Lakhmir Singh & Manjit Kaur: Light- 3 | Lakhmir Singh & Manjit Kaur Solutions: Class 8 Science PDF Download

Q.71. Name any five food items (including two fruits) which are rich in vitamin A. 

5 food items rich in vitamin A are- fish, sweet potatoes, carrots, banana, pink grapefruit, apricots.

Q.72. Explain why, too little or too much light, both are bad for eyes. 

In the presence of little light the iris has to dilate pupil to facilitate entry of more and more light to view things, which leads to strain on the eyes and causes headaches. In case of too much light, the retina may get injured and damaged thus impairing vision.

Q.73. Explain why, an owl can see well in the night (but not during the day) whereas an eagle can see well during day (but not at night). 

An owl can see well in the night but not during the day because owl is a night bird and night birds have fewer cones and more rods in the retina. Since rods function in less amount of light so an owl can see well in the night. To see well during the day, cones are required which are not present in adequate amount in an owl’s retina. An eagle can see well during the day but not during the night because eagle has more cones and lesser rods in the retina. Cones function in good amount of light so an eagle can see well during the day. To see well during the night, rods are required which are not present in adequate amount in a n eagle’s retina.

Q.74.
(A) What is ‘blind spot’ in the eye? 

Blind spot is the point in human eye where the image is not visible. This is because there are no photoreceptors, i.e., rods and cones present in the optical disk.

(B) What is night blindness? What causes night blindness?

Night blindness is the inability to see properly at night or in poor light. In scientific terms, it is called as “nyctalopia”.
The following factors cause night blindness:

• The deficiency of Vitamin A (retinol) in the diet of a person for a considerable time can lead to the disease called night blindness.
• The disorder in which the rod cells in the retina gradually lose their ability to respond to the light.
• It can also happen due to cataract, or clouding of the eye’s lens.
• Usher syndrome can also cause night blindness, which is a genetic condition that affects both hearing and vision.
• Nearsightedness or blurred vision can also cause night blindness.

Q.75. What is lateral inversion? Explain with the help of an example. 

The effect produced by a plane mirror in reversing the images from left to right and right to left while forming images is lateral inversion.
Example: The images are reversed by the plane mirror.

Q.76
(A) What is meant by a luminous object? Name two luminous objects. 

Objects that have their own light i.e., objects that emit light of their own are known as luminous objects. 2 luminous objects are- stars and sun. The sun and the stars have their own light. A light bulb is also luminous.

(B) What is meant by a non-luminous object? Name two non-luminous objects.

Non- luminous objects are those that are themselves not capable of producing their own light but can reflect light from another source. 2 non luminous objects are- wood, plastics etc. Moon also is incapable of producing its own light and it reflects sunlight and hence is non-luminous.

Q.77.
(A) What is the difference between regular reflection and diffuse reflection of light? Name one object which can produce regular reflection of light and another which produces diffuse reflection of light.

(B) Draw diagrams to show regular reflection of light and diffuse reflection of light.

(C) Which of the following will cause regular reflection of light and which diffuse reflection of light?
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard
(d) Mirror
(e) Paper
(f) Marble floor with water spread over it.

(a) polished wooden table- regular reflection since the surface is polished (plane)
(b) chalk powder-diffused reflection will occur because the surface of chalk powder is uneven
(c) cardboard- diffused reflection of light since cardboard has an uneven surface
(d) mirror- regular reflection since the surface of a mirror is plane
(e) paper- diffused reflection since the surface of paper is uneven
(f) marble floor with water spread over it- regular reflection as the floor with water will behave as a plane surface

Q.78.
(A) Draw a diagram to show the reflection of light from a plane mirror. Label the following on the diagram:
(a) Plane mirror 
(b) Incident ray 
(c) Reflected ray 
(d) Point of incidence 
(g) Angle of reflection 
(e) Normal 
(f) Angle of incidence

(B) State the laws of reflection of light.

The laws of reflection of light are

• The incident ray, the normal ray and the reflected ray all lie in the same plane, and
• The angle of incidence is always equal to angle of reflection.

Q.79.
(A) Draw a labelled diagram showing how a plane mirror forms an image of a point object placed in front of it.

(B) State the characteristics of the image formed in a plane mirror.

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.
• Image is laterally inverted.
• Image is far behind the mirror as the object is in front of the mirror.

Q.80.
(A) Draw a labelled diagram of the human eye. Label the following parts on this diagram: Cornea, Iris, Pupil, Ciliary muscles, Eye-lens, Retina, Optic nerve, Blind spot.

(B) What are the functions of the following parts of the eye?
(a) Iris
(b) Eye-lens
(c) Ciliary muscles
(d) Retina
(e) Optic nerve

Functions of parts- 
IRIS- The muscles of the iris dilate and constrict the pupil and thus increases and reduces the amount of light reaching the retina.
EYE-LENS- Lens focuses the light rays passing through it onto the retina and thus creates a clear image of the objects in sight.
CILIARY MUSCLES- The ciliary muscle fibers affect the zonular fibers in eye and thus change the lens shape and thus changes the converging power.
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.
OPTIC NERVE- The optic nerve transmits signals from photoreceptors in the retina to the brain through electric impulses.

