Free Progressive Waves MCQs with Answers

1. In a transverse wave, the particles of the medium vibrate

• A. along the direction of propagation
• B. perpendicular to the direction of propagation
• C. in circles of increasing radius
• D. not at all

Explanation: Light, waves on a string and ripples on water are transverse, with the displacement at right angles to the direction the energy travels. In a longitudinal wave such as sound the vibration is along the direction of travel, producing compressions and rarefactions. Only transverse waves can be polarised, which is the standard experimental test of which type a wave is.

2. Only transverse waves can be polarised because polarisation restricts

• A. the speed of the wave
• B. the frequency of the wave
• C. the plane in which the vibrations occur
• D. the amplitude of the wave

Explanation: A transverse wave vibrates in any plane perpendicular to its direction of travel, so a filter can select one of those planes, whereas a longitudinal wave vibrates only along the direction of travel and there is nothing to select. This is why sound cannot be polarised but light can. The fact that light can be polarised is direct evidence that it is a transverse wave.

3. Mechanical waves cannot travel through outer space because they:

• A. Have low speed in vacuum
• B. Disperse in space due to long wavelength
• C. Lose frequency in the absence of air
• D. Lose transmission without interacting particles

Explanation: A mechanical wave propagates by particles of a medium pushing on their neighbours, so with no particles there is nothing to carry the disturbance and sound simply does not exist in a vacuum. Electromagnetic waves need no medium, which is why light crosses space while sound cannot. The frequency of a wave is set by its source and is not lost.