Lesson: The Human Eye and the Colourful World

Question 1

A person cannot see distinctly objects kept beyond 2 m. This defect can be corrected by using a lens of power:

(a) + 0.5 D

(b) $-$ 0.5 D

(d) $-$ 0.2 D

Solution:

Question 2

A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his text book. Which of the following statements is correct?

(a) The near point of his eyes has receded away

(b) The near point of his eyes has come closer to him

(d) The far point of his eyes has receded away

Solution:

Question 3

A prism ABC (with BC as base) is placed in different orientations.

A narrow beam of white light is incident on the prism as shown in given figure.

In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?

a) (i)

b) (ii)

c) (iii)

d) (iv)

Solution:

Question 4

At noon the sun appears white as:

(a) Light is least scattered

(b) All the colours of the white light are scattered away

(d) Red colour is scattered the most

Solution:

Question 5

Which of the following phenomena of light are involved in the formation of a rainbow?

(a) Reflection, refraction and dispersion

(b) Refraction, dispersion and total internal reflection

(d) Dispersion, scattering and total internal reflection

Solution:

Question 6

Twinkling of stars is due to atmospheric:

(a) Dispersion of light by water droplets

(b) Refraction of light by different layers of varying refractive indices

(d) Internal reflection of light by clouds

Solution:

Question 7

The clear sky appears blue because:

(a) Blue light gets absorbed in the atmosphere

(b) Ultraviolet radiations are absorbed in the atmosphere

(d) Light of all other colours is scattered more than the violet and blue colour lights by the atmosphere

Solution:

Question 8

Which of the following statements is correct regarding the propagation of light of different colours of white light in air?

(a) Red light moves fastest

(b) Blue light moves faster than green light

(d) Yellow light moves with the mean speed as that of the red and the violet light

Solution:

Question 9

The danger signals installed at the top of tall buildings are red in colour. These can be easily seen from a distance because among all other colours, the red light:

(a) Is scattered the most by smoke or fog

(b) Is scattered the least by smoke or fog

(d) Moves fastest in air

Solution:

Question 10

Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset?

(a) Dispersion of light

(b) Scattering of light

(d) Reflection of light from the earth

Solution:

Question 11

The bluish colour of water in deep sea is due to:

(a) The presence of algae and other plants found in water

(b) Reflection of sky in water

(d) Absorption of light by the sea

Solution:

Question 12

When light rays enter the eye, most of the refraction occurs at the:

(a) Crystalline lens

(b) Outer surface of the cornea

(d) Pupil

Solution:

Question 13

The focal length of the eye lens increases when eye muscles:

(a) Are relaxed and lens becomes thinner

(b) Contract and lens becomes thicker

(d) Contract and lens becomes thinner

Solution:

Question 14

Which of the following statement is correct?

(a) A person with myopia can see distant objects clearly

(b) A person with hypermetropia can see nearby objects clearly

(d) A person with hypermetropia cannot see distant objects clearly

Solution:

Short Answer Questions

Question 15

Draw ray diagrams each showing (i) myopic eye and (ii) Hypermetropic eye.

Solution:

Question 16

A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advices will a doctor give to her? Draw ray diagram for the correction of this defect.

Solution:

As the student is unable to see the letters written on the board, it is a case of myopia (near = sightedness). The doctor would advise the student to use a concave lens of appropriate power to enable him to see distant objects clearly.

Question 17

How are we able to see nearby and also the distant objects clearly?

Solution:

Human eye has the ability to focus on distant objects as well as nearby objects on the retina by changing the focal length of its lens. This enables us to see nearby and also the distant objects clearly.

Question 18

A person needs a lens of power $-$ 4.5 D for correction of her vision.

(a) What kind of defect in vision is she suffering from?

(b) What is the focal length of the corrective lens?

Solution:

(a) Myopia

(b) The power of a lens P of focal length f (in metres) is given as,

$P = \frac{1}{f}$

As P is given as -4.5 D, the focal length,

$\begin{array}{l} f = \frac{1}{- 4.5} = - 0.22 m \end{array}$

Question 19

How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw the diagram.