Q.81. The angle of reflection is equal to the angle of incidence:
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never

Correct Answer is Option (a)
According to laws of reflection, angle of reflection is always equal to the angle of incidence. The law of reflection also states that the incident ray, reflected ray and the normal to a surface, all lie in the same plane.

Q.82. The image formed by a plane mirror is:
(a) Virtual, behind the mirror and enlarged.
(b) Virtual, behind the mirror and of the same size as the object.
(c) Real, at the surface of the mirror and enlarged.
(d) Real, behind the mirror and of the same size as the object.

Correct Answer is Option (b)
The images formed by a plane mirror are virtual, opposite in direction and are of the same size as the object.

Q.83. The least distance of distinct vision for a young adult with normal vision is about:
(a) 25 m
(b) 2.5 cm
(c) 25 cm
(d) 2.5 m

Correct Answer is Option (c)
The least distance of distinct vision means the minimum distance for an object to be viewed clearly. This minimum distance for a young adult with normal vision is 25cm.

Q.84. The angle between an incident ray and the plane mirror is 30°. The total angle between the incident ray and the reflected ray will be:
(a) 30°
(b) 60°
(c) 90°
(d) 120°

Correct Answer is Option (a)
The angle between the incident ray and the mirror is 30 degrees.

Angle of incidence = angle of reflection.

90 degrees is going directly into the mirror.

So, 90 – 30 = 60 degrees.

The total angle between the incident ray and the reflected ray will be 120 degree.

Q.85. The image of an object formed by a plane mirror is:
(a) Virtual
(b) Real
(c) Diminished
(d) Upside-down

Correct Answer is Option (a)
The image of an object formed by a plane mirror is virtual always suggesting that the light rays do not actually come from the image.

Q.86. Which of the following is a non-luminous object?
(a) Sun
(b) Star
(c) Moon
(d) Fire

Correct Answer is Option (c)
Non- luminous objects are those that are themselves not capable of producing their own light but can reflect light from another source. Moon reflects sunlight and hence is non-luminous.

Q.87. A device which works on the reflection of light from two plane mirrors arranged parallel to one another is:
(a) Electroscope
(b) Kaleidoscope
(c) Periscope
(d) Stethoscope

Correct Answer is Option (c)
Periscopes are used to see objects that are not in direct line of sight.

Q.88. The number of images formed of an object placed between two plane mirrors inclined at right angles to each other is:
(a) Two
(b) Five
(c) One
(d) Three

Correct Answer is Option (d)
The relationship between no. of images and angle of mirror is
Images = (360degrees/ angle between the mirrors) – 1
Images = (360 degree/90 degree) – 1
Images = 4 – 1
Images = 3

Q.89. As the angle between two plane mirrors is decreased gradually, the number of images of an object placed between them:
(a) Increases gradually
(b) Decreases gradually
(c) First increases then decreases
(d) First decreases then increases

Correct Answer is Option (a)
If the angle between 2 plane mirrors is decreased, the number of images of an object placed in between will increase gradually.
The relationship between no. of images and angle of mirror is
Images = (360degrees/ angle between the mirrors) – 1

Q.90. The deficiency of one of the following in the diet of a person for a considerable time can lead to a disease called night blindness. This one is:
(a) Vitamin B
(b) Vitamin D
(c) Vitamin A
(d) Vitamin C

Correct Answer is Option (c)
Vitamin A deficiencies occur from fat malabsorption, liver disorders and inadequate food intake.

Q.91. Which of the following is not a part of the human eye?
(a) Retina
(b) Auditory nerve
(c) Optic nerve
(d) Ciliary muscle

Correct Answer is Option (b)
Auditory nerve is not a part of the human eye, but a part of human ear. It transmits hearing information between the brain and cochlea.
See the diagram of human ear attached:

Q.92. How does the eye change in order to focus on near or distant objects?
(a) The lens moves in or out
(b) The retina moves in or out
(c) The lens becomes thicker or thinner
(d) The pupil becomes larger or smaller

Correct Answer is Option (c)
The shape of the lens is changed by ciliary muscles so that the image forms at a sharp focus at the retina. The lens becomes thicker to focus at near objects and becomes thinner to focus at distant objects.

Q.93. Which of the following changes occur when you walk out of bright sunshine into a poorly lit room?
(a) The pupil becomes larger
(b) The lens becomes thicker
(c) The ciliary muscle relaxes
(d) The pupil becomes smaller

Correct Answer is Option (a)
When a person walks out of bright sunshine into a poorly lit room, the pupil dilates and becomes larger to allow maximum light to enter the eye and thus helps in viewing things easily in a poorly lit room.

Q.94. An incident ray makes an angle of 65° with the surface of a plane mirror. The angle of reflection in this case will be:
(a) 65°
(b) 45°
(c) 25°
(d) 35°

Correct Answer is Option (d)
Angle of incidence is always equal to angle of refection. Hence, angle of reflection will be 35°.

