Lakhmir Singh & Manjit Kaur: Light- 2 | Lakhmir Singh & Manjit Kaur Solutions: Class 8 Science PDF Download

Q.36. Write the names of the main parts of the human eye.

The main parts of the human eye are-

• IRIS: The muscles of the iris dilate and constrict the pupil and thus increases and reduces the amount of light reaching the retina.
• RETINA: Retina receives the light focused by the lens, the photoreceptors In the retina (rods and cones) convert this light into signals and the signals are then passed onto the brain for viewing.
• LENS: Lens focuses the light rays passing through it onto the retina and thus creates a clear image of the objects in sight.
• PUPIL: Pupil controls the amount of light that enters the eye. It is controlled by the iris.
• CORNEA: Cornea acts as the outermost layer of the eye. It protects the eye with the eyelids.
• CILIARY MUSCLES: The ciliary muscle fibers affect the zonular fibers in eye and thus change the lens shape and thus changes the converging power.
• OPTIC NERVE: The optic nerve transmits signals from photoreceptors in the retina to the brain through electric impulses

Q.37. What happens to the size of the pupil of our eye in dim light? 

In dim light the size of the pupil of our eye becomes larger or dilates to increase the amount of light going inside the eye. This dilation in size is facilitated by the muscles of iris.

Q.38. What happens to the size of the pupil of our eye in bright light? 

In bright light the size of the pupil of our eye constricts and becomes small to reduce the amount of light going inside the eye. This reduction in size is facilitated by the muscles of iris.

Q.39. State whether the following statements are true or false:
(a) The moon is an illuminated object.
(b) Diffuse reflection means the failure of the laws of reflection of light.
(c) In a kaleidoscope, a pattern seen once can never be seen again.

(a) True. Moon is a non-luminous object. It does not have light of its own, it reflects the light of sun and it is an illuminated object.
(b) False. Diffuse reflection does not mean the failure of laws of reflection of light. Diffused reflection is caused by the irregularity of a surface.
(c) True. The pattern once seen through the eye hole can never be seen again in a kaleidoscope. The mirrors in the tube and the colored glass pieces continuously make different patterns.

Q.40. Fill in the following blanks with suitable words:
(a) The angle of _________ equals the angle of reflection.
(b) A person 1 m in front of a plane mirror seems to be _________ m away from his image.
(c) If you touch your _________ ear with right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with _________
(d) The size of pupil becomes _________ when you see in dim light.
(e) Night birds have _________ cones than rods in their eyes.
(f) The image of an object persists on the retina of an eye for about _________ second even after the object has disappeared.
(g) If the still pictures of a moving object are flashed on our eyes at a rate faster than _________ pictures per second, the eye perceives the object as moving.
(h) In a movie, the still pictures in proper sequence are projected on the screen usually at the rate of  _________ pictures per second.

(a) Incidence. This is by law of reflection. The angle of incidence equals the angle of reflection.
(b) 2m. Image formed by a plane mirror is far behind the mirror as the object is in front of the mirror.
(c) Left ear; left hand. Images formed by a plane mirror are opposite in direction.
(d) Large. The size of pupil dilates, facilitated by the iris, so that maximum amount of possible light enters the eye and a person views things clearly.
(e) Fewer. Night birds have fewer cones and more rods because rods function in less amount of light. Cones function in bright light.
(f) 1/16th. The phenomenon is known as Persistence of Vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16th of a second even after the object has disappeared.
(g) 16. The is due to the phenomenon Persistence of Vision. The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16th of a second, hence if 16 frames are flashed in front of the eyes, they are viewed by the retina as an ongoing action.
(h) 24. The frame rate for motion pictures is 24 pictures per second. Frame rate is the rate at which consecutive images are displayed in a display.

Q.41. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain. 

The objects in a dark room cannot be seen because there is no light. We can only see something when a certain amount of light falls on it. The objects outside the room can only be seen if there is light outside the room.

Q.42. What makes things visible to us? Why cannot we see a book which is placed (a) behind a wooden screen, and (b) in a dark room?

Light makes things visible to us. We see an object when light passing through it is reflected. We cannot see a book which is placed
(a) Behind a wooden screen because the wooden screen is opaque so it does not allow light to pass through it.
(b) In a dark room because there is no light to be reflected by the book. Thus, due to no reflection, the book is not visible.

Q.43. We can see the sun because it is glowing. How are we able to see the moon? 

We are able to see the moon because the light of sun falls on moon and is reflected onto the earth. Hence, we are able to see the moon.

Q.44. Name the two types of reflection of light. Which type of reflection makes us see an object from all directions? 

