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Wave — Refraction of Light, Lenses and the Human EyeSolved Questions· Unit 10
Science & Technology · Chapter 10
57 solved questions

Chapter 10: Wave — Refraction of Light, Lenses and the Human Eye

Every question in this chapter, answered and explained — step-by-step solutions drawn live from the chapter library across 7 question sections.

Very Short Answer Questions18Short Answer Questions9Give Reason Questions11Differentiate Between5Long Answer Questions5Numerical Problems6Q. Draw a ray diagram showing refraction of light through a glass slab and label the incident ray, refracted ray, emergent ray, angle of incidence, angle of refraction, angle of emergence, and lateral shift.3
01

Very Short Answer Questions

18
1SEE Boardqb-q0

What is refraction of light?

Ans.: Refraction of light is the bending of light as it passes from one optical medium to another, due to a change in the speed of light.
2SEE Boardqb-q1

Define refractive index.

Ans.: The refractive index of a medium is the ratio of the speed of light in air/vacuum to the speed of light in that medium.
3SEE Boardqb-q2

State Snell's law.

Ans.: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media: sin i / sin r = constant (μ).
4SEE Boardqb-q3

What is a critical angle?

Ans.: The critical angle is the angle of incidence in a denser medium for which the corresponding angle of refraction in the rarer medium is exactly 90°.
5SEE Boardqb-q4

What is total internal reflection?

Ans.: Total internal reflection is the complete reflection of light back into the denser medium when the angle of incidence exceeds the critical angle.
6SEE Boardqb-q5

What is the critical angle of glass?

Ans.: The critical angle of glass (with respect to air) is 42°.
7SEE Boardqb-q6

What is dispersion of light?

Ans.: Dispersion of light is the splitting of light into its seven constituent colours (VIBGYOR) while passing through a prism.
8SEE Boardqb-q7

Name the seven colours of the visible spectrum in order.

Ans.: Red, Orange, Yellow, Green, Blue, Indigo, Violet (VIBGYOR).
9SEE Boardqb-q8

Which colour of light bends the least, and which bends the most, in a prism?

Ans.: Red light bends the least; violet light bends the most.
10SEE Boardqb-q9

What is a lens?

Ans.: A lens is a transparent medium bounded by at least one spherical (curved) surface.
11SEE Boardqb-q10

Define the power of a lens.

Ans.: The power of a lens is the reciprocal of its focal length in metres (P = 1/f); its SI unit is the dioptre (D).
12SEE Boardqb-q11

What is the near point of a normal eye?

Ans.: The near point of a normal eye is 25 cm.
13SEE Boardqb-q12

What is the far point of a normal eye?

Ans.: The far point of a normal eye is at infinity.
14SEE Boardqb-q13

What is myopia?

Ans.: Myopia (shortsightedness) is a defect of vision in which distant objects appear blurry because their image forms in front of the retina.
15SEE Boardqb-q14

What is hypermetropia?

Ans.: Hypermetropia (long-sightedness) is a defect of vision in which nearby objects appear blurry because their image forms behind the retina.
16SEE Boardqb-q15

What is a cataract?

Ans.: A cataract is a clouding of the eye's lens, usually due to age, which causes blurry vision.
17SEE Boardqb-q16

What is night blindness?

Ans.: Night blindness (nyctalopia) is the inability to see well at night or in dim light, mainly caused by a deficiency of vitamin A affecting the rod cells.
18SEE Boardqb-q17

What is an optical fibre?

Ans.: An optical fibre is a thin, transparent medium (such as glass) used to transmit light through repeated total internal reflection.
02

Short Answer Questions

9
1SEE Boardqb-q18

Write the laws of refraction of light.

Ans.: 1) The incident ray, the normal, and the refracted ray all lie in the same plane at the point of incidence. 2) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media (Snell's Law): sin i/sin r = μ.
2SEE Boardqb-q19

Write two conditions necessary for total internal reflection of light.