Q.95. Which of the following produces diffuse reflection of light?
(a) Mirror on a dressing table
(b) Water surface of a pond
(c) Screen in a cinema hall
(d) Polished wooden table

Correct Answer is Option (b)
Others, the mirror, the screen and the polished wooden table will give regular reflection as they are even surfaces.

Q.96. The human eye forms the image of an object at its:
(a) Cornea
(b) Iris
(c) Pupil
(d) Retina

Correct Answer is Option (d)
The image of an object is formed at the retina of an eye which is the third and the innermost coat of an eye. Light rays are focused on the retina by cornea, lens and the pupil.

Q.97. The change in converging power of an eye-lens is caused by the action of:
(a) Iris
(b) Ciliary muscles
(c) Optic nerve
(d) Retina

Correct Answer is Option (b)
The ciliary muscle fibers affect the zonular fibers in eye and thus change the lens shape and thus changes the converging power.

Q.98. The size of the pupil of the eye is adjusted by:
(a) Cornea
(b) Ciliary muscles
(c) Optic nerve
(d) Iris

Correct Answer is Option (d)
The muscles of the iris dilate and constrict the pupil and thus increases and reduces the amount of light entering the eye.

Q.99. The defect of vision in which the eye-lens of a person gets progressively cloudy resulting in blurred vision is called:
(a) Myopia
(b) Night blindness
(c) Cataract
(d) Hypermetropia

Correct Answer is Option (c)
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.

Q.100. A person cannot see the distant objects clearly (though he can see the nearby objects clearly). He is suffering from the defect of vision called:
(a) hypermetropia
(b) myopia
(c) night blindness
(d) cataract

Correct Answer is Option (b)
Myopia is the defect of vision wherein a person cannot see the distant objects clearly as the light is unable to focus on the retina and thus distant objects appear to be blurred.

Q.101. A man stands 10 m in front of a large plane mirror. How far must he walk before he is 5 m away from his image?

The object distance is equal to image distance in a plane mirror.

For the distance of man from his image to be 5m

The distance of man from mirror + distance of image from the mirror = 5m

Since object distance = image distance

Therefore, 2*distance of man from the mirror = 5m

Distance of man from the mirror = 5/2 = 2.5m

The man is 10m from the mirror at the starting, so he will have to walk 10 – 2.5 = 7.5m towards the mirror for being 5m away from his image.

Q.102. A ray of light strikes a plane mirror XY at an angle of incidence of 65°, is reflected from this plane mirror and then strikes a second plane mirror YZ placed at right angles to the first mirror. What is the angle of reflection for the mirror YZ?

The mirrors XY and YZ are at right angles, i.e., perpendicular. When a ray of light strikes the XY mirror with 65 degree angle, the incident angle will be 90 – 65 = 25 degree on the YZ mirror. Since angle of incidence = angle of reflection, the angle of reflection for YZ mirror will be 25 degrees.

Q.103. The eye of a person exhibits a phenomenon X due to which it can see the image of an object for a short duration of Y even after the object has disappeared from his view. It is due to the phenomenon X that we are able to see moving Z on a television screen. What are X, Y and Z?

The phenomenon X is 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.

Short duration of Y is 1/16 seconds.

Z is pictures. Due to the phenomenon of persistence of vision, we are able to see moving pictures on a television or a cinema screen.

Q.104. Man A has a defect of vision due to which he cannot see the nearby objects clearly (though he can see the distant objects clearly). On the other hand, man B has a defect of vision due to which he cannot see the distant object clearly (though he can see the nearby objects clearly). The defect in man A can be corrected by using spectacles containing lenses C whereas the defect in man B can be corrected by using spectacles containing lenses D.
(a) Name the defect of vision in man (i) A, and (ii) B.
(b) What type of lenses are (i) C, and (ii) D?

(a) (i) Man A suffers from hypermetropia wherein the image of a nearby object is formed behind the retina so the person has a blurred vision while looking at nearby objects.
(ii) Man B suffers from myopia wherein Light from a distant object forms an image before it reaches the retina so the person can see the nearby objects clearly but not the distant objects.
(b) C are Convex lens. these lenses are placed in front of a hyper-metropic eye which moves the image forward and focuses directly onto the retina which clears the vision of a nearby object.
D are concave lenses. These lenses are placed in front of a myopic eye which moves the image back to the retina and clarifies the image of a distant object.

Q.105. A student makes a device P by using three long and narrow strips of plane mirrors inclined at 60° to one another which enables him to see beautiful patterns made by pieces of coloured glass bangles. On the other hand, another student makes a device Q by using two plane mirrors arranged parallel to each other which helps him to see a football match clearly even when some very tall persons are sitting in front of him in the ground. What are P and Q?

Device P is a Kaleidoscope which consists of 3 plain mirror strips are arranged inside a tube (cylinder) with colored glass pieces at one end and a glass to view on the other. The device Q is a Periscope. A periscope is a device that reflects the reflected ray again. It consists of a tube attached to a set of 2 mirrors. A periscope is used to see over, through or around any object. So, the student is able to see a football match clearly even when some very tall persons are sitting in front of him in the ground because the periscope helps him in seeing over, through and around the tall persons sitting in front of him in the ground.