Light: Mirrors and Lenses Class 8: Spherical Mirrors, Laws of Reflection and Lenses

What Light: Mirrors and Lenses is about

Key concepts

Spherical mirrors: concave and convex

NCERT section 10.1 What Are Spherical Mirrors?

A shiny steel spoon is a curved mirror (Activity 10.1): its inner (inward-curved) side gives an inverted image of your face, its outer (bulging) side an erect, smaller image.

Spherical mirrors have reflecting surfaces shaped like part of a hollow sphere.

• Concave mirror: the reflecting surface curves inwards (like the inside of a spoon).
• Convex mirror: the reflecting surface curves outwards (like the back of a spoon).
• In diagrams, the non-reflecting side is shaded.

Such mirrors are not cut from glass spheres; flat glass is ground and polished into a curve, then coated (for example with aluminium). Coating the outer curved surface makes the inner side reflect (concave); coating the inner surface makes a convex mirror.

Activity 10.2: placed face-up on a table and viewed from the side at eye level, a concave mirror's surface dips inwards and a convex mirror's bulges outwards.

Worked example

Which side of a steel spoon acts as a concave mirror and which as a convex mirror? What image of your face does each give when held at arm's length?

Try it first, then show the solutionSolution

1. The inner (hollow) side curves inwards: concave mirror. At arm's length it gives an inverted image.
2. The outer (bulging) side curves outwards: convex mirror. It gives an erect, smaller image.

Answer: Inner side concave (inverted image); outer side convex (erect, diminished image).

Images formed by spherical mirrors

NCERT section 10.2 What Are the Characteristics of Images Formed by Spherical Mirrors?

Activity 10.3: an object is placed 3 to 4 cm in front of a concave and a convex mirror, then moved slowly away.

Mirror	Object close	Object moved away
Concave	erect and enlarged	becomes inverted; first enlarged, then smaller and smaller
Convex	erect and diminished	still erect and diminished, getting slightly smaller
Plane	erect, same size	erect, same size

• A concave mirror's image can be enlarged, the same size or diminished, erect or inverted, depending on the object's distance.
• A convex mirror's image is always erect and diminished.
• Lateral inversion (left and right swapped) is seen in all three types.

So you can identify a mirror from its images: same size every time (plane), always smaller (convex), enlarged or upside down (concave).

Worked example

A pen is held very close to a concave shaving mirror and then moved slowly away until it is across the room. Describe how its image changes.

Try it first, then show the solutionSolution

1. Very close to the mirror, the image is erect and enlarged.
2. As the pen moves away, the image becomes inverted, first enlarged.
3. Moved farther still, the inverted image gets smaller and smaller.

Answer: Erect and enlarged when close; then inverted, becoming smaller as the pen moves away.

Uses of concave and convex mirrors

NCERT section 10.2 What Are the Characteristics of Images Formed by Spherical Mirrors?

Concave mirrors:

• reflectors in torches and vehicle headlights (they send light out as a strong beam);
• dentist's mirror: held close to teeth, it gives an enlarged view;
• the main mirror of most modern reflecting telescopes;
• solar concentrators and solar cookers (they converge sunlight to produce heat).

Convex mirrors:

• side-view mirrors of vehicles: always erect, diminished images and a much wider view of the road behind;
• road-safety mirrors at sharp bends and intersections, so drivers see traffic coming from the other side;
• surveillance mirrors in big shops to watch a large area.

Because a convex mirror makes vehicles look smaller, they seem farther away than they really are, which is why side mirrors carry the warning 'Objects in mirror are closer than they appear'.

Worked example

Why do side-view mirrors carry the warning 'Objects in mirror are closer than they appear'?

Try it first, then show the solutionSolution

1. Side-view mirrors are convex, which always form erect, diminished images.
2. A smaller image makes a vehicle look farther away than it is.
3. The warning reminds the driver that the vehicle behind is actually nearer.

Answer: Convex mirrors shrink images, so vehicles seem farther than they are.

Laws of reflection

NCERT section 10.3 What Are the Laws of Reflection?

Light is shown by rays: straight lines with arrows along the path of light.

• Incident ray: the ray falling on the mirror. Reflected ray: the ray coming back.
• Point of incidence (O): where the incident ray strikes.
• Normal: a line at 90° to the mirror at the point of incidence.
• Angle of incidence (i): between the incident ray and the normal. Angle of reflection (r): between the reflected ray and the normal.

Activity 10.4: a comb with one slit and a torch give a thin beam on a plane mirror standing on white paper; the rays are drawn, the normal added and the angles measured for several beams. **Law 1: $\angle i=\angle r$.** A ray falling along the normal ($i=0°$) reflects straight back ($r=0°$).

