Light Energy ICSE Class 8: Refraction, Curved Mirrors, Ray Diagrams and Dispersion

What Light Energy is about

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Physics: your school sets and marks the exam. Light Energy sits in the Theme 5: Light Energy theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Refraction of light

Refraction is the change in direction (bending) of light when it passes obliquely from one transparent medium into another, such as from air into water or glass. It happens because light travels at different speeds in different media.

• Light going from air into water or glass (a denser medium) bends towards the normal.
• Light going from water or glass into air bends away from the normal.
• A ray that meets the surface along the normal (at right angles) goes straight through without bending.

Everyday examples: a pencil in a glass of water looks bent at the surface; a swimming pool looks shallower than it is; a coin in a bowl of water appears raised; stars twinkle.

Worked example

Why does a straight pencil partly dipped in water look bent at the water surface?

Try it first, then show the solutionSolution

1. Light from the part under water passes from water into air and bends away from the normal.
2. To the eye, this light seems to come from a point higher than the real one, so the dipped part looks raised and the pencil looks bent.

Answer: Because of refraction of light at the water surface

Concave and convex mirrors and their terms

Term	Meaning
Pole (P)	centre of the mirror's surface
Centre of curvature (C)	centre of the sphere the mirror is part of
Radius of curvature (R)	distance PC
Principal axis	line through P and C
Principal focus (F)	point on the axis where rays parallel to the axis meet (concave), or seem to come from (convex), after reflection
Focal length (f)	distance PF

For a spherical mirror, F lies half-way between P and C: $f=\frac{R}{2}$.

A concave mirror has a real focus in front of it (it converges light); a convex mirror has a virtual focus behind it (it diverges light).

Worked example

A concave mirror has a radius of curvature of 24 cm. Find its focal length.

Try it first, then show the solutionSolution

1. $f=\frac{R}{2}$.
2. $f=\frac{24}{2}=12$ cm.

Answer: 12 cm

Rules for ray diagrams

Use any two of these rays from the top of the object; the image is where they meet after reflection.

Concave mirror:
1. A ray parallel to the principal axis passes through F after reflection.
2. A ray through F becomes parallel to the principal axis after reflection.
3. A ray through C falls along the normal and comes back along the same path.
4. A ray striking the pole is reflected making an equal angle with the principal axis.

Convex mirror: a parallel ray reflects as if coming from F behind the mirror; a ray directed towards C reflects back along itself.

Draw rays with arrows, using a ruler; show virtual (apparent) parts with dotted lines.

Worked example

A ray of light parallel to the principal axis falls on a concave mirror. Where does it go after reflection? Where does a ray passing through C go?

Try it first, then show the solutionSolution

1. The parallel ray passes through the principal focus F after reflection.
2. A ray through C strikes the mirror along the normal, so it reflects back along the same path.

Answer: Through F; back along itself

Real and virtual images

Real image	Virtual image
formed where reflected rays actually meet	formed where reflected rays only appear to meet
can be taken on a screen	cannot be taken on a screen
always inverted (for one mirror)	always erect
formed in front of the mirror	formed behind the mirror

A plane mirror and a convex mirror always give virtual, erect images. A concave mirror gives a real, inverted image whenever the object is beyond F.

Worked example

The image of a candle flame formed by a mirror is obtained on a screen and is upside down. Is the image real or virtual, and which kind of mirror is it?

Try it first, then show the solutionSolution

1. It can be taken on a screen and is inverted, so it is real.
2. Only a concave mirror (of the three types) forms real images.

Answer: Real; a concave mirror

Real images formed by a concave mirror

Position of object	Position of image	Nature and size
At infinity (very far, e.g. the Sun)	at F	real, inverted, highly diminished (a point)
Beyond C	between F and C	real, inverted, diminished
At C	at C	real, inverted, same size
Between C and F	beyond C	real, inverted, magnified
At F	at infinity	real, inverted, highly magnified

(An object between F and P gives a virtual, erect, magnified image behind the mirror: the shaving mirror case.)

Draw-and-label checklist: principal axis with P, F and C marked (F half-way between P and C); object as an upright arrow; two rays from its tip using the rules; image as an arrow where the rays meet; arrows on every ray.