The two types of reflection of light are Regular reflection of light and Diffused reflection of light. Regular reflection of light makes us see an object from all directions as the reflected rays are all parallel and in a particular direction so the image of the object can be viewed from all directions. In diffused reflection of light the reflected rays are neither parallel nor in a specific direction so the image of the object isn’t visible from any direction.

Q.45. A wall reflects light and a mirror also reflects light. What difference is there in the way they reflect light? 

Reflection of light from a wall is diffused reflection because the surface of the wall is uneven so the light is reflected in different directions and also the reflected rays are not parallel Reflection of light from a mirror is regular reflection because the surface of mirror is an even surface which is polished. The light gets reflected from the mirror surface in one particular direction and the reflected rays are all parallel.

Q.46. Explain why, a book lying on a table in a room can be seen from all the parts of the room. 

A book lying on a table in a room can be seen from all the parts of the room because of reflection by the light falling on the book. The light is reflected by the book and it reaches our eye at any part of the room.

Q.47. What is the full form of i and r? What is the relation between them? 

i = angle of incidence and r = angle of reflection
By law of reflection, the angle of incidence is equal to the angle of reflection.

Q.48. You see your image in a plane mirror? State two characteristics of the image so formed.

Characteristics of the image formed by a plane mirror are The images formed by a plane mirror are virtual and erect Image formed is of the same size as the object and is laterally inverted. mage is far behind the mirror as the object is in front of the mirror.

Q.49. What is a periscope? How many mirrors are there in a periscope? 

A periscope is a device that reflects the reflected ray again. It consists of a tube attached to a set of mirrors. There are 2 mirrors in a periscope. The light from the object falls on one mirror that is placed at 45o to the object and the light gets reflected. This reflected light in turn falls on another mirror and is again reflected until it reaches the eyepiece.

Q.50. State the various uses of a periscope.

• A periscope is used to see over, through or around any object.
• It can be used to see over a wall, to see around a corner.
• Periscopes are also used in warfare to keep an eye on the enemy.

Q.51. Explain how, a hair dresser makes you see hair at the back of your head after the hair cut is complete. 

The phenomenon that applies here is multiple reflection that provides the image of back of the head to be viewed. A person is made to stand in front of a mirror and another mirror is held at the back of the head. The reflection from the mirror at the back is visible in the mirror in front of the person and hence the hair at the back of the head is visible to the person in the front mirror.

Q.52. How many images of an object will be formed when the object is placed between two plane mirrors which are inclined at the following angles to one another?
(a) 120°
(b) 45°
(c) 180°
(d) 60°
(e) 90°

The formula to calculate the no. of images of an object placed between 2 plane mirrors – (360/θ) − 1; where theta is the angle of inclination.
(a) 2 (360/120) – 1 = 3 – 1 = 2
(b) 7 (360/45) – 1 = 8 – 1 = 7
(c) 1 (360/180) – 1 = 2 – 1 = 1
(d) 5 (360/60) – 1 = 6 – 1 = 5
(e) 3 (360/90) – 1 = 4 – 1 = 3

Q.53. Two plane mirrors are set at right angles to each other. A coin is placed in-between these two plane mirrors. How many images of the coin will be seen?

When 2 plane mirrors are set at right angles to each other and a coin is placed in-between these two plane mirrors, then three images will be formed, see the diagram below:

The formula for calculating the number of images, when two are kept at an angle θ, is given as: (360/θ) − 1

Now,θ = 90 degree.

Thus, the number of images formed will be

= (360/θ) − 1

= (360/90) – 1

= 4 – 1

= 3

Q.54. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm? 

Infinite number of images of the candle will be formed because the mirrors are placed parallel to each other.

Q.55. Explain why, when an object is placed between two plane mirrors inclined at an angle, then multiple images are formed.

Multiple images are formed due to multiple reflection when an object is placed between two plane mirrors at any angle.
N= (360/ θ) – 1
Say, the angle is 120
N = (360/120) – 1
N = 3 – 1
N = 2

Q.56. How can you show that white light (say, sunlight) consists of seven colors?

White light (say, sunlight) consists of seven colors can be shown by passing this light through a prism. The sunlight passes through the prism and splits into a band of 7 colors, also known as the spectrum.

Q.57. What information do you get about sunlight from the formation of a rainbow? 

The information that we get about sunlight from the formation is about dispersion of light. The separation of white light into seven different colors is known as dispersion.

Q.58. What is meant by dispersion of light’? Name a natural phenomenon which is caused by the dispersion of sunlight in the sky. 

