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Physics Form 4 Answers

the gap will be diffracted • Radio waves with longer (c) sin 45 = 0.7071
0
–1
in the children’s area [1] wavelength will diffract r = sin 0.4877 = 29.2°
• The smaller amplitude around the mountain Q4 (a) The speed of light in glass block
of the diffracted waves more compared with is slower than the speed of
causes the sea to be radio waves with shorter light in water. This is because
calmer. [1] wavelength [1] the refractive index of glass is
10 (iii) • Example 2 [1] larger. Therefore, its density is
• There is a vacuum greater than that of water.
Section C between the Sun and the c air
8. (a) Refraction of waves is a Earth [1] (b) n = c medium
phenomenon that occurs when • The fact that ultraviolet c air
there is a change of direction rays reach the 1.33 = c water
of the propagation of waves Earth shows that c air
travelling from one medium to electromagnetic waves and 1.52 = c glass
another due to a change of travel through vacuum [1] c air
speed. [1] Max 7 c glass c water 1.33
(b) (i) Sound waves travel faster in c water = c air 1.52 = 0.88
air when the temperature of c glass
the air is higher. [1]
(ii) • At daytime, when it is hot, Chapter Checkpoint 6.2
the surface of the Earth 6 Light and Optics
heats up faster than the Q1 (a) The angle of incidence ray BC
air. [1] Checkpoint is larger than the critical angle.
• Hence, the hot surface of 6.1 Thus, total internal reflection
the Earth causes the layer Q1 i = 140° – 90° = 50° occurs.
of air near the surface to r = 125° – 90° = 35° (b) Light ray DE refract away
be warmer than the upper sin i from normal because it leaves
layer. [1] n = sin r from glass prism to air, from a
• This causes the sound sin 50° denser medium to a less dense
waves to be refracted = sin 35° medium.
away from the Earth. [1] = 1.34 1 1
• On a cool night, the Q2 n = sin c = sin 42° = 1.49
surface of the Earth cools Q2 D a = 12 – 10 = 2 cm
down faster than the air. D r D r 1.49 = sin P
[1] n = D a → 1.50 = 2 sin 40°
• Hence, the air near the D r = 1.5 × 2 = 3.0 cm sin P = 1.49 × sin 40°
surface of the Earth = 0.9578
becomes cooler than the \The thickness of glass block is P = 73.3°
upper layer. [1] 3.0 cm Q = 90° – 40° = 50°
• This causes the sound Q3 i sin i r sin r
waves to be refracted R = Q = 50°
towards the Earth. [1] 22 0.3746 15 0.2588 S = 90° – 50° = 40°
6 28 0.4695 19 0.3256 1.49 sin 40° = 1 sin T
–1
(c) (i) • Transverse waves [1] 33 0.5446 22 0.3746 T = sin (1.49 sin 40°)
• Travel with a speed of 35 0.5736 26 0.4384 T = 73.3°
3 × 10 m s in
8
–1
vacuum [1] 41 0.6561 27 0.4540 Q3
• Transfer energy from one
place to another sin i
(Other characteristics
acceptable) [1] 0.9
3
(ii) 1. Sound wave does not
travel in vacuum. [1] 0.7 • Periscope is used to see an
2. Sound wave is a object behind the observer.
longitudinal wave. [1] • Image formed is inverted and
2 0.5 same size as object.
(d) (i) • Example 3 [1]
• The microwaves 0.3 sin r Checkpoint 6.3
transmitted will be 0.2 0.3 0.4 0.5 0.6 Q1 (a) The focal point is the point on
reflected by the plane and (a) Set of reading for angle of the principal axis where the light
return to the receiver [1] incident = 35° rays parallel to the principal axis
(ii) • Example 1 [1] after passing through the lens
• The mountain is an (b) n = 0.454 will converge on it.
obstacle [1] 0.6561
= 1.45

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ANS FOC PHYSICF F4 1P.indd 261 29/01/2020 2:04 PM