Reflection, refraction and diffraction of waves — GCSE Physics practice questions

Question 1

Out at sea, straight waves travel towards a long straight beach at an angle to it.
Near the beach the waves turn, so that their crests arrive almost parallel to the shoreline.

What causes the waves to turn?

Accepted Answer

A: The water becomes shallower, so the waves slow down.. Water waves travel more slowly in shallower water. The part of each crest nearer the beach is in shallower water and moves more slowly than the rest, so the crest swings round towards the shoreline. This is refraction, caused by the change of speed; the frequency does not change.

Question 2

At an outdoor light show, a beam of green laser light shines at an angle onto the still surface of a pond.
Light travels more slowly in water than in air.

Which statement is correct?

Accepted Answer

B: The light in the water has the same frequency as the light in air but a shorter wavelength.. The frequency is set by the source (the laser) and does not change at the boundary, so the colour stays green. In water the speed is smaller, so λ = v / f is shorter. Part of the beam is reflected at the surface with its wavelength unchanged, so not all of the energy enters the water.

Question 3

Music is playing in a hall. A person stands outside, to one side of an open doorway 0.85 m wide.
She hears bass notes of frequency 85 Hz clearly, but high notes of frequency 3400 Hz sound much quieter. The speed of sound in air is 340 m / s.

Which statement is **not** correct?

Accepted Answer

C: The high notes are quieter because they travel more slowly than the bass notes.. λ = v / f gives 340 / 85 = 4.0 m for the bass and 340 / 3400 = 0.10 m for the high notes. The bass wavelength is larger than the doorway, so it diffracts strongly round to the side; the high notes, with a wavelength much smaller than the doorway, spread out less, and light (wavelength below a micrometre) spreads even less. All sounds travel at the same speed in the same air, so statement C is not correct.