Ans.: 1) Light must be travelling from a denser medium to a rarer medium. 2) The angle of incidence in the denser medium must be greater than the critical angle for that pair of media.
3SEE Boardqb-q20

Why do stars twinkle?

Ans.: Light from stars passes through several layers of the atmosphere with continuously changing refractive index before reaching an observer's eyes. This causes repeated bending of light, which changes the brightness and apparent position of the star, making it appear to twinkle.
4SEE Boardqb-q21

Why is a diamond seen to sparkle?

Ans.: A diamond has a very high refractive index and hence a very small critical angle (24°). Light entering the diamond easily exceeds this critical angle at its cut faces and undergoes multiple total internal reflections before emerging, making it sparkle.
5SEE Boardqb-q22

What is the role of total internal reflection in fibre optic internet?

Ans.: In fibre optic internet, data is converted into light signals and sent through optical fibres. The light undergoes repeated total internal reflection at the core-cladding boundary (since the angle of incidence there is always greater than the critical angle), allowing it to travel through the fibre — even around bends — without escaping, enabling very fast and secure data transmission.
6SEE Boardqb-q23

What is the cause of dispersion of light?

Ans.: Different colours of light have different wavelengths and therefore travel at different speeds in a medium like glass. Since light bends twice while passing through a prism, the colours separate by different amounts — red (longest wavelength, fastest) bends least, and violet (shortest wavelength, slowest) bends most.
7SEE Boardqb-q24

What is accommodation of the eye?

Ans.: Accommodation of the eye is the process by which the ciliary muscles relax or contract to adjust the focal length (curvature) of the eye lens, so that a clear image of an object forms on the retina regardless of the object's distance.
8SEE Boardqb-q25

Why does an object under water appear to be at a lesser depth than its real depth?

Ans.: Light rays from the object travel from water (denser) to air (rarer) and bend away from the normal at the surface. When these refracted rays are extended backward, they appear to come from a point closer to the surface than the object's actual position, making the apparent depth less than the real depth.
9SEE Boardqb-q26

Write two uses of optical fibres in the medical field.

Ans.: 1) Endoscopy — examining internal organs like the stomach and oesophagus without surgery. 2) Keyhole (laparoscopic) surgery — operating inside the body through a small incision using a laparoscope.
03

Give Reason Questions

11
1SEE Boardqb-q27

Between glass and water, glass is considered a denser medium and water is a rarer medium.

Ans.: Glass and water are compared by the speed of light in them, not by their physical density. The speed of light in glass (2.00 × 10⁸ m/s) is less than in water (2.25 × 10⁸ m/s), so glass is optically denser and water is optically rarer, regardless of their physical mass density.
2SEE Boardqb-q28

When a coin is placed in a glass containing water, it appears to rise a bit.

Ans.: Light rays reflected by the coin travel from water (denser) to air (rarer) and bend away from the normal at the water surface. When these refracted rays are extended backward, they appear to come from a point higher than the coin's actual position, so the coin appears to have risen.
3SEE Boardqb-q29

When letters written on paper are observed from the top of a glass slab, the letters appear to be slightly raised.

Ans.: Light from the letters travels from glass (denser) to air (rarer) as it leaves the top of the slab and bends away from the normal. The refracted rays, when traced backward, appear to originate from a point closer to the top surface than the actual letters, making them appear slightly raised.
4SEE Boardqb-q30

Stars twinkle.

Ans.: Starlight undergoes successive refraction through atmospheric layers of continuously changing refractive index before reaching the observer's eye. This causes the star's apparent brightness and position to change rapidly and randomly, which we perceive as twinkling.
5SEE Boardqb-q31

The sun appears on the horizon about two minutes before the actual sunrise.