Activity 10.5: with chart paper extending beyond the table edge, the reflected beam is seen on the flat paper but disappears when the extension is bent down. Law 2: the incident ray, the reflected ray and the normal drawn where the ray strikes are all in one plane.

Both laws hold for all mirrors: plane, concave and convex. More than 800 years ago, in Bhāskara II's time, astronomers used reflections in shallow bowls of water to measure positions of stars and planets.

Worked example

A ray makes an angle of 52° with the normal to a plane mirror. Find the angle of reflection, the angle the reflected ray makes with the mirror, and the angle between the incident and reflected rays.

Try it first, then show the solutionSolution

1. By the first law, $\angle r=\angle i=52°$.
2. The normal is at 90° to the mirror, so the reflected ray makes $90°-52°=38°$ with the mirror.
3. Angle between the rays $=52°+52°=104°$.

Answer: 52°; 38° with the mirror; 104° between the rays.

Converging and diverging mirrors

NCERT section 10.3 What Are the Laws of Reflection?

Activity 10.6: leaving several slits of the comb open gives multiple parallel beams.

• On a plane mirror, the reflected beams stay parallel.
• On a concave mirror, they come closer together: the mirror converges light.
• On a convex mirror, they spread out: the mirror diverges light.

Each ray still obeys the laws of reflection; the curved surface means the normal points in a different direction at each point, so parallel rays reflect at different angles.

Activity 10.7 (with an adult): a concave mirror facing the Sun focuses sunlight into a sharp bright spot on paper; held steady, the paper may smoke and burn, because the light energy is concentrated on a small area. Never look at the Sun or its reflection.

Solar concentrators use mirrors and lenses to focus sunlight to heat liquids for steam (electricity), for large-scale cooking and for solar furnaces that can even melt steel.

Worked example

Why can a concave mirror burn a piece of paper in sunlight while a convex mirror cannot?

Try it first, then show the solutionSolution

1. A concave mirror converges parallel sunlight to a small bright spot, concentrating its energy and producing enough heat to ignite paper.
2. A convex mirror diverges sunlight, spreading it out, so the energy never concentrates.

Answer: Concave converges and concentrates heat; convex spreads light out.

Lenses: convex and concave

NCERT section 10.4 What Is a Lens?

Activity 10.8: a drop of water on an oiled glass strip has an outward-curved surface; letters seen through it look larger. The drop acts as a simple lens, like a magnifying glass.

A lens is a piece of transparent material (usually glass or plastic) with curved surfaces. Light passes through a lens; we see things through a lens rather than in it (as with a mirror).

• Convex lens: thicker in the middle than at the edges.
• Concave lens: thicker at the edges than in the middle.

Activity 10.9: looking at an object through each lens:

• Convex lens: object close to the lens looks erect and enlarged; farther away it looks inverted, first enlarged, then diminished.
• Concave lens: always erect and diminished.

A magnifying glass is a convex lens held close to the text.

Worked example

A student looks at a distant tree through a lens held at arm's length and sees it upside down and small. Through the same lens held just above her thumbnail, the nail looks upright and large. Which kind of lens is it? Explain.

Try it first, then show the solutionSolution

1. A concave lens always shows objects erect and diminished, so it cannot give an inverted image.
2. A convex lens shows a nearby object erect and enlarged, and a distant object inverted.
3. Both observations fit a convex lens.

Answer: A convex lens: upright and enlarged up close, inverted for distant objects.

Converging and diverging lenses and their uses

NCERT section 10.4 What Is a Lens?

Activity 10.10: multiple parallel beams pass through a thin glass plate, a convex lens and a concave lens, each held upright between two books.

• Glass plate: beams pass straight through, still parallel.
• Convex lens: beams converge, so it is a converging lens.
• Concave lens: beams diverge, so it is a diverging lens.

Activity 10.11: a convex lens can focus sunlight to a bright spot that burns paper, like a concave mirror. Do not look at the Sun through a lens.

Uses of lenses: spectacles, cameras (including smartphone cameras), microscopes and telescopes. The eye has a convex lens that can change its shape, letting us see both near and far objects.

Compare: concave mirror and convex lens both converge light and can give enlarged or inverted images; convex mirror and concave lens both diverge light and always give erect, diminished images.

Worked example

Three unlabelled pieces of glass are tested with sunlight on paper: A gives a bright spot, B gives a dim spread-out patch, C gives a patch the same size as the piece. Identify each.

Try it first, then show the solutionSolution

1. A converges sunlight to a spot: convex lens.
2. B spreads light out: concave lens.
3. C lets light pass straight through: a flat glass plate.

