8th Science Guide Unit 3 Light

8th Standard Science Guide Lesson 3 Light | English Medium

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8th Science Guide Unit 3 Light

I. Choose the best answer

1. Mirrors having a curved reflecting surface are called as

1. plane mirrors
2. spherical mirrors
3. simple mirrors
4. None of the above

Ans: spherical mirrors

2. The spherical mirror with a reflecting surface curved inward is called

1. convex mirror
2. concave mirror
3. curved mirror
4. None of the above

Ans: convex mirror

3. The centre of a sphere of which the reflecting surface of a spherical mirror is a part is called

1. pole
2. centre of curvature
3. cradius of curvature
4. aperture

Ans: centre of curvature

4. The spherical mirror used as a rear view mirror in the vehicle is

1. concave mirror
2. convex mirror
3. plane mirror
4. None of the above

Ans: convex mirror

5. The imaginary line passing through the centre of curvature and pole of a spherical mirror is called

1. centre of curvature
2. pole
3. principal axis
4. radius curvature

Ans:  radius curvature

6. The distance from the pole to the focus is called

1. Pole length
2. focal length
3. principal axis
4. None of the above

Ans : focal length

7. Focal length is equal to half of the

1. centre of curvature
2. axis
3. radius of curvature
4. None of the above

Ans : radius of curvature

8. If the focal length of a spherical mirror is 10 cm, what is the value of its radius of curvature?

1. 10 cm
2. 5 cm
3. 20 cm
4. 15 cm

Ans : 20 cm

9. If the image and object distance is same, then the object is placed at

1. infinity
2. at F
3. between f and P
4. at C

Ans : at C

10. The refractive index of water is

1. 0
2. 33
3. 44
4. 52

Ans : 1.0

II. Fill in the blanks.

1. The spherical mirror used in a beauty parlour as make-up mirror is _______. Ans : Concave mirror

2. Geometric centre of the spherical mirror is ________. Ans : pole
3. Nature of the images formed by a convex mirror is _________.,Ans : smaller, virtual & erect
4. The mirror used by the ophthalmologist to examine the eye is _______. Ans : concave mirror
5. It the angle of incidence is 45°, then the angle of reflection is _____. Ans : 45°
6. Two mirrors are parallel to each other, then the number of images formed is _______. Ans : infinite

III. Match the following:

1. Convex mirror – Radio telescopes
2. Parobolic mirror – wall
3. Regular reflection – rear – view mirror
4. Irregular reflection – Plane mirror

Ans: 1 – b, 2 – d, 3 – c, 4 – a

1. Snell’s law – Kaleidoscopce
2. Dispersion of light – sin i/sin r =μ
3. Refractive index – Rainbow
4. Multiple reflection – c/v = μ

Ans: 1 – b, 2 – c, 3 – d, 4 – a

VI. Answer in brief

1. What is called a spherical mirror?

• Spherical mirrors are one form of curved mirrors.
• If the curved mirror is a part of a sphere, then it is called a ‘spherical mirror’.

2. Define focal length?

• The distance between the pole and the principal focus is called focal length (f) of a spherical mirror.

focal length = Radius of curvature / 2

3. The radius of curvature of a spherical mirror is 25 cm. Find its focal length.

radius of curvature, R	= 25 cm
focal length f	= R / 2
f	= 25 / 2 = 12.5 cm
focal length f	= 12.5 cm

4. Give two applications of a concave and convex mirror.

Concave mirrors:

• Concave mirrors are used while applying make-up or shaving, as they provide a magnified image.
• They are used in torches, search lights and head lights as they direct the light to along distance.

Convex mirrors :

• Convex mirrors are used in vehicles as rear view mirrors because they give an upright image and provide a wider field of view as they are curved outwards.
• They are found in the hallways of various buildings including hospitals, hotels, schools and stores.
• They are usually mounted on a wall or ceiling where hallways make sharp turns.