Dispersion of light is the separation of white light or visible light into different colors is known as dispersion. Formation of rainbow is a natural phenomenon which is caused by the dispersion of sunlight in the sky. The sunlight splits into 7 colors, namely, violet, indigo, blue, green, yellow, orange and red.

Q.59. How many plane mirror strips are there in a kaleidoscope? How are they arranged? 

Kaleidoscope consists of 3 plain mirror strips which are arranged inside a tube (cylinder) with colored glass pieces at one end and a glass to view on the other.

Q.60. How does eye adjust itself to deal with light of varying intensity? 

The iris adjusts the size of pupil according to the light of varying intensity. When the amount of light is very high, the iris contracts the pupil and reduces the amount of light entering the eye. Whereas, when the amount of light is less, the iris expands the pupil so that more light can enter the eye and things can be viewed clearly.

Q.61. Explain why, we cannot see our surroundings clearly when we enter a darkened cinema hall from bright sunshine but our vision improves after some time.

In bright sunshine the size of the pupil of the eye is very small so when we enter a darkened cinema hall from bright sunshine very little amount of light enters the eye. The vision improves at some time later because the pupil dilates and more amount of light enters the eye easily.

Q.62. How does the eye-lens differ from the ordinary convex lens made of glass? 

The eye lens is made up of living cells and the focal length of eye lens is adjustable. Whereas, an ordinary convex lens is made of glass and its focal length cannot be changed as it is fixed.

Q.63. Name the part of the eye:
(a) which controls the amount of light entering the eye.
(b) which converges light rays to form the image.
(c) on which image is formed.
(d) which carries the image to brain.
(e) which changes the curvature (or thickness) of eye-lens to focus objects lying at various distances.

(a) Iris controls the amount of light entering the eye by constricting and dilating the pupil in bright and dim light respectively.
(b) Lens. The lens may diverge or converge the light rays to form an image.
(c) Retina. An upside-down image is formed on the retina.
(d) Optic nerve carries the image to brain in form of electric impulses.
(e) Ciliary muscle changes the lens shape to focus objects lying at various distances.

Q.64. Name the cells on the retina of an eye:
(a) which are sensitive to bright light.
(b) which are sensitive to dim light.
(c) which produce sensation of colour.

(a) Cones are sensitive to bright light. Cone shaped cells present in the retina are sensitive to bright light.
(b) Rods are sensitive to dim light. Rod shaped cells present in the retina are sensitive to dim light. They are most important for vision in dim light
(c) Cones are the cells of retina that produce color sensation. These photoreceptor cells are responsible for color vision.

Q.65. What are rods and cones in the retina of an eye? 

Rods are rod shaped cells present in the retina are sensitive to dim light. They are most important for vision in dim light. Cones are cone shaped cells present in the retina are sensitive to bright light. These photoreceptor cells are responsible for color vision.

Q.66. Name any one defect of the eye. How is it corrected? 

Myopia (nearsightedness) is the defect of vision wherein a person is unable to see the distant objects clearly as the light is unable to focus on the retina and thus distant objects appear to be blurred. The problem of nearsightedness is that the light is focused in front of the retina
Myopia is corrected by wearing glasses or contact lenses. A concave lens(diverging lens) with a minus power is used in the glass, which moves the image back to retina and thus clears the image. The diverging lens will diverge the light before it reaches the retina. The light will then be converged by cornea and lens thus producing an image on the retina.

Q.67. What is cataract? How can the vision of a person having cataract be restored? 

Cataract is the clouding of the lens of human eye, that lies behind the iris and pupil. It leaves to a blurred vision or decrease in vision. The lens that has becomes cloudy is replaced by an artificial lens to restore the normal and clear vision.

Q.68. What is meant by ‘persistence of vision’? 

Persistence of Vision- The property of this phenomenon is that the image formed on the retina of the eye persists for about 1/16th of a second, hence if 16 frames are shown on a screen in succession, they are viewed by the retina as an ongoing action.

Q.69. Explain how you can take care of your eyes.

One can take care of eyes by following or practicing some simple steps-

• Watch television from a distance.
• If a dust particle or an insect gets into our eye, we should never rub the eye, instead wash the eye with cold water.
• Do not look at the sun directly as too much of light i.e. bright light may injure the retina.
• Read from a normal distance of vision.
• Never read or study in dim light or low light as it causes headaches.

Q.70. What should we do if something like a dust particle or an insect gets into our eye?

If something like a dust particle or an insect gets into our eye, we should never rub the eye, instead wash the eye with cold water. If washing with cold water does not help, we should consult a doctor immediately.