Answer: A convex lens, B concave lens, C plane glass.

Science in real life: Light: Mirrors and Lenses

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.

Topics in this chapter

• Laws of Reflection and Angle Problems
• Spherical Mirrors and Lenses: Images, Convergence and Uses

Light: Mirrors and Lenses: formulas and key facts

First law of reflection

$\angle i=\angle r$

Angle with the mirror

$\text{angle with mirror}=90°-\text{angle with normal}$

Angle between incident and reflected rays

$\angle i+\angle r=2\angle i$

Ray along the normal

$\angle i=0°\Rightarrow \angle r=0°$

Converge and diverge

$\text{concave mirror, convex lens: converge};\text{convex mirror, concave lens: diverge}$

Always erect and diminished

$\text{convex mirror and concave lens}$

Light: Mirrors and Lenses: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Light: Mirrors and Lenses

The question patterns to expect, and the method that works for each.

Tricks to remember Light: Mirrors and Lenses

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

• Concave caves in

  A conCAVE mirror is hollow like a cave; convex bulges out.

  Use it when: Naming mirrors.

• Angles from the normal

  'Normal people measure from the normal', never from the mirror.

  Use it when: Angle of incidence and reflection.

• Twin pairs

  Concave mirror and convex lens are twins (converge, can magnify); convex mirror and concave lens are twins (diverge, always small and upright).

  Use it when: Comparing mirrors and lenses.

• Fat middle focuses

  A lens fat in the middle (convex) brings light together.

  Use it when: Convex lens converges.

• Through, not in

  You look IN a mirror but THROUGH a lens.

  Use it when: Mirror versus lens.

Light: Mirrors and Lenses: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Light: Mirrors and Lenses

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

• ✗Mistake: Measuring the angle of incidence from the mirror

  The mirror line is easier to see.

  Wrong working

  Ray makes 30° with the mirror, so the angle of incidence is 30°.

  Right working

  Angle of incidence $=90°-30°=60°$ from the normal; angle of reflection is also 60°.

  ✓Do this: Always convert to the angle with the normal.

  Usually costs: 1-2 marks.

• ✗Mistake: Saying no reflection happens along the normal

  The ray does not appear to bend.

  Wrong working

  A ray along the normal is not reflected.

  Right working

  It reflects straight back along the same path; $i=r=0°$.

  ✓Do this: Zero angle still means reflection.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up which mirror is used as a side-view mirror

  Concave mirrors magnify, which seems helpful.

  Wrong working

  Vehicles use concave mirrors to see traffic behind.

  Right working

  Convex mirrors: always erect images and a wider field of view.

  ✓Do this: Wide view, always upright: convex.

  Usually costs: 1 mark.

• ✗Mistake: Calling a convex lens diverging because a convex mirror diverges

  Same word, opposite behaviour.

  Wrong working

  A convex lens spreads light like a convex mirror.

  Right working

  A convex lens converges light; a convex mirror diverges it.

  ✓Do this: Learn the twin pairs.

  Usually costs: 1 mark.

• ✗Mistake: Forgetting the second law

  Only $\angle i=\angle r$ is remembered.

  Wrong working

  There is only one law of reflection.

  Right working

  Second law: incident ray, normal and reflected ray lie in the same plane.

  ✓Do this: Learn both laws with their activities.

  Usually costs: 1 mark.

NCERT exercise guide: Light: Mirrors and Lenses

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Light: Mirrors and Lenses

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

1. Q1 · 1 mark · MCQExam level

   The image formed by a convex mirror is always
   (a) inverted and enlarged
   (b) erect and diminished
   (c) erect and enlarged
   (d) inverted and diminished

   Hint

   Side-view mirror.

   Answer

   (b) erect and diminished

2. Q2 · 1 mark · MCQExam level

   Which lens is called a diverging lens?
   (a) Convex
   (b) Concave
   (c) Both
   (d) Neither

   Hint

   Thicker at the edges.

   Answer

   (b) Concave

3. Q3 · 1 mark · Very short answerExam level

   Name the mirror used in car headlights.

   Hint

   Strong beam.

   Answer

   Concave mirror.

4. Q4 · 2 marks · Very short answerExam level

   How can you tell a convex lens from a concave lens by touch?

   Hint

   Thickness.

   Answer

   A convex lens is thicker in the middle than at the edges; a concave lens is thinner in the middle and thicker at the edges.

5. Q5 · 2 marks · Very short answerHarder exam level

   A ray of light makes an angle of 60° with a plane mirror. Find the angle of reflection.

   Hint

   Convert to the normal.

