SAQ for Human Eye and the Colourful World Class 10 Science NCERT
Important Questions1
Answer
(i) Iris : Gives colour to eyes, control size of pupil.
(ii) Pupil : Regulate amount of light.
(iii) Cornea : Refracts most of the light into eyes.
2
What is meant by the term "power of accommodation' of human eye? How does it help a person to see nearby as well as distant objects clearly?
OR
How do we able to see nearby and also the distant objects clearly
Answer
Power of accommodation of human eye: The ability of eye lens to adjust its focal length to form the sharp image of the object at varying distance on the retina is called its power of accommodation. The curvature of eye lens can be changed it. Change in its focal length) to some extent by the ciliary muscles. When we are looking at the nearby objects, the ciliary muscles contract, they increase the thickness of eye lens. As a result, the focal length of eye lens decreases in such a way that the clear sharp image of nearby object is formed on the retina. Thus, the object is seen clearly to us. When we are looking at the distance objects, these muscles are in relaxed position, the eye lens becomes thinner, focal length of the lens increases. Therefore, the parallel rays coming from the distant object are focused on the retina and object is seen clearly to us. Thus, the accommodation power of an eye helps a person to see nearby as well as distant objects clearly3
(i) What is meant by 'least distance of distinct vision ?
(ii) How does iris controls the size of the pupil in bright light and dim light?
Answer
(i) The minimum distance at which objects can be seen distinctly without strain is called the least distance of distinct vision.
(ii) In bright light, the iris contracts the pupil to allow less light to enter the eye. In dim light the iris expands the pupil to allow more light to enter the.
4
With the help of ciliary muscles the human eye can change its curvature and thus alter the focal length of its lens. State the changes that occur in the curvature and focal length of the eye lens while viewing: (a) a distant object, (b) nearby objects.
Explain, why a normal eye is not able to see distinctly the object placed closer than 25 cm, without putting any strain on the eye.
Answer
(a) When we were distant objects, the chary muscles expand to death curvature and thereby in the focal length of them. Hence, the lens becomes thin. This enables us to see the distant object clearly. Thus, the focal length of the eye lens in while seeing distant objects.
(b) To see the nearby objects clearly, the focal length of the lens should be shorter. For this, the ciliary muscles contract to increase the curvature and thereby decrease the focal length of them. Hence, the lens becomes thick. This enables you to the nearby clearly.
A normal eye is not able to see distinctly the objects placed closer than 5 cm, without putting any strain on the eye. This is because the ciliary muse of eyes are unable to contact beyond a certain limit. If the objct are placed at a distance less than 25 cm from the eye, then the objects appear blurred because light rays coming from the object meet beyond the retina
5
(i) What are the value of (a) near point and (b) by far point of vision of a normal adult person ?
(ii) A person with a myopic we cannot see objects beyond 12 m directly. What should be the type of the corrective lens used? What would be its focal length and power?
Answer
(i) 25cm, (b) Infinity (∞)
(ii) To an object has to be bought at an ∞ has to be brought at an image to 120 cm.
1/f = 1/-120 – 1/(- ∞) = 1/-120
f = - 120 cm
P = 100/-120
= -5/6 D
= - 0.83 D
A concave lens focal length 120 cm and power -0.83 D is to be used.
6
(i) Make a ray diagram to show how the eye defect myopia is corrected by using a suitable lens.
(ii) State two reason due to which this wye detect may be caused.
(ii) A person with myopic eye cannot see objects beyond a distance of 1.5 m. What is the power of the lens required to correct the problem?
Answer
(i) 
(ii) (a) Elongation of the eye
(b) Decrease in focal length of eye lens.
(iii) u = ∞, v = - 1.5 m
P = 1/f = 1/v – 1/u
= 1/-1.5
= -0.66 D
7
(i) Draw a diagram to show the formation of image of a distant object by a myopic eye. How can such an eye-defect be rectified ?
(ii) State two reasons due to which this eye defect may be caused.
Answer
(i) Diagram.
It can be rectified by concave lens.
(ii) Due to excessive curvature of lens or elongation of eye ball.
8
Answer
The ability of the lens to adjust its focal length is called power of accommodation.
The focal length cannot change. Thus the image distance cannot change. Thus the image distance is equal to distance from the centre of the eye lens to the retina.
9
Answer
u = - 25 cm
v = - 75 cm
⇒ 1/v – 1/u = 1/f
⇒ 1/-75 – 1/-25 = 1/f
⇒ 1/-75 + 1/25 = 1/f
2/75 = 1/f
⇒ f = 75/2 = 37.5 cm
So, P = 100/37.5 cm
10
Answer
A concave lens of suitable focal length diverges the parallel rays from the distant objects as if they are coming from the far point of the myopic eye. This helps the eye lens to form a clear image at the retina.11
Answer
When a hypermetropic person wearing the spectacles looks at a distant object, the parallel rays from the distant object get converged in front of the retina. The image appears blurred, in order to avoid this, the person prefers to remove his spectacles.12
(i) Ravi kept a book at a distance of 10 cm from the eyes of his friend Hari. Hari is not able to read anything written on the book. Explain why?
