ScienceCBSE Class 8 · Light: Mirrors and Lenses

Spherical Mirrors and Lenses: Images, Convergence and Uses (Class 8)

Updated 17 October 2026 · Fact-checked

A concave mirror curves inwards and a convex mirror outwards. A concave mirror can form enlarged, same-size or diminished images, erect or inverted, depending on object distance, and converges light; a convex mirror always forms an erect, diminished image and diverges light. A convex lens (thicker in the middle) converges light and a concave lens (thinner in the middle) diverges it.

Understand Spherical Mirrors and Lenses: Images, Convergence and Uses

Spherical mirrors. A shiny spoon is like a curved mirror: its inner surface gives an inverted image, its outer bulging surface an erect, smaller image. Mirrors whose reflecting surfaces are parts of a hollow sphere are spherical mirrors. One whose reflecting surface curves inwards is a concave mirror; one whose reflecting surface curves outwards is a convex mirror. They are made by grinding and polishing flat glass; a reflecting coating on the outer curved surface gives a concave mirror, on the inner curved surface a convex mirror. Placed with the reflecting side up and viewed from the side, the curve inwards or outwards identifies them (Activity 10.2).

Images (Activity 10.3).

*Plane mirror:* always erect, same size as the object.
*Concave mirror:* when the object is close, the image is erect and enlarged; moving the object away, the image becomes inverted and is first enlarged and then keeps getting smaller. Images can thus be enlarged, diminished or the same size, erect or inverted.
*Convex mirror:* the image is always erect and diminished; it shrinks slightly as the object moves away.
Lateral inversion is seen in all three kinds of mirrors.

So the images tell us what kind of mirror we are looking at.

Converging and diverging (Activity 10.6). Parallel beams stay parallel after a plane mirror, converge after a concave mirror and diverge after a convex mirror. A concave mirror held facing the Sun concentrates the light into a bright spot on paper and can ignite it (Activity 10.7, with strong safety rules: never look at the Sun or its reflection). Solar concentrators use mirrors and lenses to heat a liquid for steam, large-scale cooking or solar furnaces that can melt steel.

Uses of mirrors. Concave: reflectors in torches, headlights of cars and scooters, the dentist's mirror (enlarged view of teeth), and the main mirror of most modern reflecting telescopes. Convex: side-view mirrors of vehicles (erect, smaller images and a wider field of view), road-safety mirrors at bends and intersections, and shop surveillance mirrors.

Lenses. A lens is a piece of transparent material (glass or plastic) with curved surfaces. A water drop on oiled glass, with an outward curved surface, acts as a simple lens and enlarges the text below. A convex lens is thicker in the middle than at the edges; a concave lens is thicker at the edges than in the middle. We see things through a lens, not in it.

*Convex lens (Activity 10.9):* at a small distance the object looks erect and enlarged; as the object moves farther the image becomes inverted, initially enlarged and then diminished. Images can be enlarged, diminished or the same size; erect or inverted.
*Concave lens:* image always erect and diminished; the size changes with distance.
*Beams (Activity 10.10):* a thin glass plate lets parallel beams pass unchanged, a convex lens converges them (a converging lens) and a concave lens diverges them (a diverging lens). A convex lens can also burn paper (Activity 10.11).

Lenses are used in eyeglasses, cameras (including smartphone cameras), telescopes and microscopes; the eye itself has a convex lens that can change its shape.

The laws that every reflected ray obeys are covered in the previous topic.

Spherical Mirrors and Lenses: Images, Convergence and Uses: formulas and key facts

Concave mirror: Reflecting surface curves inwards
Converges light
Image enlarged, same size or diminished; erect or inverted; Uses: torch reflector, headlights, dentist's mirror, telescope.
Convex mirror: Reflecting surface curves outwards
Diverges light
Image always erect and diminished; Uses: vehicle side mirrors, road safety mirrors, shop surveillance.
Plane mirror: Parallel beams stay parallel | Image erect and same size; Lateral inversion in all mirrors.
Convex lens: Thicker in the middle | Converges light (converging lens); Near object: erect and enlarged; far object: inverted.
Concave lens: Thicker at the edges | Diverges light (diverging lens); Image always erect and diminished.

How to solve Spherical Mirrors and Lenses: Images, Convergence and Uses questions

Use this method to identify a mirror or lens or to choose a device.

1Look at the image: always erect and smaller means convex mirror or concave lens.
2If the image is enlarged and erect for a close object, it is a concave mirror or convex lens.
3If the image becomes inverted as the object moves away, it is a concave mirror or a convex lens.
4To decide between mirror and lens, ask: do we see the image in the surface (mirror) or through it (lens)?
5For uses, match the need: wide view (convex mirror), concentrating light or enlarged view (concave mirror), enlargement (convex lens).
6Add the beam behaviour: converge or diverge.

Spherical Mirrors and Lenses: Images, Convergence and Uses in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Choosing a mirror for a hairpin bend

Suppose a town plans to fix a mirror at a sharp hill bend so that drivers coming from either side can see each other, and a dentist needs a mirror to view a back tooth.