Worked example

An object is placed 25 cm in front of a concave mirror of focal length 8 cm. Describe the image.

Try it first, then show the solutionSolution

1. $C$ is at $2f=16$ cm, so the object at 25 cm is beyond C.
2. The image forms between F (8 cm) and C (16 cm).
3. It is real, inverted and diminished.

Answer: Between F and C; real, inverted, diminished

Uses of curved mirrors and dispersion

Concave mirrors: reflectors in torches, car headlights and searchlights (a bulb at F gives a strong parallel beam); shaving and make-up mirrors (enlarged, erect image when held close); dentists' mirrors; solar cookers (sunlight concentrated at F).

Convex mirrors: rear-view mirrors in vehicles and mirrors at sharp road bends or in shops; they always give an erect, diminished image and show a wide field of view.

Dispersion: when white light passes through a glass prism, it splits into seven colours: VIBGYOR (violet, indigo, blue, green, yellow, orange, red). The band of colours is a spectrum. Each colour bends by a different amount: violet the most, red the least. A rainbow is formed by dispersion of sunlight in raindrops.

Worked example

Why is a convex mirror used as a rear-view mirror in vehicles?

Try it first, then show the solutionSolution

1. It always forms an erect image, so traffic behind looks the right way up.
2. The image is diminished, so a much wider area of the road behind can be seen in a small mirror.

Answer: Erect image and a wide field of view

Physics in real life: Light Energy

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

• Concave and Convex Mirrors: Image Formation and Uses
• Refraction and Dispersion of Light

Light Energy formula sheet

Focal length

$f=\frac{R}{2}$

Same for concave and convex mirrors.

Colours of the spectrum

$\text{V, I, B, G, Y, O, R}$

Violet bends most, red least.

Refraction at a surface

$\text{air}\to \text{glass: towards normal}$

Glass → air: away from normal.

Light Energy: diagrams to practise

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

How the school exam asks Light Energy

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

Tricks to remember Light Energy

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; a convex mirror bulges out.

  Use it when: Telling the mirrors apart.

• F is half of C

  f = R/2: the focus sits half-way between the pole and the centre of curvature.

  Use it when: Mirror terms and diagrams.

• Parallel ↔ focus

  Parallel in, through F out; through F in, parallel out.

  Use it when: Ray-diagram rules.

• VIBGYOR, violet bends most

  Violet is at the bottom of the spectrum because it bends the most.

  Use it when: Dispersion.

Light Energy: 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 B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 8-mark question split into 2 + 3 + 3 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 Energy

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

• ✗Mistake: Putting F at C

  F is half-way between P and C.

  ✓Do this: Mark C first, then F at its mid-point.

  Usually costs: The whole ray diagram.

• ✗Mistake: No arrows on rays

  Arrows show the direction of light; without them the diagram is incomplete.

  ✓Do this: Put an arrow on every ray, before and after reflection.

  Usually costs: 1 mark.

• ✗Mistake: Saying a convex mirror forms real images

  A convex mirror always forms a virtual, erect, diminished image.

  ✓Do this: Only a concave mirror can form real images.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up the colours that bend most and least

  Violet bends most, red least.

  ✓Do this: Remember VIBGYOR from the bottom of the spectrum upwards.

  Usually costs: 1 mark.

• ✗Mistake: Calling reflection 'refraction'

  Reflection is bouncing back from a surface; refraction is bending while passing into another medium.

  ✓Do this: Mirror → reflection; water, glass, prism → refraction.

  Usually costs: 1 mark.

Practice questions on Light Energy

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) real and inverted
   (b) virtual and erect
   (c) real and magnified
   (d) virtual and inverted

   Hint

   It diverges light.

   Answer

   (b)

2. Q2 · 1 mark · Very short answerExam level

   Find the radius of curvature of a mirror whose focal length is 10 cm.

   Hint

   R = 2f.

   Answer

   20 cm.

3. Q3 · 2 marks · Short answerExam level

   Name the seven colours of white light in order and state which is deviated least by a prism.

   Hint

   VIBGYOR.

   Answer

   Violet, indigo, blue, green, yellow, orange, red. Red is deviated least.

