ICSE-X-Physics
04: Refraction of Light at Plane Surfaces
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- #ICSE
Answers for Chapter 4 Refraction of Light at Plane Surfaces Class 10 Selina Physics
- #Section : I
- Qstn #1A Water pond appears to be 2.7 m deep. If the refractive index of water is 4/3, find the actual depth of the pond.Ans : 1 Apparent depth = 2.7 m
μw = 4/3
Real depth = apparent depth x μw
= 2.7 × 4/3 = 3.6 m
- Qstn #2A coin is places at the bottom of a beaker containing water (refractive index = 4/3) to a depth of 12 cm. By what height the coin appears to be raised when seen from vertically above?Ans : 2 Refractive index of the water, μw = 4/3
Real depth at which the coin is places = 12 cm
Shift in the image = ?
Shift = real depth × (1 - 1/μ)
Shift = 12 × (1 - 3/4)
R = 12/4 = 3cm
- Qstn #3A postage stamp kept below a rectangular glass block of refractive index 1.5 when viewed from vertically above it, appears to be raised by 7.0 mm. calculate the thickness of the glass block.Ans : 3 Refractive index of the glass block, μg = 1.5
Shift in the image = 7mm or 0.7 cm
Thickness of glass block or real depth = ?
Shift = Real depth × (1 - 1/μ)
0.7 = R × (1 - 1/15)
R = (0.7 × 1.5)/0.5 = 2.1 cm
Exercise 4 D
- #Section : ii
- Qstn #1Explain the term critical angle with the aid of a labelled diagram.Ans : 1 Critical angle: The angle of incidence in the denser medium corresponding to which the angle of refraction in the rarer medium is 90° is called the critical angle.
- Qstn #2How is the critical angle related to the refractive index of a medium?Ans : 2 The critical angle is related to the refractive index of a medium by the relation
μ = 1/sinic = cosecic
- #3-aglass-air surfaceAns : The critical angle for glass-air surface is
For glass, refractive index aμg = 3/2
sin ic = 1/aμg = 2/3 or ic = 42°
- #3-bwater-air surface.Ans : The critical angle for water-air surface is
For water, refractive index aμw = 4/3
∴ sin ic = 1/aμw= ¾ or ic = 49°
- Qstn #4What is meant by the statement the critical angle for diamond is 24°?Ans : 4 The critical angle for diamond is 24°. This implies that at an incident angle of 24° within the diamond the angle of refraction in the air will be 90°. And if incident angle will be more than this angle then the ray will suffer total internal reflection without any refraction.
- Qstn #5A light ray is incident from a denser medium on the boundary separating it from a rarer medium at an angle of incident equal to the critical angle, what is the angle of refraction for the ray?Ans : 5 When a ray is incident from a denser medium to a rarer medium at angle equal to critical angle (i=ic), the angle of refraction becomes 90°
- Qstn #6Name two factors which affect the critical angle for a given pair of media. State how do the factors affect it.Ans : 6 The factors which affect the critical angle are:
(i) The colour (or wavelength) of light, and
(ii) The temperature
(i) Effect of colour of light: The critical angle for a pair of media is less for the violet light and more for the red light. Thus critical angle increases with increase in wavelength of light.
(ii) Effect of temperature: The critical angle increases with increase in temperature because on increasing the temperature of medium, its refractive index decreases.
- Qstn #7The critical angle for glass-air is 45° for the light of yellow colour. State whether it will be less than, equal to, or more than 45° for (i) ref light, (ii) blue light?Ans : 7 As the wavelength decreases (or increases) refractive index becomes more (or less) and critical angle becomes less (or more).
(i) For red light the critical angle will be more than 45Ëš and
(ii) For blue light the critical angle will be less than 45Ëš.