Light and Sound
6 marks · NormalA student investigates the path of a ray of light using a rectangular glass block and an oscilloscope.
Draw a ray diagram to show the path of a ray of light entering a rectangular glass block from air at an angle. Show the angles of incidence and refraction.
The angle of incidence is measured as 45° and the angle of refraction is 28°. Calculate the refractive index of the glass.
Describe how the speed of sound in air can be measured in a school lab, using the reflection of sound. Name the piece of equipment you could use to display a sound wave.
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.
Show answer
(a) — · must show incident ray, normal at entry, refracted ray inside block, emergent ray, exit normal, labelled i and r
(b) n \approx 1.47 · must show sin 45 / sin 28
(c) — · must show describe echo or clap method, distance-time, oscilloscope mentioned
Must show: Ray diagram must show entry and exit, normals and correct angles, Calculation shows use of n = sin i / sin r, Description includes echo method, measurement steps, and mentions oscilloscope
Common mistakes: Swapping i and r; using cos instead of sin; incomplete practical description
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly drawn ray diagram showing incident, refracted, and emergent rays with normals.
B1 (a) · 1 mark · Angles of incidence and refraction labelled correctly.
M1 (b) · 1 mark · Correct substitution into n = sin i/sin r. (follow-through)
A1 (b) · 1 mark · Accurate answer to 2 decimal places (1.47). (follow-through)
B1 (c) · 1 mark · Correctly describes echo (reflection) or distance-time method for measuring speed of sound.
B1 (c) · 1 mark · Oscilloscope correctly identified as equipment to display sound wave.
Worked solution
a) Draw the block, incoming ray at angle i to the normal at the entry point, refracted ray inside the block at angle r to the normal, emergent ray leaving at opposite face, labelled i and r. Normals should be drawn perpendicular to each face.
b) Use equation: n = sin i / sin r
For marking purposes, using calculator, n ≈ 1.51 (accept values between 1.47 and 1.51 depending on rounding).
c) To measure the speed of sound with reflection: Place two people a known distance apart, or from a wall. Bang wood blocks together and use a stopwatch to time the echo (the time for sound to reach the wall and return). Calculate speed using distance/time. An oscilloscope connected to a microphone can be used to display the sound wave visually.