4. Q4 · 2 marks · Short answerHarder exam level

   Give two uses each of a concave mirror and a convex mirror.

   Hint

   Light beams and wide views.

   Answer

   Concave: torch reflectors, shaving mirrors (also headlights, solar cookers). Convex: rear-view mirrors, mirrors at road bends (also shop security mirrors).

5. Q5 · 1 mark · Assertion-reasonHarder exam level

   Assertion (A): A concave mirror is used as a reflector in torches.
   Reason (R): Light from a source placed at the focus of a concave mirror is reflected as a parallel beam.
   (a) (A) is true and (R) is false.
   (b) (A) is false and (R) is true.
   (c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).

   Hint

   Ray rule 2.

   Answer

   (c)

6. Q6 · 2 marks · Short answerHarder exam level

   An object is placed 40 cm from a concave mirror of focal length 15 cm. State the position and nature of the image.

   Hint

   C is at 30 cm.

   Answer

   The object is beyond C, so the image is between F and C: real, inverted and diminished.

7. Q7 · 2 marks · Short answerHarder exam level

   A ray of light passes from air into a glass block obliquely. Does it bend towards or away from the normal? What happens if it enters along the normal?

   Hint

   Air to a denser medium.

   Answer

   Towards the normal. Along the normal it goes straight through without bending.

8. Q8 · 2 marks · Short answerHarder exam level

   Why does a coin at the bottom of a bowl of water appear raised?

   Hint

   Light leaving water.

   Answer

   Light from the coin bends away from the normal as it passes from water into air, so it seems to come from a point higher than the coin.

9. Q9 · 3 marks · Short answerHarder exam level

   An object is placed between C and F of a concave mirror. Describe the image and name one situation where this arrangement is useful.

   Hint

   Image beyond C.

   Answer

   The image is beyond C, real, inverted and magnified. This is useful when an enlarged real image is wanted on a screen or wall, for example to throw a magnified picture of a small, brightly lit object onto a wall.

10. Q10 · 3 marks · Short answerHarder exam level

    Draw and label a diagram to show the dispersion of white light by a prism.

    Hint

    Red least, violet most.

    Answer

    A ray of white light enters one face of a triangular prism and comes out of the other face spread into a band: red (least bent) at the top through to violet (most bent) at the bottom, labelled VIBGYOR on a screen.

11. Q11 · 3 marks · Short answerHOTS / competency

    The image of a distant tree is obtained on a screen 20 cm from a mirror. Name the mirror, its focal length and its radius of curvature, and describe the image.

    Hint

    Distant object → image at F.

    Answer

    Concave mirror; $f=20$ cm, $R=40$ cm. The image is real, inverted and highly diminished, at F.

12. Q12 · 3 marks · Short answerHOTS / competency

    Explain why the image of an object at the centre of curvature of a concave mirror forms at the same place.

    Hint

    Use the ray through F and the ray parallel to the axis; think of symmetry.

    Answer

    A ray from the top of the object parallel to the axis passes through F; a ray through F returns parallel. Because C is at distance $2f$, these rays meet at the same distance $2f$, below the axis: the image is at C, inverted and the same size.

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.Define refraction.Show answer

   The bending of light as it passes obliquely from one transparent medium to another.

2. 2.Relation between f and R?Show answer

   f = R/2.

3. 3.Which mirror is used in torches?Show answer

   Concave.

4. 4.Which mirror is used as a rear-view mirror?Show answer

   Convex.

5. 5.Where is the image when an object is at C of a concave mirror?Show answer

   At C (real, inverted, same size).

6. 6.Name the seven colours of the spectrum in order.Show answer

   Violet, indigo, blue, green, yellow, orange, red.

7. 7.Which colour bends most in a prism?Show answer

   Violet.

8. 8.Can a virtual image be taken on a screen?Show answer

   No.

Quick revision checklist

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

• ☐Can you label P, F, C and the principal axis?
• ☐Is F half-way between P and C on your diagram?
• ☐Can you draw the two standard rays with arrows?
• ☐Do you know the image position for an object beyond C, at C and between C and F?
• ☐Can you give two uses each of concave and convex mirrors?
• ☐Can you write VIBGYOR and say which colour bends most?

Light Energy: frequently asked questions