Ans.: Sunlight passes from the rarer upper layers of the atmosphere to the denser lower layers, bending towards the normal at each layer. Because of this successive refraction, light from the sun (which is still below the horizon) reaches the observer's eye as if coming from above the horizon, so the sun appears to rise about two minutes early.
6SEE Boardqb-q32

A diamond appears to shine, but a piece of glass cut to the same shape does not shine.

Ans.: Diamond has a much higher refractive index than glass, giving it a much smaller critical angle (24° vs 42°). Light entering a diamond easily exceeds this small critical angle at its many faces and undergoes multiple total internal reflections, making it sparkle. In glass, with its larger critical angle, most light simply refracts out through the opposite face instead of being totally internally reflected.
7SEE Boardqb-q33

Sunlight is refracted (dispersed) when it is passed through a prism.

Ans.: Sunlight (white light) is a mixture of seven colours of different wavelengths, each of which travels at a slightly different speed in glass. Since light bends twice on passing through a prism, the different colours are refracted by different amounts and separate out — this dispersion produces the visible spectrum.
8SEE Boardqb-q34

A convex lens converges light rays.

Ans.: A convex lens is thicker at the middle than at the edges. It can be thought of as a combination of prisms with their bases facing inward (toward the centre); since a prism always bends light towards its base, and the central portion refracts light more strongly than the edges, parallel rays passing through a convex lens are bent inward and meet at a single point (the focus).
9SEE Boardqb-q35

A concave lens diverges the rays of light.

Ans.: A concave lens is thinner at the middle than at the edges. It can be thought of as a combination of prisms with their bases facing outward (away from the centre); since a prism always bends light towards its base, parallel rays passing through a concave lens are bent outward and spread apart (diverge) after refraction.
10SEE Boardqb-q36

Deficiency of vitamin A in the body is one of the main causes of night blindness.

Ans.: The rhodopsin pigment in the rod cells of the retina (which are responsible for vision in dim light) is made from a type of protein and vitamin A. When the body lacks vitamin A, this pigment becomes deficient, impairing the rod cells' function and causing night blindness.
11SEE Boardqb-q37

Colour blindness occurs when the cone cells of the retina stop functioning.

Ans.: The retina contains three types of cone cells (blue, red, and green) that detect and distinguish different colours of light. If these cone cells are defective or stop functioning properly (mainly due to heredity, but also mutation or damage), the eye cannot distinguish between certain colours, resulting in colour blindness.
04

Differentiate Between

5
1SEE Boardqb-q38

Reflection of Light and Total Internal Reflection of Light

Reflection of LightTotal Internal Reflection of Light
Occurs at the boundary of any two media, from either medium.Occurs only when light travels from a denser to a rarer medium.
Only part of the light is reflected; the rest is refracted/transmitted.All (100%) of the light is reflected back into the denser medium.
Can occur at any angle of incidence.Occurs only when the angle of incidence exceeds the critical angle.
2SEE Boardqb-q39

Concave Lens and Convex Lens

Concave LensConvex Lens
Thinner at the middle than at the edges.Thicker at the middle than at the edges.
Diverges light rays (diverging lens).Converges light rays (converging lens).
Always forms a virtual, erect, diminished image.Forms real or virtual images depending on object position.
Has negative power (in dioptres).Has positive power (in dioptres).
3SEE Boardqb-q40

Near Point of the Eye and Far Point of the Eye

Near PointFar Point
The nearest distance at which the eye sees objects clearly.The farthest distance at which the eye sees objects clearly.
25 cm for a normal eye.Infinity for a normal eye.
Eye lens has maximum curvature (thickest, shortest focal length) here.Eye lens has minimum curvature (thinnest, longest focal length) here.
4SEE Boardqb-q41

Shortsightedness (Myopia) and Long-sightedness (Hypermetropia)

Shortsightedness (Myopia)Long-sightedness (Hypermetropia)
Distant objects appear blurry; near objects are clear.Nearby objects appear blurry; distant objects are clear.
Image forms in front of the retina.Image forms behind the retina.
Corrected using a concave (diverging) lens.Corrected using a convex (converging) lens.
Often caused by an elongated eyeball.Often caused by a shortened eyeball, or occurs with old age.
5SEE Boardqb-q42