(a) Which mirror for the bend, and why? (b) Which for the dentist, and why? (c) State the images that each mirror gives. (d) Why does the car-mirror warning say objects are closer than they appear?

Try it first, then show the solutionSolution

Convex mirror: it gives a much wider area of the road and an erect image.
Concave mirror: when held close, it gives an enlarged erect view of the teeth.
Convex: always erect and smaller. Concave held close: erect and enlarged.
Convex mirrors make images smaller, which makes the objects look farther than they are (an explanation derived from the diminished image; the chapter only poses the question).

Answer: (a) Convex, wide field; (b) concave, enlarged erect view; (c) erect and smaller; erect and enlarged; (d) images look small, so vehicles seem farther away.

Memory hook: Convex sees wide; concave sees big.

Burning paper with a lens

Imagine a student focuses sunlight through a convex lens onto a sheet of paper (safety: never look at the Sun through it).

(a) What does the lens do to the parallel sunrays? (b) Why does the paper start to smoke? (c) Which type of lens would not do this? (d) Which mirror could do the same job?

Try it first, then show the solutionSolution

A convex lens converges the rays to a small bright spot.
The concentrated light produces enough heat at that point to ignite the paper.
A concave lens diverges light, so the light is spread out.
A concave mirror, as in Activity 10.7.

Answer: (a) Converges; (b) heat concentrated in a small spot; (c) concave lens; (d) concave mirror.

Memory hook: Converge to concentrate; diverge to disperse.

Tricks to remember Spherical Mirrors and Lenses: Images, Convergence and Uses

Short memory aids. Say each one aloud once, then use it on a question the same day.

Concave = cave
A concave mirror is like a cave: the reflecting surface goes inwards.
Use it when: Recognising concave.
Convex lens is fat in the middle
Convex lens: thick belly. Concave lens: thin waist.
Use it when: Lens shape.
Convex mirror: small and always upright
Convex mirror = always erect, always smaller.
Use it when: Image of a convex mirror.
Mirror similarities
Concave mirror and convex lens both converge; convex mirror and concave lens both diverge.
Use it when: Pairing mirrors and lenses.

Spherical Mirrors and Lenses: Images, Convergence and Uses: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Distinguish concave and convex mirror · 2 marks · Short answerBoard level

Give two ways to distinguish a concave mirror from a convex mirror.

Try it first, then show the solutionSolution

Look at the side view with the reflecting surface up: the concave surface curves inwards, the convex outwards. (1)
Look at the image: a convex mirror always gives an erect, diminished image, while a concave mirror gives an enlarged, erect image for a close object and an inverted image for a far one. (1)

Answer: By the shape of the surface and by the nature of the image.

Example 2: Which lens · 1 mark · MCQBoard level

A lens that is thicker at the edges than at the middle is a
(a) convex lens that converges light
(b) concave lens that diverges light
(c) convex lens that diverges light
(d) concave lens that converges light

Try it first, then show the solutionSolution

Thicker at the edges means concave; a concave lens diverges light.

Answer: (b)

Example 3: Match the use to the device · 5 marks · Long answerHarder board level

Choose the mirror or lens and give a reason for each use: (a) side-view mirror of a vehicle, (b) torch reflector, (c) magnifying glass, (d) shop surveillance mirror, (e) headlight reflector of a scooter.

Try it first, then show the solutionSolution

(a) Convex mirror: erect smaller images and wide field of view. (1)
(b) Concave mirror: its reflector converges light into a beam. (1)
(c) Convex lens: gives an erect, enlarged image of a close object. (1)
(d) Convex mirror: monitors a large area. (1)
(e) Concave mirror: the reflector converges the lamp's light into a beam. (1)

Answer: (a) Convex mirror; (b) concave mirror; (c) convex lens; (d) convex mirror; (e) concave mirror.

Example 4: Observations with an unknown device · 4 marks · Case-basedHarder board level

Read the passage and answer.

A student looks at a small toy through two unknown devices, P and Q. Through P the toy looks erect and enlarged when close, and inverted when far. Through Q it always looks erect and smaller, whatever the distance. Both are thick at one place and thin at another.
(a) What could P be if it is a transparent device through which we look? (1)
(b) What could Q be? (1)
(c) Predict how a parallel beam behaves when it passes P and Q. (1)
(d) If P were a mirror instead, what mirror would give the same images? (1)

Try it first, then show the solutionSolution

(a) A convex lens. (1)
(b) A concave lens. (1)
(c) The beam converges through P and diverges through Q. (1)
(d) A concave mirror. (1)

Answer: (a) Convex lens; (b) concave lens; (c) converge, diverge; (d) concave mirror.

Example 5: Solar cooker design · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

A community wants to cook food using sunlight. Group X suggests a large concave mirror that focuses the light on the pot. Group Y suggests a large convex mirror. Group Z suggests a large convex lens above the pot.
(a) Which suggestions can concentrate sunlight? (1)
(b) Why will Group Y's design fail? (1)
(c) Name this type of device. (1)
(d) Give two safety precautions when testing with a concave mirror or lens. (1)

Try it first, then show the solutionSolution

(a) X (concave mirror) and Z (convex lens), since both converge light. (1)
(b) A convex mirror diverges light and spreads it. (1)
(c) A solar concentrator. (1)
(d) Do not look at the Sun or its reflection; focus only on paper or a pot, never towards faces or eyes; adult supervision. (1)

Answer: (a) X and Z; (b) convex mirror diverges; (c) solar concentrator; (d) never look at the Sun; do not aim at faces.