5. State the laws of reflection.

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

6. If two plane mirrors are inclined to each other at an angle of 45°, find the number of Images formed.

Angle of inclination	= 45°
No of images formed	= 360° / θ -1
	= 360° / 45° – 1
	= 4 – 1 = 3
No of images formed	= 3

7. Define the refractive index of a medium.

• The amount of refraction of light in a medium is denoted by a term known as refractive index of the medium, which is the ratio of the speed of light in the air to the speed of light in that particular medium.

8. State the Snell’s law of refraction.

Refraction of light rays, as they travel from one medium to another medium, obeys two laws, which are known as Snell’s laws of refraction. They are:

• The incident ray, the refracted ray and the normal at the point of intersection, all lie in the same plane.
• The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is equal to the refractive index of the medium, which is a constant.

sin / sin r = µ

 

V. Answer in detail:

1. Explain the images formed by a concave mirror?

Concave mirrors forms a real image and it can be caught on a screen unlike convex mirrors, concave mirrors show different image types.

Depending on the position of the object in front of the mirror, the position, size and nature of image will vary.

This table provides a summary of image formed by a concave mirror

Position of the object	Position of the image	Image size	Nature of the image
At infinity	At F	Highly diminished point sized	Virtual and erect
Between infinity and the pole(p)	Between P and F	Diminished	Virtual and erect
At infinity	At F	Highly diminished	Real and inverted
Beyond C	Beyond C and f	Diminished	Real and inverted
At C	At C	Same size as the object	Real and inverted
Between C and F	Beyond C	Magnified	Real and inverted
At F	At infinity	Highly magnified	Real and inverted
Between F and P	Behind the mirror	Magnified	Virtual and erect

2. What is reflection? Write short notes on regular and irregular reflection?

• A ray of light, falling on a body having a shiny polished and smooth surface alone is bounced back. This bouncing back of the light rays as they fall on the smooth, shiny and polished surface is called reflection.

Regular reflection:

• When a beam of light (collection of parallel rays) falls on a smooth surface, it gets reflected.
• After reflection, the reflected rays will be parallel to each other. Here, the angle of incidence and the angle of reflection of each ray will be equal.
• Hence, the law of reflection is obeyed in this case and thus a clear image is formed. This reflection is called ‘regular reflection’ or ‘specular reflection’.

Irregular reflection:

• In the case of a body having a rough or irregular surface, each region of the surface is inclined at different angles.
• When light falls on such a surface, the light rays are reflected at different angles.
• In this case, the angle of incidence and the angle of reflection of each ray are not equal.
• Hence, the law of reflection is not obeyed in this case and thus the image is not clear. Such a reflection is called ‘irregular reflection’ or ‘diffused reflection’.

3. Explain the working of a periscope.

• It is an instrument used for viewing bodies or ships, which are over and around another body or a submarine.
• It is based on the principle of the law of reflection of light.
• It consists of a long outer case and inside this case mirrors or prisms are kept at each end, inclined at an angle of 45°.
• Light coming from the distant body, falls on the mirror at the top end of the periscope and gets reflected vertically downward.
• This light is reflected again by the second mirror kept at the bottom, so as to travel horizontally and reach the eye of the observer.
• In some complex periscopes, optic fibre is used instead of mirrors for obtaining a higher resolution.
• The distance between the mirrors also varies depending on the purpose of using

4. What is dispersion? Explain in detail.

• Splitting of white light into its seven constituent colours (wavelength), on passing through a transparent medium is known as dispersion of light.
• Dispersion occurs because, light of different colours present in white light have different wavelength and they travel at different speeds in a medium.
• Refraction of a light ray in a medium depends on its speed.

5. Speed of light in air is 3 x 108 ms-1 and the refractive index of a medium is 1.5. Find the speed of light in the medium.

Given :

Speed of light in air c	= 3 x 108 ms-1
Refractive index of a medium µ	= 1.5
Speed of light in medium v	= ?

Solution:

µ	= cv
1.5	= 3 × 108 / v
v	3 × 108 / 1.5
v	= 2 x 108 ms-1