Colour Blindness and Night Blindness

Colour BlindnessNight Blindness
Caused by a defect in the cone cells of the retina.Caused by a problem with the rod cells of the retina.
Inability to distinguish certain colours (e.g., red-green).Inability to see clearly in dim light or at night.
Mainly hereditary.Mainly caused by vitamin A deficiency (also heredity, disease, or injury).
05

Long Answer Questions

5
1SEE Boardqb-q43

State and explain the laws of refraction of light. Define refractive index.

Ans.The laws of refraction of light are: 1) The incident ray, the normal at the point of incidence, and the refracted ray all lie in the same plane. 2) The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media — this is Snell's Law: sin i / sin r = μ (a constant).

This constant μ is called the refractive index of the pair of media. The refractive index of a medium (with respect to air/vacuum) is also equal to the ratio of the speed of light in air/vacuum (c) to the speed of light in that medium (v): μ = c/v. A higher refractive index means a lower speed of light in that medium, and a more optically denser medium.

2SEE Boardqb-q44

What is total internal reflection? Explain its conditions with reference to the critical angle, and describe two of its applications.

Ans.Total internal reflection (T.I.R.) is the phenomenon in which light travelling from a denser to a rarer medium, at an angle of incidence greater than the critical angle, is completely reflected back into the denser medium instead of being refracted. The critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is exactly 90°.

Conditions: (1) light must travel from a denser to a rarer medium, and (2) the angle of incidence must exceed the critical angle for that pair of media.

3SEE Boardqb-q45

Explain the formation of a rainbow with the help of a diagram.

Ans.A rainbow forms when sunlight is behind water droplets suspended in the air after rain. Each droplet acts like a tiny prism. Sunlight entering a spherical water droplet first refracts, splitting into its seven constituent colours. These separated colours then undergo total internal reflection at the far (inner) surface of the droplet. As they exit the droplet, they refract a second time, separating further. Red light, which is deviated the least, emerges to form the outer/upper part of the arc, while violet light, deviated the most, forms the inner/lower part.

Since millions of droplets across the sky each send their red, orange, etc. light to the observer's eye at slightly different angles, the overall effect is a circular arc of colours — appearing semicircular from the ground, but as a full circle when viewed from an aircraft or high altitude.

4SEE Boardqb-q46

Describe the main parts of the human eye and their functions, and explain how accommodation of the eye works.

Ans.The main parts of the eye are: the cornea (transparent front layer that does most of the light refraction), the pupil (opening through which light enters, size controlled by the iris), the iris (coloured muscle controlling pupil size), the eye lens (a natural convex lens made of crystalline protein), the ciliary muscles (attached to the lens, controlling its thickness), the retina (light-sensitive layer with rod cells for brightness and cone cells for colour), and the optic nerve (carries the image signal to the brain).

Accommodation is the process by which the ciliary muscles relax or contract to change the thickness (curvature) and hence the focal length of the eye lens, so that images of objects at different distances always focus exactly on the retina, since the distance between the lens and retina is fixed. When viewing distant objects, the ciliary muscles relax, making the lens thin with a longer focal length. When viewing nearby objects, the ciliary muscles contract, making the lens thick with a shorter focal length.

5SEE Boardqb-q47

Explain the causes of myopia (shortsightedness) and describe how it is corrected.

Ans.In myopia, distant objects appear blurry because their images form in front of the retina instead of exactly on it. This happens for two main reasons: (1) the eyeball becomes elongated, increasing the distance between the lens and the retina; or (2) the ciliary muscles do not relax enough when viewing distant objects, so the lens remains too thick and its focal length becomes shorter than required. As a result, the far point of a myopic eye is not at infinity but at some finite distance.