   Answer

   Angle of incidence $=90°-60°=30°$, so angle of reflection $=30°$.

6. Q6 · 2 marks · Very short answerHarder exam level

   Why are concave mirrors used in torches?

   Hint

   Direction of light.

   Answer

   The concave reflector collects light from the bulb and sends it forward as a strong, nearly parallel beam, so the torch lights a distant spot brightly.

7. Q7 · 2 marks · Very short answerHarder exam level

   Match: (i) solar concentrator, (ii) shop surveillance mirror, (iii) magnifying glass, (iv) a lens that always shows things upright and smaller; with (a) convex mirror, (b) concave mirror, (c) convex lens, (d) concave lens.

   Hint

   Converge or diverge; mirror or lens.

   Answer

   (i)-(b), (ii)-(a), (iii)-(c), (iv)-(d).

8. Q8 · 3 marks · Short answerHarder exam level

   Describe how to demonstrate the first law of reflection with a torch, comb, plane mirror and paper.

   Hint

   Activity 10.4.

   Answer

   Cover all comb gaps except one to make a slit; shine a torch through it to get a thin beam along white paper onto an upright plane mirror. Mark the incident and reflected rays, remove the mirror, draw the normal at the point of incidence and measure both angles with a protractor. Repeat at several angles: i equals r each time.

9. Q9 · 3 marks · Short answerHarder exam level

   Why can a convex lens and a concave mirror both burn paper in sunlight, but a concave lens cannot?

   Hint

   Converge.

   Answer

   The convex lens (by letting light through) and the concave mirror (by reflecting it) both converge parallel sunlight to a small spot, concentrating energy enough to ignite paper. A concave lens diverges light, spreading the energy out.

10. Q10 · 5 marks · Long answerHarder exam level

    Compare images seen through a convex lens and a concave lens as an object is moved slowly away from each (Activity 10.9). Mention erect or inverted and size changes for both, and one use of each lens.

    Hint

    Twin pairs.

    Answer

    Convex lens: object close looks erect and enlarged; farther away it looks inverted, first enlarged then diminished. Concave lens: always erect and diminished, with size changing as distance changes. Uses: convex lens in magnifying glasses, cameras and microscopes; concave lens in some spectacles.

11. Q11 · 3 marks · Short answerHOTS / competency

    Two plane mirrors are at right angles. A ray strikes the first mirror at 30° to its normal. Find the angle of reflection at the first mirror and the angle of incidence on the second mirror.

    Hint

    Draw it; angles in a triangle.

    Answer

    At the first mirror, r = 30°, so the reflected ray makes 60° with the first mirror. In the triangle formed with the corner (90°), it meets the second mirror at $180°-90°-60°=30°$ to that mirror, so the angle of incidence there is $90°-30°=60°$.

12. Q12 · 2 marks · Very short answerHOTS / competency

    The lens in our eye can change its shape. Why is this useful, and what kind of lens is it?

    Hint

    Near and far.

    Answer

    It is a convex lens. Changing shape lets it adjust how strongly it converges light, so we can see both near objects (like a book) and distant ones clearly.

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. 1.What is a concave mirror?Show answer

   A spherical mirror whose reflecting surface curves inwards.

2. 2.What kind of image does a convex mirror always form?Show answer

   Erect and diminished.

3. 3.What is the normal?Show answer

   A line at 90° to the mirror at the point of incidence.

4. 4.What is the angle of incidence for a ray falling along the normal?Show answer

   0°.

5. 5.Which mirror converges a parallel beam?Show answer

   A concave mirror.

6. 6.Name one use of a convex mirror.Show answer

   Side-view mirrors of vehicles (also road-bend and shop surveillance mirrors).

7. 7.Define a lens.Show answer

   A piece of transparent material with curved surfaces.

8. 8.Which lens is thicker in the middle?Show answer

   A convex lens.

9. 9.What type of lens is a magnifying glass?Show answer

   Convex.

10. 10.Which lens always shows objects erect and diminished?Show answer

    A concave lens.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you tell concave from convex mirrors by shape and by image?
• ☐Can you describe how a concave mirror's image changes as an object moves away?
• ☐Can you give two uses each of concave and convex mirrors with reasons?
• ☐Can you define incident ray, reflected ray, normal, i and r?
• ☐Can you state both laws of reflection and the activity for each?
• ☐Can you solve angle questions when the angle is given with the mirror?
• ☐Can you draw parallel beams on plane, concave and convex mirrors?
• ☐Can you tell convex from concave lenses by shape and image?
• ☐Can you explain why a convex lens can burn paper?
• ☐Can you name devices that use lenses?

Light: Mirrors and Lenses: frequently asked questions