CBSE Class 10 Physics The Human Eye and the Colourful World Worksheet Set C

ONE MARK QUESTIONS

Question: What is the number of receptors contained in the retina?
Answer: Retina contains more than 125 millions receptors which are of two types, rods and cones.

Question: Which liquid fills the space between eye lens and retina?
Answer: Vitreous humour.

Question: Name the essential parts of human eye,
Answer: (a) Retina (6) Eye lens (c) Cornea (d) Pupil (e) Iris

Question: Why is blind spot so called?
Answer: An image formed at this point is not sent to brain.

Question: Which liquid fills the space behind the cornea?
Answer: Aqueous humour is filled in the space behind the cornea.

Question: How is the sense of vision carried from the eye to the brain?
Answer: Through optical nerve, sense of vision is carried fromeye to the brain.

Question: Why can you not see an object clearly if it is placed very close to your eye?
Answer: For a human eye, there is a near point, 25 cm. If the distance of the object is lesser than this distance, eye cannot see object clearly.

Question: What is meant by far point?
Answer: The farthest point upto which an normal human eye can see objects clearly is known as far point.

Question: When you enter from a bright sunshine into a dark room, you are not able to see clearly for sometime, why?
Answer: In bright light, pupil had become small.

Question: What is eye lens made of ?
Answer: Eye lens is made of transparent and flexible jelly like material.

Question: Which part of the eye controls the amount of light entering the eye?
Answer: Pupil controls the amount of light entering the eye.

TWO MARKS QUESTIONS

Question: How can change of size of eyeball be one of the reason for: (a) Myopic (b) Hypermetropic Compare the size of eyeball with that of a normal eye in each case. How does this change of size affect the position of image in each case?
Answer: (a) The eye suffering from myopia, has long eye ball than that of normal eye due to which the retina is at a larger distance from the eye lens. This results in the formation of the image in front of the retina.
(b) The eye suffering from hypermetropia has short eye ball than that of normal eye due to which the retina is smaller distance from the eye lens. This results in the formation of the image behind the retina.

Question: State the function of pupil and ciliary muscles.
Answer:
a. Pupil controls the amount of light entering the eye.
b. Ciliary muscles help in accommodation of power of eye lens.

Question: Why do we observe the seven colours when white light passes through a glass prism? Which component of white light deviates the least?
Answer: Since angle of deviation for different colour is different hence different colour suffers different deviation.
Deviation for red colour is minimum.

Question: Mention the factor on which scattering of light depends. Why does the sky appear dark in space?
Answer: Scattering of light depends on the presence of atmosphere which contains water droplets, molecules of air, smoke and dust particles. In the space, there is no atmosphere to scatter light.

Question: (a) What is meant by least distance of distinct vision?
(b) How does the thickness of the eye lens change when we shift looking from a distant tree to reading a book?
Answer: 
(a) Least distance of distinct vision means the minimum distance upto which an eye can see clearly.
(b) To see distant object focal length of eye lens must be more so it becomes thinner when we want to read a book eye lens is comparatively thicker.

Question: Sushil went to an eye specialist for check up. He prescribed him to use spectacle lens of + 0.5D power. a. Name the defect of vision he is suffering from.
b. Find the focal length of spectacle lens.
Answer: a. Hypermetropia.
b. Power of spectacle P = + 0.5D
f =1/p = +1/0.5 m
or f =+2 m positive sign means it is a convex lens.

Question: What is dispersion of light? Name the (i) component of white light that deviates the least, (ii) component of white light that deviates the most, while passing through a glass prism.
Answer: a. Splitting of white light into its constituent colours when passed through a prism called dispersion of light.
b. Violet colour deviates the most while passing through the glass prism.

Question: List four common defects of vision that can be corrected with the use of spectacles.
Answer: (i) Myopia (ii) Hypermetropia (iii) presbyopia (iv) Astigmatism.

Question: Why do we observe difference in colours of the sun during sunrise, sunset and noon?
Answer: 
During the sunrise or sunset, the sun is at horizon. Most of the blue light and other light of shorter wavelength are scattered most. Only the longer wavelength (red) reaches to us. That’s why sun appears red during the sunrise or sunset.
At noon, the Sun appears white because it is at our head and all wavelength reaches to us due to little of the blue and violet colours are scattered due to least distance travelled by fight.

Question: Why does it take sometimes to see the objects in a dim room when we enter the room from bright sunlight outside?
Answer: In bright sunlight, pupil of eye contracts to control the light. When we enter a dim room, pupil takes some time to expand to allow more light to see the objects clearly.