Solution:

In order to get a narrow beam of light incident on one prism and white light emerging out of the second prism, we can arrange the two identical prisms in such a way that one prism is placed inverted with respect to the other.

Question 20

Draw a ray diagram showing the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also indicate the order of the colours of the spectrum obtained.

Solution:

The separation of white light into individual colour components is known as dispersion of light.

Question 21

Is the position of a star as seen by us its true position? Justify your answer.

Solution:

The stars appear to be higher than they actually are due to atmospheric refraction.

Light from a star is refracted as it enters the earth’s atmosphere. Hot air is less dense and has lesser refractive index than cool air. Objects appear to move when viewed through hot air or smoke because the air, which is the refractive medium in this case, is not stationary in its position. This changing refractive index makes the objects appear to be moving around their position. Thus, stars appear to us in a higher position than they actually are:

Question 22

Why do we see a rainbow in the sky only after rainfall?

Solution:

A rainbow is formed when bright sunlight passes through suspended droplets of water in the air. It is also necessary to view the rainbow from the correct angle, or it will not be visible.

A ray of light from the sun passes through the suspended water droplets which cause refraction of the light. The light is then reflected off the inside of the droplet and refracts again when exiting the droplet.

Upon entering the droplet the light loses its speed and bends towards the normal and when the ray exits the droplet, it gains speed and bends away from the normal.

Thus, after refracting twice and reflecting once, the light ray is dispersed and bent downwards towards an observer on earth's surface.

Question 23

Why is the colour of the clear sky blue?

Solution:

When white light from the sun reaches the atmosphere, the higher frequencies of light like blue, indigo and violet get scattered by the gas molecules in the atmosphere. On the other hand, the lower frequencies of light such as red, orange and yellow do not get scattered as much.

The scattering of the higher frequencies of light illuminates the sky with light on the BIV end of the visible spectrum. Since the light at the blue end of the spectrum is scattered more than the light at the red end, blue light comes to us from all directions and so the sky appears blue; whichever angle we are looking at the sky from.

If the earth had no atmosphere, there would have been no scattering. In that case, the sky would have looked dark. This is the reason, why the sky appears dark to passengers flying at very high altitudes, as scattering of light is not prominent at such heights.

Question 24

What is the difference in colours of the Sun observed during sunrise/sunset and noon? Give explanation for each.

Solution:

During Sunset/ sunrise:

The white light from the sun travels a greater distance through the atmosphere and has to pass through more atmospheric particles during sunset or sunrise. Since this causes much more scattering of light, and since the higher frequencies tend to get scattered more, only the lower frequencies of light (red and orange) pass through and reach our eyes, so the sky appears red at sunset.

During Mid-day:

At mid-day, the lower frequencies of light (red, orange, yellow, green) tend to pass through the atmosphere and reach our eyes directly, while the higher frequencies get scattered. These lower frequency colours combine to form yellow and since they are most visible to us, the sun appears to be yellow in colour, even though the sunlight is actually white.

Long Answer Questions

Question 25

Explain the structure and functioning of Human eye. How are we able to see nearby as well as distant objects?

Solution:

Hint- Give explanation of each part and discuss power of accommodation.

The main parts of the human eye are: cornea, iris, pupil, ciliary muscles, eye lens, retina and optic nerve.

Cornea: The cornea is the clear, transparent, thin membrane in the front portion of the eye, through which light enters. It is the main refractive surface of the eye.

Iris: The iris is a coloured, circular membrane located behind the cornea and in front of the lens. It regulates the amount of light entering the eye by adjusting the size of the pupil. In bright light, iris muscles contract the pupil whereas in darkness, these muscles expand the pupil.

Pupil: The pupil is the opening or a hole in the middle of the iris that allows light to enter. It appears black, as it absorbs all the light entering it.

Ciliary muscles: Ciliary muscles change the thickness of the eye lens while focussing. In other words, it helps in changing the focal length of the eye lens.

