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VIII-EM-LIGHT
LIGHT
TEXTBOOK EXERCISE
I. CHOOSE THE BEST ANSWER.
1. Which of the following has a curved reflecting surface?
a) plane mirrors b) spherical mirrors
c) simple mirrors d) None of the above
Ans : b) spherical mirror
2. The spherical mirror with a reflecting surface curved inward is called
a) convex mirror b) concave mirror
c) curved mirror d) None of the above
Ans :a) convex mirror
3. The spherical mirror used as a rear view mirror in the vehicle is
a) concave mirror b) convex mirror
c) plane mirror d) None of the above
Ans :b) convex mirror
4. The imaginary line passing through the centre of curvature and pole of a spherical mirror is called
a) centre of curvature b) pole
c) principal axis d) radius curvature
Ans :d) radius curvature
5. The distance from the pole to the focus is called
a) pole length b) focal length
c) principal axis d) None of the above
Ans :b) focal length
6. If the image and object distance is same, then the object is placed at
a) infinity b) at F
c) between f and P d) at C
Ans :d) at C
7. If the focal length of a spherical mirror is 10 cm, what is the value of its radius of curvature?
a) 10 cm b) 5 cm
c) 20 cm d) 15 cm
Ans :c) 20 cm
II. FILL IN THE BLANKS.
1. The spherical mirror used in a beauty parlour as a 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 :virtual and errect
4. The mirror used by the ophthalmologist to examine the eye is ___________.
Ans :concave mirror
5. If the angle of incidence is 45°, then the angle of reflection is ________.
Ans :45°
6. If an object is placed between two mirrors which are parallel to each other, the number of images formed is __________.
Ans :infinite
III. MATCH THE FOLLOWING.
Ans :
IV. ANSWER BRIEFLY.
1. Define focal length.
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
2. Give any two applications of a concave and convex mirror.
Ans :
Concave mirrors:
a. Concave mirrors are used while applying make-up or shaving, as they provide a magnified image.
b. They are used in torches, search lights and headlights as they direct the light to a long distance.
Convex mirrors :
a. Convex mirrors are found in the hallways of various buildings including hospitals, hotels, schools and stores.
b. They are also used on roads where there are sharp curves and turns.
3. State the laws of reflection.
Ans :
The incident ray, the reflected ray and the normal at the point of incidence, all lie in the same plane.
The angle of incidence (i) and the angle of reflection (r) are always equal.
4. Define the refractive index of a medium.
Ans :
The refractive index of the medium is defined as the ratio of the speed of light in the air to the speed of light in that particular medium.
It is denoted by the Greek letter 'µ'
Speed of light in air (c)
μ = ----------------------------------------------
Speed of light in the medium (v)
5. State Snell’s law of refraction
Ans :
a) The incident ray, the refracted ray and the normal at the point of intersection, all lie in the same plane.
b) 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 i
------- = µ
sin r
V. ANSWER IN DETAIL.
1. Explain the images formed by a concave mirror.
2. What is reflection? Write a short note on regular and irregular reflection.
All bodies can not reflect light rays. When a beam of light falls on a smooth surface, it gets reflected.The amount of reflection of light depends on the nature of the reflecting surface of the body.
Based on the nature of the surface, reflection can be classified into two types namely,
a)regular reflection and
b)irregular 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. This reflection is called ‘regular reflection’ or ‘specular reflection’.
Example: Reflection of light from the surface of still water.
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. Such a reflection is called ‘irregular reflection’ or ‘diffused reflection’.
Example: Reflection of light from a wall.
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 varies depending on the purpose.
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.
It is because light of different colours present in white light have different wavelengths and they travel at different speeds in a medium. You know that refraction of a light ray in a medium depends on its speed. As each coloured light has a different speed, the constituent coloured lights are refracted at different extents, inside the prism. Moreover, refraction of a light ray is inversely proportional to its wavelength. Thus, the red coloured light,which has a large wavelength, is deviated less while the violet coloured light, which has a short wavelength, is deviated more.
VI. NUMERICAL PROBLEMS.
1. The radius of curvature of a spherical mirror is 25 cm. Find its focal length.
Focal length = Radius of curvature / 2
= R/2
= 25/2 = 12.5cm
Focal length =12.5cm
2. If two plane mirrors are inclined to each other at an angle of 45°, find the number of images formed.
Angle of inclination = 45°
360°
Number of images formed = ------ - 1
𝛉
360
=-------- -1
45
= 8 - 1 = 7
Number of images formed = 7 images
3. Speed of light in air is 3 × 108 m s–1 and the refractive index of a medium is 1.5. Find the speed of light in the medium.
According to Snell's Law:
sin i n2 v1
------ = ---- = ------
sin r n1 v2
Refractive Index for air is ≈1
Substituting
1.5 3 × 108
----- = ----------
1 v2
3 × 108
Therefore v2 = ---------- = 2 × 108 m/s
1.5
Speed of light in the medium = 2 × 108 m/s
