mirror's image looks the same wherever the object is.
  2. (b)Yes, and its image always becomes larger whenever the object is moved farther away.
  3. (c)Only in which way up it is, since its size is fixed by the mirror.
  4. (d)Yes, in both its size and which way up it is.
Show answer

(d) Yes, in both its size and which way up it is.

A plane mirror's image is always the same size as the object, which is where the habit comes from. A concave mirror is different: far away the image is inverted, and close in it is erect and enlarged, so both the size and the orientation change.

Images formed by a convex mirror

Question 8

An object stands in front of a convex mirror. What is its image like?

  1. (a)Erect and smaller than the object.
  2. (b)Erect and smaller, but without lateral inversion because only plane mirrors show that.
  3. (c)Erect and larger than the object.
  4. (d)Inverted and smaller than the object.
Show answer

(a) Erect and smaller than the object.

A convex mirror always gives an image that is the right way up and smaller than the object, whatever the distance. Unlike a concave mirror it never inverts the image and never enlarges it.

The angle of incidence equals the angle of reflection

Question 9

Light strikes a plane mirror perpendicularly. What happens to the light?

Light rayPlane mirror
  1. (a)It bounces off to one side at an angle.
  2. (b)It is not reflected at all.
  3. (c)It goes straight back along the same path.
  4. (d)It travels along the mirror surface.
Show answer

(c) It goes straight back along the same path.

The incoming light travels along the normal, the line perpendicular to the mirror. Its angle of incidence is 0 degrees, so its angle of reflection is also 0 degrees. The light is reflected straight back along its own path.

The incident ray, the normal and the two angles

Question 10

In ordinary ray drawings, a student uses one ray in one sketch and five in another. What does the number of drawn rays tell us about brightness?

  1. (a)That the first diagram is of a plane mirror and the second of a curved one.
  2. (b)That the second diagram has five times as much light in it.
  3. (c)That the first sketch always represents brighter light.
  4. (d)Nothing about the amount of light; a ray only shows the direction light travels.
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(d) Nothing about the amount of light; a ray only shows the direction light travels.

A ray is a line showing the direction in which light travels. It is a way of drawing, not a measure of brightness. Several rays are drawn together only when we want to follow what happens to a beam at different points.

The incident ray, the normal and the reflected ray lie in one plane

Question 11

A ray strikes a plane mirror obliquely. It is moved out of its original plane while still striking the same point. What happens to the plane containing both rays and the normal?

Incident ray changes planeA plane mirror has fixed incidence point P and a fixed normal. The shaded original incident plane contains the original incoming ray; the moved incoming ray is outside it. Outgoing rays and the new plane are omitted. QUESTION DIAGRAM Incident ray changes plane Two incoming positions only · reflected rays are not drawn P Normal Original plane Original incident ray Moved ray Plane mirror 3D position guides
  1. (a)It changes, and the incident ray, normal, and reflected ray still lie in one plane.
  2. (b)It stays where it was, and the reflected ray leaves that plane.
  3. (c)The law no longer holds, because it applies only to one direction of incidence.
  4. (d)The reflected ray disappears whenever the incident ray changes direction.
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(a) It changes, and the incident ray, normal, and reflected ray still lie in one plane.

The incident ray and the normal determine the plane. Moving the incident ray out of the original plane changes this plane. The reflected ray follows the second law.

Concentrating sunlight with a concave mirror

Question 12

A concave mirror is held in sunlight so that a small bright spot falls on a sheet of paper, and the paper chars. What supplied the heat?

  1. (a)The mirror’s material, which must be hotter than the paper before any sunlight arrives.
  2. (b)The Sun, whose light and heat the mirror gathered into a small area.
  3. (c)The mirror, which produces heat when sunlight falls on it.
  4. (d)The paper, which becomes hot when it is looked at through glass.
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(b) The Sun, whose light and heat the mirror gathered into a small area.

The paper absorbs energy from sunlight and heats up. The concave mirror concentrates that sunlight into a small area; it does not create the energy.

A convex lens converges and a concave lens diverges

Question 13

A convex lens concentrates strong sunlight into a small spot on paper. What can happen after sufficient exposure?

  1. (a)Nothing, because a concave lens would be needed to bring the light to a point.
  2. (b)The paper can char because sunlight is concentrated into a small area.
  3. (c)Nothing, because only a mirror can concentrate sunlight in this way.
  4. (d)It becomes cooler, because the lens takes the heat out of the light.
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(b) The paper can char because sunlight is concentrated into a small area.

A convex lens converges sunlight into a small spot. The paper absorbs this concentrated light and heats up. With sufficient exposure, it can char.

What a lens is

Question 14

A round glass tumbler is filled with water, and a pencil seen through it looks wider. Is the water acting as a lens?

  1. (a)Yes, because every transparent object acts as a lens regardless of its shape.
  2. (b)No, because water cannot change the direction of light.
  3. (c)Yes, because it is a transparent material with a curved surface.
  4. (d)No, because a lens has to be made of glass.
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(c) Yes, because it is a transparent material with a curved surface.

Water is transparent and can act as a lens when its shape gives it curved boundaries. Water in a round tumbler can make a pencil appear wider.

Where lenses are used

Question 15

Light from a sign passes through one lens of a pair of eyeglasses and enters one eye. Which lenses does it pass through?

  1. (a)The eyeglass lens and the lens inside that eye.
  2. (b)One, the lens in the eyeglasses, since the eye has no lens of its own.
  3. (c)None, because eyeglasses work by reflection.
  4. (d)Only the eye lens, because eyeglasses do not contain lenses.
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(a) The eyeglass lens and the lens inside that eye.

The eye contains a lens of its own, which is what forms the image inside it. Eyeglasses add a second lens in front to help that image fall where it should.

Common misconception in Light: Mirrors and Lenses

Open full misconception guide →

Why do mirror images look reversed?

Wrong

A plane mirror swaps left and right in space.

Why it feels right. My reflection looks like someone facing me with a different raised hand.

Right

A plane mirror reverses depth relative to its surface.

Worked example

The mirror flips the direction toward or away from its surface. An upward arrow along the surface still points up.
  1. Hold an arrow pointing toward the mirror.
  2. Its image points back toward you.
  3. An arrow along the mirror keeps its sideways direction.