Light and Sound
6 marks · NormalWhen a ray of light passes from air into glass, part of the light is refracted and part is reflected. Sounds can also be reflected by surfaces.
Complete the ray diagram below for a ray passing from air into a glass block and emerging on the other side. Draw the path through the block, marking the normals, angles of incidence and refraction.
A ship uses echo sounding (sonar) to measure ocean depth. If the sound pulse returns after 3.0 s and the speed of sound in seawater is 1500 m/s, calculate the depth of the ocean.
Compare the laws of reflection for light and sound. Give one difference in how each wave behaves when it meets a surface.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) — · must show diagram: incident ray, normal at entry, refracted ray, emergent ray, normals, labelled angles
(b) 2250 m · must show depth = (v × t)/2
(c) — · must show state 'angle of incidence equals angle of reflection' for both; give one physical difference (e.g., light reflects from smooth surfaces, sound can reflect from rough)
Must show: Diagram includes all correct rays and labels; use s = v × t for distance; comparison mentions angle laws and difference in absorption/diffusion
Common mistakes: Depth calulated without dividing by 2; angles missing on diagram
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correct ray path with entry, exit, normals and angles.
B1 (a) · 1 mark · Correctly labels angle of incidence and refraction.
M1 (b) · 1 mark · Correct formula: depth = (speed × time)/2. (follow-through)
A1 (b) · 1 mark · Correct calculation: (1500×3.0)/2 = 2250 m. (follow-through)
B1 (c) · 1 mark · States angle of incidence = angle of reflection for both sound and light.
B1 (c) · 1 mark · States a difference (e.g., light reflects from smooth surfaces, sound can reflect from rough).
Worked solution
a) Draw glass block with incident ray at angle to surface, normal at entry, refracted ray inside glass, and emergent ray exiting into air. Label angles.
c) Both light and sound obey the law: 'angle of incidence equals angle of reflection.' A difference: light needs a smooth surface for clear reflection (mirror); sound can reflect from rough surfaces (walls causing echoes).