(ii) A lens of focal length 5.0 cm is being used by a student in the laboratory as a magnifying glass. His least distance of distinct vision is 25 cm. What magnification is the student getting?
Answer
(i) Because least distance of distinct vision is 25 cm, and book is kept at a distance of 10 cm.
(ii) u = ?, v = - 25 cm, f = 5 cm.
1/f = 1/v – 1/u
⇒ 1/5 = 1/(-25) – 1/u
⇒ 1/u = 1/(-25) – 1/5
⇒ u = -25/6
m = v/u
⇒ m = (-25) × 6/-25
⇒ m = 6
13
Explain why?
(i) Myopic person prefers to remove his spectacles while reading a book
(ii) A hypermetropic person prefers to remove the spectacles while looking at the sky ?
Answer
(i) Myopic person does not need spectacles while reading a book as he has the near point at 25 cm. If such person reads the book with corrective lens (concave), he will have to keep the book at distance greater than 25 cm so that the image of book will be formed by the concave lens at 25 cm and moreover, the size of the book that appear to him is also smaller. Therefore, the person prefers to remove his spectacles while reading a book.
(ii) Hypermetropic person does not need spectacles to see distant objects as he has the far point at infinity. If such person uses spectacles (convex lens) to see the distant objects, the image will be formed, before retina due to increase in converging power and hence the person cannot see distant object distinctly. Therefore, such person prefers to remove spectacles while looking at the sky.
14
Answer
This person has defect of Hypermetropia which is far sightedness that means the image is formed beyond the retina so it requires a convergent lens to get focused at retina.
Now corrective measure is :
u = - 25 cm
v = - 50 cm
1/f = 1/v – 1/u
⇒ 1/f = 1/-50 – 1/-25
p = 1/f
⇒ p = 100/50 = 2 D
Focal length = 50 cm,
Power = 2D
15
Answer
Cause of dispersion of white light : Different colours of light bend through different angles with respect to the incident ray as they pass through a prism.
Violet light bends the most and red the least. Thus, the each colour emerges along different paths.
16
What is meant by scattering of light? The sky appears blue and the sun appears reddish at sunrise and sunset. Explain these phenomena with reason.
OR
What is the difference of colours of the sun observed during sunrise/sunset and noon? Give explanation of each?
Answer
(i) Scattering of light-Phenomenon of spreading of light (diffused reflection) by minute particles in a medium.
(ii) The sky appears blue because the blue colour of sunlight scatters much more than the red colour by particles in atmosphere/ air due to its shorter wavelength.
(iii) At sun-rise and sun-set most of the blue light and shorter wavelengths are scattered away by the particles in the atmosphere as the light from the sun near the horizon passes through thick layers of air and larger distance. The light that reaches us is of longer wavelength (red colour) giving a reddish appearance.
17
Answer
Since the atmosphere consists of varying densities the apparent position of the object, as seen through the hot air fluctuates. This wavering of light is an effect on atmospheric refraction.
The twinkling of a star is due to atmosphere refraction of star light. The atmospheric refraction of light occurs in a medium of gradually changing refractive index.
The planets are much closer to the earth and are thus seen as extended sources. A planet is considered as a collection of large number of point sized sources of light, the total variation in the amount of light entering our eye from all individual point sized sources will average out to zero, thereby nullifying the twinkling effect.
18
Answer
The angle between the extended incident ray and the emergent ray is called the angle of deviation.
This is because the different colours travel through a glass prism at different speeds.
19
Answer
Atmospheric refraction.
The sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction.
20
Name the phenomenon associated with the following:
(i) The sky appears blue.
(ii) Formation of a rainbow in the sky.
(iii) Twinkling of stars.
Answer
(i) (a) Atmospheric refraction
(b) Scattering of light
(ii) (a) Atmospheric refraction
(b) Change in the density of the atmosphere due to temperature and other conditions.
21
Answer
During sunrise and sunset, Sun is red in colour while at noon, the Sun appears white. At the time of sunrise and sunset, the Sun is near the horizon. The rays from the Sun have to travel a much larger part of the atmosphere to reach an observer on earth. So, most of the blue light is scattered away. The red colour which has the largest wavelength is scattered the least and enters into our eyes. Hence, the Sun appears red at the time of sunrise and sunset. At noon, the sun is nearly overhead. The sunlight has to pass through much smaller portion of Earth's atmosphere. The scattering is much less and the Sun looks white.22
Answer
(i) Due to atmospheric refraction, the sun is visible to us about two minutes before the actual sunrise and about two minutes after the actual sunset.23
Answer
The various phenomena observed in nature due to scattering of light are the following:
(i) Sun appears red near the horizons (during sunrise and sunset) and white when seen overhead.
(ii) Bluish colour of the sky.
(iii) Visible path of light as it enters a dark room.
