Light Book Back Questions 8th Science Lesson 3
8th Science Lesson 3
3] Light
Book Back Questions with Answer and Do You Know Box Content
Do You Know:
Method of coating a glass plate with a thin layer of reflecting metals was in practice during the 16th century in Venice, Italy. They used an amalgam of tin and mercury for this purpose. Nowadays, a thin layer of molten aluminium or silver is used for coating glass plates that will then become mirrors.
Convex mirrors used in vehicles as rear-view mirrors are labels with the safety warning: ‘Objects in the mirror are closer than they appear’. This is because inside the mirrors, vehicles will appear to be coming at a long distance.
The principle behind the working of a parabolic mirror has been known since the Greco-Roman times. The first mention of these structures was found in the book, ‘On Burning Mirrors’, written by the mathematician Diocles. They were also studied in the 10th century, by a physicist called Ibn Sahl. The first parabolic mirrors were constructed by Heinrich Hertz, a German physicist, in the form of reflector antennae in the year 1888.
Problem 1: The radius of curvature of a spherical mirror is 20 cm. Find its focal length.
Solution: Radius of curvature = 20 cm
Focal length (f) = Radius of curvature/2
= R/2 = 20/2 = 10 cm.
Problem 2: Focal length of a spherical mirror is 7 cm. what is its radius of curvature?
Solution: Radius of curvature (R) = 2 x Focal length = 2 x 7 = 14 cm.
Silver metal is the best reflector of light. That is why a thin layer of silver is deposited on the side of materials like plane glass sheets, to make mirrors.
Problem 3: If two plane mirrors are inclined to each other at an angle of 90o, find the number of images formed.
Solution: Angle of inclination = 90o,
Number of images formed = (360o/ɵ) -1 = (360o/90o) – 1 = 4 – 1 = 3.
Problem 4: Speed of light in air is 3 x 108 ms-1 and the speed of light in a medium is 2 x 108 ms-1. Find the refractive index of the medium with respect to air.
Solution: Refractive index (µ) = Speed of light in air (c) / Speed of light in the medium (v)
µ = (3 x 108) / (2 x 108) = 1.5.
Problem 5: Refractive index of water is 4/3 and the refractive index of glass is 3/2. Find the refractive index of glass with respect to the refractive index of water.
Solution: w µ g Refractive index of glass/Refractive index of water = (3/2) / (4/3) = 9/8 = 1.125.
The information if rainbow is an example of dispersion if white light. This can be seen on the opposite side of the Sun. after rainfall, large number of droplets still remain suspended in the air. When white light passes through them, it is split into seven colours. Dispersion of white light from a large number of droplets eventually forms a rainbow.
Choose the best answers:
1. Which of the following has curved reflecting surface?
(a) Plane mirrors
(b) Spherical mirrors
(c) Simple mirrors
(d) None of the above
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
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 these
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) Principle axis
(d) Radius curvature
5. The distance from the pole to the focus is called
(a) Pole length
(b) Focal length
(c) Principle axis
(d) None of the above
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
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
Fill in the blanks:
1. The spherical mirror used in a beauty parlour as make-up mirror is _______________.
2. Geometric centre of the spherical mirror is _______________.
3. Nature of the images formed by a convex mirror is ____________.
4. The mirror used by the ophthalmologist to examine the eye is _____________.
5. If the angle of incidence is 45o, then the angle of refection is ____________.
6. If an object is placed between two mirrors which are parallel to each other, the number of images formed is ___________.
Match the following:
1. Convex mirror – Radio telescopes
2. Parobolic mirror – Rear – view mirror
3. Snell’s law – Kaleidoscope
4. Dispersion of light – sin i/sin r = µ
5. Multiple reflection – Rainbow
Answers:
Choose the best answers:
1. Spherical mirrors 2. Concave mirror 3. Convex mirror 4. Principal axis 5. Focal length 6. at C 7. 20 Cm
Fill in the blanks:
1. Concave mirror 2. Pole 3. Smaller, Virtual & erect 4. Concave mirror 5. 45o 6. Infinite
Match the following:
1.mirror – Rear – view mirror
2.Convex Parobolic mirror – Radio telescopes
3.Snell’s law – sin i /sin r =μ
4.Dispersion of light – Rainbow
5.Multiple reflection – Kaleidoscope