Question: (a) Explain how a normal human eye is able to see distinctly the object placed at a distance as well as those placed at a nearer distance.
(b) What is the far point and near point of a normal human eye?
Answer: (a) Ciliary muscles change the curvature of eye lens as per requirement of the eye. This changes the focal length of eye lens. When muscles are relaxed, lens becomes thin and distant objects are seen clearly.
While seeing closer objects, muscles contract, this thickens the eye lens and focal length decreases.
(b) Far point — Infinity. Near point — 25 cm from the eye.

Question: State the function of pupil and ciliary muscles.
Answer: a. Pupil controls the amount of light entering the eye.
b. Ciliary muscles help in accommodation of power of eye lens. 

Question: (a) What is ‘power of accommodation of the eye?
(b) What happens to the image distance when the object being viewed is moved away from the eye?
Answer:
(a) Power of accommodation of the eye means the ability to change the focal length of the eye lens to see near and distant objects.
(b) Image distance remains fixed which is equal to distance between retina and eye lens.

Question: Sushil went to an eye specialist for check up. He prescribed him to use spectacle lens of + 0.5D power.
a. Name the defect of vision he is suffering from.
b. Find the focal length of spectacle lens.
Answer:
a. Hypermetropia.
b. Power of spectacle P = + 0.5D
f = 1/P = +1/0.5 . m or f =+ 2 m positive sign means it is a convex lens.

Question: What is meant by persistence of vision?
Answer: On removing object seen, its impression persists on the retina for 16 1 th second. The inability of human brain to perceive more than two images formed on the retina within interval of 1/16 th of a second is called persistence of vision. In this case, both the images merge and give a sensation of continuity.

Question: Draw a neat diagram to show the refraction of a light ray through a glass prism, and label on it the angle of incidence and angle of deviation.
Answer: EF–Refracted ray ∠r –Angle of refraction
FS–Emergent ray ∠e–Angle of emergence
∠A–Angle of prism ∠D–Angle of deviation

Question: What is meant by least distance of distinct vision? How does this vary between the very young and old people?
Answer: Least distance of distinct vision is the minimum distance between eye and object to see it clearly.
 In young, the least distance of distinct vision is lesser and for old people it is more.

Question: Explain why do stars twinkle and planets do not?
Answer: The light coming from star get refracted through the atmospheric layers. Due to this, the star appear slightly higher than its actual position. Since the densities of atmospheric layers changes frequently and hence the position of the star appear changes continuously stars are very distant point-sized objects. As the path of rays of light coming from star is continuously changes.
So sometimes light enters into our eyes and some times it faints. This is twinkling of the star. Since planets are closer to us, their size is also big. Our eyes receive light from different points of the planets. Hence planets do not twinkle.

Question: Define dispersion of white light and name the colours of white light in order.
Answer: When white light is passed through a prism, it splits into its seven constituent colours. Splitting of white light into its constituent colours is called dispersion.Seven constituent colours are – violet, indigo, blue, green, yellow, orange and red. (VIBGYOR).

FIVE MARKS QUESTIONS

Question: (a) Write the function of each of the following parts of human eye:
(i) Cornea (ii) Iris
(iii) Crystalline lens (iv) Ciliary muscles
(b) Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an astronaut on the Moon? Give reason to justify your answer.
Answer: (a) Function of Human eye:
(i) Cornea focuses light ray to enter the eye.
(ii) Iris control and regulate amount of light entering the eye and control the size of pupil.
(iii) Crystalline lens is like a convex lens which converge light rays on the retina.
(iv) Ciliary muscles adjust focal length of eye lens by contraction and relaxation so that sharp image can be obtained on the retina.
(b) During morning and evening time, sun is at the horizon, sunlight has to travel longer distance in atmosphere. Violet and blue colours etc. of shorter wavelength scatter in the space only. The longer wavelength red colour reaches to the observer on earth and sun appears reddish.
No, because there is no atmosphere on moon and no scattering of light takes place there.

Question: (a) Write the function of each of the following parts of human eye:
(i) Cornea (ii) Iris
(iii) Crystalline lens (iv) Ciliary muscles
(b) Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an astronaut on the Moon? Give reason to justify your answer.
Answer:
(a) Function of Human eye:
(i) Cornea focuses light ray to enter the eye.
(ii) Iris control and regulate amount of light nentering the eye and control the size of pupil.
(iii) Crystalline lens is like a convex lens which converge light rays on the retina.
(iv) Ciliary muscles adjust focal length of eye lens by contraction and relaxation so that sharp image can be obtained on the retina.
(b) During morning and evening time, sun is at the horizon, sunlight has to travel longer distance in atmosphere. Violet and blue colours etc. of shorter wavelength scatter in the space only. The longer wavelength red colour reaches to the observer on earth and sun appears reddish.
No, because there is no atmosphere on moon and no scattering of light takes place there.