(iv) Danger signals or stop signals are usually red.
(v) Blueness of distant mountains.
24
Explain in brief the reason for each of the following :
(i) The sun appears reddish during sun-rise.
(ii) At noon the sun appears white.
(iii)To an astronaut the sky appears dark instead of blue.
Answer
(i) During morning sun rays travel large distances in the earth's atmosphere, and in this process the shorter wavelengths scatter away and only large wave length (red light) reaches us.
(ii) At noon, sun is overhead and light rays travel comparatively smaller distance and only little of blue/violet light scatter, so sun appears white.
(iii) No atmosphere in outer space for scattering, so sky appears dark.
25
(i) "Stars seem higher than they actually are
(ii) "The sky appears dark to passengers flying at very high altitudes". Justify these statements with reason.
Answer
(i) Light coming from the stars are supposed to travel in a straight line. But the refractive index of air is not same throughout. It changes as hot air is rarer than cold air. So refraction takes place. In this case, star light continuously travels from a rarer medium to a denser medium. Hence, it continuously bends towards the normal. 19 The continuous bending of starlight towards the normal results in a rise of the apparent position of the star.
(ii) Scattering of light take place because of the particles present in atmosphere. At high altitude due to absence of atmosphere scattering of light does not take place and hence sky appears dark to passengers flying at high altitude.
26
Answer
Description of activity: When a glass prism is used to obtain a spectrum of sunlight, a second identical prism in an inverted position with respect to the first position will allow all the colours of spectrum to recombine. Thus a beam of white light will emerge from the other side of the second prism.
27
Answer
Sunlight reaches the Earth's atmosphere and is scattered in all directions by the gases and particles in the air. Blue light is scattered more than the other colours because it has the shortest wavelength. This is why the sky appears blue to an observer from the surface of the Earth. For an astronaut staying in the international space station orbiting the Earth, the colour of the sky will be black because the light reaching it does not scatter.28
Answer
Light coming from the stars undergoes refraction on entering the Earth's atmosphere. This refraction continues until it reaches the Earth's surface. This happens because of uneven heating of atmospheric air. Hence, the atmospheric air has changing refractive index at various altitudes. In this case, star light continuously travels from a rarer medium to a denser medium. Hence, it continuously bends towards the normal.
The refractive index of air medium gradually increases with a decrease in altitude. The continuous bending of starlight towards the normal results in a slight rise of the apparent position of the star.
Since the physical conditions of the Earth’s atmosphere keeps changing, the aparent position of the star is not stationary. The star changes its position continuously, which makes it twinkle. This happens because star light travels a very large distance before reaching the observer. However, the path varies continuously because of uneven atmospheric conditions. Hence, the stars seem to be fluctuating, sometimes appearing brighter and sometimes fainter. All this together, gives rise to the twinkling effect of stars.
29
Answer
The phenomenon of scattering of white light by colloidal particles is known as Tyndall effect.
Examples :
(i) A fine beam of sunlight entering a smoke filled room through a hole. Smoke particles scatter the white light and hence the path of light beam becomes visible.
(ii) (a) Sunlight passing through the trees of forest.
(b) Tiny water droplets in mist scattering white light.
30
A narrow beam PQ of white light is passing through a glass prism ABC as shown in the diagram.
Trace it on your answer sheet and show the path of the emergent beam as observed on the screen DE.
(i) Write the name and cause of the phenomenon observed.
(ii) Where else in nature is this phenomenon observed?
(iii) Based on this observation, state the conclusion which can be drawn about the constituents of white light.
Answer
(i) The phenomenon of the splitting up of the white light into its constituent colours is called dispersion of light.
Dispersion of light is caused because different constituent colours of light offer different refractive indices to the material of the prism.
(ii)The formation of rainbow is caused by the dispersion of the white sunlight into its constituent colours.
(iii) Based on the dispersion of white light into its constituent colours, we can conclude that (a) The white light consists of seven colours
(b) The violet light suffers maximum deviation and the red light suffers minimum deviation.
31
Answer
The reason is that for a given angle of incidence, the angle of reflection is same for all the wavelengths of white light while the angle of refraction is different for different wavelengths.32
Answer
Tyndall effect: When a beam of light is passed through a colloidal solution, placed in a dark room, the path of beam becomes illuminated when observed through a microscope placed perpendicular to the path of light. This effect is called Tyndall effect.
Cause of Tyndall effect : The size of the colloidal particle is relatively larger than the solute particle of a true solution. The colloidal particles first absorb energy from the incident light and then scatter a part of this energy from their surfaces. Thus, Tyndall effect is due to scattering of light by the colloidal particles and the colloidal particles are seen to be moving as points of light moving against a dark background.
Some daily phenomena based on Tyndall effect are as follows:
(i) When a fine beam of sunlight enters a smoke filled room through a small hole, the smoke particles become visible due to the scattering of light.
(ii) When sunlight passes through a canopy of a dense forest, the tiny water droplets in the mist scatter light and become visible.