Lakhmir Singh Physics Class 10 Solutions Page No:271
Question 33: A person walking in a dark corridor enters into a brightly lit room :
(a) State the effect on the pupil of the eye.
(b) How does this affect the amount of light entering the eye ?
Solution : (a) Pupil becomes smaller.
(b) The amount of light entering the eye is reduced.
Question 34: Ciliary muscles of human eye can contract or relax. How does it help in the normal functioning of the eye ?
Solution : Ciliary muscles get relaxed and the eye lens becomes thin when the eye is looking at a distant object, and these muscles contract and make the eye-lens thick when the eye is looking at a nearby object. Thus, ciliary muscles help in the normal functioning of the eye by changing the thickness of the eye-lens while focussing.
Question 35: Describe and explain, how a normal eye can see objects lying at various distances clearly.
Solution : To focus on distant objects, the ciliary muscles of the eye get fully relaxed and pull the suspensory ligaments attached to the eye-lens tightly. This, in turn, stretches the eye-lens and the eye-lens becomes thin. This thin eye-lens has large focal length and small converging power sufficient to converge the parallel rays of light coming from a distant object to form an image on the retina.
An eye focused on a distant object (at infinity).
To focus on nearby objects, the ciliary muscles of the eyes contract and make the suspensory ligaments loose. The ligaments then stop pulling the eye-lens. The eye-lens bulges under its own elasticity and becomes thick. This thick eye-lens has small focal length and large converging power which converges the diverging rays coming from the nearby object to form an image on the retina.
An eye focused on a nearby object
Question 36: There are two types of light-sensitive cells in the human eye :
(a) Where are they found ?
(b) What is each type called ?
(c) To what is each type of cell sensitive ?
Solution : (a) The two types of light-sensitive cells are found in the retina.
(b) They are called rods and cones.
(c) Rods are sensitive to dim light and cones are sensitive to bright light and colours.
Question 37: What are rods and cones in the retina of an eye ? Why is our night vision relatively poor compared to the night vision of an
Solution : Rods are the rod-shaped cells present in the retina of an eye which are sensitive to dim light.
Cones are the cone-shaped cells present in the retina of an eye which are sensitive to bright light.
Our night vision is relatively poor compared to the night vision of an owl due to the presence of relatively smaller number of rod cells in the retinas of our eyes.
Question 38: (a) How does the convex eye-lens differ from the ordinary convex lens made of glass ?
(b) List, in order, the parts of the eye through which light passes to reach the retina.
Solution : (a) The focal length of the convex eye-lens can be changed by the action of ciliary muscles, but the focal length of the ordinary convex lens made of glass is fixed.
(b) Cornea, pupil, eye-lens, retina.
Question 39: (a) What happens to the size of pupil of our eye
(b) Name the cells on the retina of an eye which are sensitive to
Question 40: (a) Draw a simple diagram of the human eye and label clearly the cornea, iris, pupil, ciliary muscles, eye-lens, retina, optic nerve and blind spot.
(b) Describe the working of the human eye with the help of the above diagram.
(c) How does the eye adjust itself to deal with light of varying intensity ?
(b) Working of the human eye
The light rays coming from the object kept in front of the eye enter the cornea, pass through the pupil and fall on the eye lens. The eyes lens is convex lens, so it converges the light rays and produces a real and inverted image of the object on the retina. The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision.
(c) The eye adjusts itself to deal with ight of vaying intensity with the help of the iris. The iris automatically adjusts the size of the pupil according to the intensity of light received by the eye. If the intensity of light is large, then iris contracts the pupil and reduces the amount of light entering the eye. And, if the intensity of light is small, then iris expands the pupil so that more light may enter the eyes.
Question 41: (a) Explain the functions of the following parts of the eye :
(a) cornea (b) iris (c) pupil (d) ciliary muscles (e) eye-lens(f) retina (g) optic nerve
(b) If you walk from a dark room into sunlight and back again into dark room, how would your pupils alter in size ? What makes this happen ?
(c) Explain why, we cannot see our seats first when we enter a darkened cinema hall from bright light but gradually they become visible.
Solution : (a) a. Cornea: It is the front part of the eye. The light coming from objects enters the eye through cornea.
b. Iris: It controls the amount of light entering the eye.
c. Pupil: It controls the illumination in the eye.
d. Ciliary muscles: The focal length of the eye-lens can be changed by changing its shape by the action of ciliary muscles.
e. Eye-lens: It focuses light on to the retina.
f. Retina: It is a delicate membrane having a large number of light sensitive cells called ‘rods’ and ‘cones’ which respond to the intensity of light and colour of objects respectively.
g. Optic nerve: It conveyes the image formed on the retina to the brain.
(b) If we walk from a dark room into sunlight, the pupil of the eye contracts. On again entering the dark room, the pupil of the eye expands.
(c) When we enter a darkened cinema hall from bright sunshine, at first we cannot see our seats clearly but gradually they become visible. This is because in bright sunshine the pupil of our eye is small and when we just enter the darkened room very little light enters our eye due to which we cannot see properly. After a while, when the pupil of our eye expands, more light enters our eye and we can see clearly.