O-LevelPhysicsWaves5054/21 · Oct/Nov 2018 · Q915 Marks

Some waves are longitudinal and some waves are transverse.

(a)State how a longitudinal wave differs from a transverse wave.[1]
(b)(i)A vibrating rod produces a water wave in a ripple tank. Fig. 9.1 shows the crests of the wave passing into the right-hand section of the tank where the depth of the water is different from the depth in the rest of the tank. The arrows on Fig. 9.1 show the direction of travel of the wave in the two sections of the ripple tank. In the left-hand section of the tank, the wavelength of the wave is 0.019m and it travels at 0.17 m/s. Calculate the frequency of the wave. frequency = [2]
(b)(ii)State what happens to the frequency of the wave, as it passes into the right-hand section of the tank.[1]
(b)(iii)Using Fig. 9.1, state and explain what happens to the speed of the wave as it passes into the right-hand section of the tank.[2]
(c)(i)Fig. 9.2 shows light, in air, striking the vertical side of a rectangular glass block at an angle of incidence of 60°. The refractive index of the glass is 1.6. The light travels in the glass and strikes side XY at P. Underline all the terms that describe a light wave. electromagnetic longitudinal transverse[1]
(c)(ii)At the point where the light enters the glass, the angle of refraction is r. Calculate angle r. r=[2]
(c)(iii) 1.Calculate the critical angle c for light travelling in the block. C = [2]
(c)(iii) 2.At P, the angle θ between the ray and the normal is given by θ = 90° − r. State and explain what happens to the light when it strikes side XY.[2]
(c)(iii) 3.On Fig. 9.2, draw the path of the light after it strikes side XY at P and the path of the light when it is again travelling in the air.[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

[B1] the vibration direction is parallel to the wave / energy travel direction or longitudinal waves have compressions and rarefactions or transverse waves have crests and troughs or longitudinal waves cannot be polarised
[C1] (f =) v/λ or 0.17 / 0.019
[A1] 8.9 Hz
[B1] (frequency) stays constant or does not change
[B1] (speed) decreases
[B1] (Fig. 9.1 shows that) wavelength decreases or refraction is towards normal or i > r or top of wave (in shaded region) lags behind bottom of wave
[B1] electromagnetic and transverse underlined
[C1] n = sin(i) / sin(r) or (r =) sin⁻¹(sin(i) / n) or (r =) sin⁻¹(sin(60°) / 1.6)
[A1] 33(°)
[C1] n = 1 / sin(c) or (c =) sin⁻¹(1 / n) or sin⁻¹(1 / 1.6)
[A1] 39°
[B1] total internal reflection
[B1] θ > c or 57(°) > 39(°) or angle of incidence greater than the critical angle
[B1] reflected ray at P (correct angle by eye)
[B1] light in air at 30° (by eye) to vertical surface