Correction: A concave (diverging) lens of suitable focal length is placed in front of the eye. It diverges the incoming parallel rays slightly before they enter the eye, so that after refraction by the cornea and eye lens, the rays converge exactly on the retina, forming a clear image.

06

Numerical Problems

6
1SEE Boardqb-q48

If the speeds of light in air and glass are 3 × 10⁸ m/s and 2 × 10⁸ m/s respectively, calculate the refractive index of glass with respect to air.

Ans.Answer:

Given: speed of light in air (c) = 3 × 10⁸ m/s, speed of light in glass (v) = 2 × 10⁸ m/s

2SEE Boardqb-q49

The refractive index of a diamond is 2.42. If the speed of light in air is 3 × 10⁸ m/s, calculate the speed of light in a diamond.

Ans.Answer:

Given: μ = 2.42, c = 3 × 10⁸ m/s

3SEE Boardqb-q50

When a ray of light falls on the surface of a plastic block, the angle made by the ray with the normal and the angle of refraction are found to be 45° and 33° respectively. Calculate the refractive index of the plastic.

Ans.Answer:

Given: angle of incidence (i) = 45°, angle of refraction (r) = 33°

4SEE Boardqb-q51

Calculate the power of a lens having a focal length of 25 cm.

Ans.Answer:

Given: focal length (f) = 25 cm = 0.25 m

5SEE Boardqb-q52

The power of the lens used in the spectacles worn by a student is −6D. Calculate the focal length of the lens. Also mention the type of lens.

Ans.Answer:

Given: Power (P) = −6 D

6SEE Boardqb-q53

Additional SEE Practice — Q6. The critical angle for a certain medium with respect to air is 30°. Calculate its refractive index. (Hint: at the critical angle, angle of refraction = 90°.)

Ans.Answer:

Given: critical angle (C) = 30°, angle of refraction at critical angle = 90°

07

Q. Draw a ray diagram showing refraction of light through a glass slab and label the incident ray, refracted ray, emergent ray, angle of incidence, angle of refraction, angle of emergence, and lateral shift.

3
1SEE Boardqb-q54

Draw ray diagrams showing the image formed by a convex lens when the object is: (i) beyond 2F, (ii) at 2F, (iii) between F and 2F, and (iv) between F and the optical centre.

Ans.
  1. (i) Object beyond 2F: image forms between F and 2F on the other side — real, inverted, diminished (as in a camera).
  2. (ii) Object at 2F: image forms at 2F on the other side — real, inverted, same size as object.
  3. (iii) Object between F and 2F: image forms beyond 2F on the other side — real, inverted, magnified (as in a projector).
  4. (iv) Object between F and optical centre: image forms on the same side as the object — virtual, erect, magnified (as in a magnifying lens).
2SEE Boardqb-q55

The given ray diagram shows the dispersion of a light ray through a triangular prism, with colours labelled X (upper) and Y (lower) after the prism. Identify colours X and Y and explain why Y bends more than X.

Ans.X (bending least, appearing on the upper part of the spectrum) is red light; Y (bending most, appearing on the lower part) is violet light. Violet light has a shorter wavelength than red light, and travels more slowly through glass. Since the amount of bending (refraction) depends on the change in speed, violet light — with the greater change in speed relative to its speed in air — bends more than red light, which has the smallest change in speed.
3SEE Boardqb-q56

Identify the type of defect of vision shown in a ray diagram where diverging rays from a nearby object (25 cm away, the normal near point) are focused behind the retina. Write two causes and describe its correction.

Ans.This is hypermetropia (long-sightedness), since light from a nearby object at the normal near point (25 cm) is focused behind the retina rather than on it.

Causes: (1) The eyeball is too short, decreasing the distance between the lens and retina. (2) The ciliary muscles cannot contract enough, so the lens cannot become thick enough for near objects, increasing the focal length beyond what is needed.

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