Eye lens: The lens is the transparent tissue behind the iris which bends and focuses light rays onto the retina. Its curvature can be changed by the ciliary muscles, thus changing the focal length.

Retina: The retina is the light-sensitive tissue at the back of the eye which transmits visual signals through the optic nerve to the brain. It consists of two light receptors, the cones and the rods. The cones are colour-sensitive and absorb stronger light whereas the rods absorb softer light in black and white.

Optic nerve: The optic nerve sends the visual signals from the eye to the brain, where these signals are interpreted into images.

How we see objects:

a) The light coming from outside enters the eye through the cornea.

b) The pupil regulates and controls the amount of light entering into the eyes.

c) The eye lens forms an inverted real image of the object on the retina. The light-sensitive cells of the retina are activated upon illumination. It generates electrical signals.

d) These signals are sent to the brain via the optic nerves.

e) The brain interprets these signals, processes the information and helps us perceive objects as they are.

Question 26

When do we consider a person to be myopic or hypermetropic?

Explain using diagrams how the defects associated with myopic and hypermetropic eye can be corrected?

Solution:

A person is considered to be myopic when he can see nearby objects clearly but cannot see distant objects distinctly.

In a myopic eye, the image of a distant object is formed in front of the retina.

Concave lens of suitable power helps in correcting myopia by bringing the image back on to the retina.

A person is considered to be suffering from hypermetropia when he can see distant objects clearly but cannot see nearby objects distinctly.

In such cases, the images of a nearby objects is formed behind the retina.

Convex lens of suitable power helps in correcting hypermetropia by bringing the image back on to the retina.

Question 27

Explain the refraction of light through a triangular glass prism using a labelled ray diagram. Hence define the angle of deviation.

Solution:

Consider the refracting surface of the prism ABC. We have shown the following:

PE - incident ray

EF - refracted ray

FM - emergent ray

$∠ MFS = e = the angle of deviation$

Angle of deviation is the angle between the incident ray and the emergent ray when a ray of light passes through a glass prism.

The ray PE of light is incident on the glass prism. It refracts bending towards the normal. This refracted ray (EF) is incident on the second side of the prism and away from the normal.

Here, $∠ MFS = e = the angle of deviation$

Angle of deviation is the angle between the incident ray and the emergent ray when a ray of light passes through a glass prism.

Question 28

How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?

Solution:

During sunrise and sunset the sun appears reddish. At noon, the sun appears white.

The reddish appearance of the sun at sunrise or sunset is due to the scattering of light. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the sun.

At noon, the sun is overhead. The light from the overhead sun travels relatively shorter distance.

During this time, the sun appears white as only a very little of the blue and violet colours are scattered. So, we are able to see the sunlight consisting of all the colours.

Question 29

Explain the phenomenon of dispersion of white light through a glass prism, using suitable ray diagram.

Solution:

When white light passes through a glass prism, it splits into its seven component colours. This is called dispersion of light.

Reason for the dispersion:

Different colours bend at different angles with respect to the incident rays, as they pass through the prism. This is shown in the following diagram.

The red light bends the least while the violet the most. Thus, we get seven distinct colours.

Question 30

How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?

Solution:

The atmosphere surrounding the earth has layers of gases. All the air in the atmosphere is not at the same temperature. The density of the air layers vary with temperature. This results in having air layers of different optical densities. Sunlight passing through air layers of different optical densities refracts. This refraction is called atmospheric refraction.

When the light coming from a star enters the earth’s atmosphere, it undergoes refraction due to its path travelling through different densities of air layers. The density of air continuously changes and refracts the light from the stars in varying proportion.

When more light is refracted, the star appears to be bright and when it is less the star appears to be dim. Thus, we see the star twinkling.

Planets on the other hand are nearer to the earth and do not appear as a point source. A planet can be considered as a combination of many point sources of light.

So, the dimness of one source is nullified by the brightness of another source.

Therefore, a planet does not appear to be twinkling.