Light and Sound
4 marks · EasyA student uses a ray box and a rectangular glass block to investigate how light travels through glass and how sound travels through air.
Draw and label a simple ray diagram to show the path of a ray of light entering and leaving a rectangular glass block. Mark the normal, angle of incidence and angle of refraction.
The student claps blocks of wood at a known distance from a wall. The time between clapping and hearing the echo is measured as 2.0 s, and the distance to the wall is 170 m. Calculate the speed of sound in air.
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) Ray diagram clearly shows: air to glass to air, normal, angles marked as i (incidence) and r (refraction). · must show Correct arrows for incident, refracted, and emergent rays, Normal at interface, Angles labelled i and r
(b) Speed of sound = 170 m × 2 / 2.0 s = 170 m/s (wrong); correct: distance = 170×2 = 340 m; speed = 340/2 = 170 m/s · must show Correct calculation for distance travelled by sound (there and back), Correct formula application
Common mistakes: Forgetting to divide distance by 2 for echoes
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Marking points
D1 (a) · 1 mark · Direction of incident ray, refracted ray, emergent ray shown.
D2 (a) · 1 mark · Normal and angles (i, r) clearly labelled.
M1 (b) · 1 mark · Writes correct speed formula: speeds = distance / time, or double the distance to wall for total sound path.
A1 (b) · 1 mark · Substitutes and calculates correct value (speed = 170×2 / 2 = 170 m/s, ±5 m/s).
Worked solution
a) The incident ray approaches the block at an angle to the normal; as it enters, it bends towards the normal (refraction), then exits bending away. Normal is drawn at the entry surface. Angles i (incidence) and r (refraction) are marked between rays and normal.
b) Total distance travelled by sound: 170 m to wall, 170 m back, total 340 m. Time = 2.0 s. Speed = distance/time = 340/2.0 = 170 m/s.