Where students lose marks in Spherical Mirrors and Lenses: Images, Convergence and Uses

The slips examiners see most often, and what to write instead.

Mistake: Writing that a concave mirror always gives an enlarged image.
The close-object case is remembered.
Wrong working
A concave mirror always gives a magnified image.
Right working
A concave mirror can give enlarged, same-size or diminished images, erect or inverted, depending on the object's distance.
Do this: Quote all possibilities.
Usually costs: 1 mark.
Mistake: Swapping converging and diverging.
Concave and convex names are confusing.
Wrong working
Concave lens converges light.
Right working
Convex lens converges; concave lens diverges. Concave mirror converges; convex mirror diverges.
Do this: Learn the pairs.
Usually costs: 1 mark.
Mistake: Mixing up the shape of lenses.
Mirrors are described by curving, lenses by thickness.
Wrong working
A convex lens is thinner in the middle.
Right working
A convex lens is thicker in the middle than at the edges.
Do this: Lens: look at thickness.
Usually costs: 1 mark.
Mistake: Omitting safety in the Sun-focusing activity.
Students describe only the observation.
Wrong working
Focus sunlight on paper using a mirror.
Right working
Do not look at the Sun or at the reflected sunlight; focus only on paper, under supervision.
Do this: Include a safety line.
Usually costs: ½ mark.

Exam tips

Use a table with columns: surface, image, converge or diverge, uses.
For convex mirror write 'always erect and diminished'.
For the concave mirror and convex lens, mention both near-object and far-object images.
Connect the use with the property (wide view, enlarged view, concentrating light).
Include the safety rule when you describe sunlight experiments.

Practice questions on Spherical Mirrors and Lenses: Images, Convergence and Uses

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 2 marks · Very short answerBoard level
Name the type of mirror used in (a) vehicle side-view mirrors, (b) a dentist's mirror, and give one reason for each.
Hint
Wide view versus enlarged view.
Answer
(a) Convex: erect, diminished images with a wide view (1). (b) Concave: enlarged view of the teeth when held close (1).
Q2 · 1 mark · MCQBoard level
Which of the following always forms an erect and diminished image?
(a) Plane mirror
(b) Concave mirror
(c) Convex mirror
(d) Convex lens
Hint
Think of vehicle mirrors.
Answer
(c)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): A concave lens is called a diverging lens.
Reason (R): A concave lens spreads parallel beams of light falling on it.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Spreading means diverging.
Answer
(a) R explains why it is called a diverging lens.
Q4 · 4 marks · Short answerHarder board level
A student records what she sees of a small object moved away from a device: (i) erect and enlarged, (ii) inverted and enlarged, (iii) inverted and diminished. (a) Is this a convex mirror, a concave mirror or a plane mirror? Give a reason. (b) Name a lens that gives the same sequence. (c) In which stages is the image enlarged? (d) State one use of the mirror in daily life.
Hint
Which mirror can give an inverted image?
Answer
(a) A concave mirror: only it gives inverted images and enlarged images (a convex or plane mirror is always erect). (b) A convex lens. (c) Stages (i) and (ii). (d) Dentist's mirror or torch reflector (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A driver sees a bus in her side-view mirror and thinks it is farther than it is. A shopkeeper uses a mirror in the corner of his shop to watch all aisles.
(a) Name the mirror type used by both. (b) Give two properties of the image. (c) Explain why the same mirror suits both uses. (d) Would a plane mirror serve the shopkeeper as well? Explain.
Hint
Convex mirrors give a wide area.
Answer
(a) Convex mirror. (b) Erect and diminished. (c) It covers a large area of view. (d) No: a plane mirror gives a same-size image and a much smaller field of view (1 mark each).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Which mirror is used in the side of a vehicle?Show answer
A convex mirror.
2.Which lens is thicker in the middle?Show answer
The convex lens.
3.What does a concave mirror do to parallel rays?Show answer
It converges them.
4.What does a concave lens do to parallel rays?Show answer
It diverges them.
5.What is lateral inversion?Show answer
The left-right reversal seen in images in all three types of mirrors.

Spherical Mirrors and Lenses: Images, Convergence and Uses: frequently asked questions

Is a water drop a lens?

A drop on oiled glass has an outward curved surface and acts as a simple lens that makes the text look larger.

Do we see images in a lens or through it?

Through it, since light passes through lenses; in mirrors we see the image in the mirror.

Which type of telescope uses curved mirrors?

Most modern telescopes are reflecting telescopes with a large concave main mirror.

How can I tell the type of a mirror just by the image?

An always erect, same-size image means plane; always erect and smaller means convex; images that can be enlarged or inverted mean concave.