Question: (a) A student is unable to see clearly the words written on the black board placed at a distance of approximately 3 m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.
(b) Why do stars twinkle? Explain.
Answer: (a) He is suffering from myopia. Causes of myopia:
(i) elongation of eye ball.
(ii) excessive curvature of cornea.
Concave lens of suitable power is used to correct myopia.
(b) Light coming from stars when enter in the Earth’s atmosphere suffers refraction from the atmospheric layers. Since the densities of atmospheric gases changes frequently. Since the stars are point sized and at a far distance so sometimes star appear brighter and sometimes dimmer which gives the impression as the stars twinkling.

Question: With the help of scattering of light, explain the reason for the difference in colours of the Sun as it appears during sunrise/sunset and noon.
Answer:  At the time of sunrise or sunset it appears red where as at noon it appear white when it is overhead because at sunrise or sunset blue colour and other shorter wavelength light get scattered away while passing through the atmosphere. Scattering of longer wavelength is least which reach to the earth. Due to this sun appears red during morning and evening time.
When sun is overhead at noon, sun rays travels smaller distance, due to little scattering of blue and violet colour wavelength almost all wavelengths reach to earth and due to this sun appears white.

Question: (a) Explain the phenomenon of scattering of light. 
State the factor on which colour of scattered light depends.
(b) List any two natural phenomenon based on scattering of light.
Answer: (a) Scattering of light is the phenomenon due to which light gets deflected by the atoms or molecules of different atmospheric gases and suspended particles present in atmosphere. Scattering depends upon size of the particles of the medium through which light passes.
(b) Two natural phenomenon based on scattering are (i) Reddish colour of sky during sunrise and sunset, (ii) Blue colour of sky.

Question: What is meant by scattering of light? Use this phenomenon to explain why the clear sky appears blue or the sun appears reddish at sunrise.
Answer: a. Scattering of light is phenomenon by which beam
of light is spreaded in many direction when itinteracts with particle of matter. When sunlight strikes molecules in atmosphere, the light is redirected in many direction.
b. Scattering of blue colour is most due to shorter wavelength, where as scattering of red colour is least. All colours scattered in the sky and red colour light reaches to earth, due to this sun appears reddish.

Question: State one function each of iris, pupil, and cornea.
Answer:
a. Iris: It controls the amount of light entering the eye by changing the size of pupil.
b. Pupil: Pupil is a part of the eye through which light enters in the eye.
c. Cornea: It is a thin membrane which covers the eye ball. Light enters the eye through cornea where it is refracted most.

Question: (a) Calculate maximum power of accommodation of a person having normal vision.
(b) A person needs to use glasses for reading newspaper. Identify the defect in her vision and the type of lens she would need to correct it.
(c) Sometimes when we enter into a dark room from bright sunlight we are unable to see objects clearly. Why?
Answer: (a) Least distance of distinct vision for a normal eye
D = 25 cm.
p=1/f=100/25=4D

For a person having normal vision, the power of accommodation is 4 D.
(b) Hypermetropia, to correct hypermetropia convex lens is needed.
(c) In bright light the pupil contract its size becomes small to control the light, but when we enter into a dark room it take sometime for the pupil to expand to allow more light in dark room. 

Question: (a) Calculate maximum power of accommodation of a person having normal vision.
(b) A person needs to use glasses for reading newspaper. Identify the defect in her vision and the type of lens she would need to correct it.
(c) Sometimes when we enter into a dark room from bright sunlight we are unable to see objects clearly. Why?
Answer:
(a) Least distance of distinct vision for a normal eye
D = 25 cm.

p = 1/f = 100/25 = 4D

For a person having normal vision, the power of accommodation is 4 D.
(b) Hypermetropia, to correct hypermetropia convex lens is needed.

(c) In bright light the pupil contract its size becomes small to control the light, but when we enter into a dark room it take sometime for the pupil to expand to allow more light in dark room.

Question: State one function each of iris, pupil, and cornea.
Answer: a. Iris: It controls the amount of light entering the
eye by changing the size of pupil.
b. Pupil: Pupil is a part of the eye through which light enters in the eye.
c. Cornea: It is a thin membrane which covers the eye ball. Light enters the eye through cornea where it is refracted most.