Wave Motion

Question 1

When calculating the apparent frequency when an object is moving TOWARDS the observer, what formula is used?

Answer 1

(actual frequency) x (speed of wave)
—————————
(speed of wave) - (speed of source)

Question 2

What are travelling waves?

Give two examples and explain why they are classed as ‘travelling’ waves.

Answer 2

Travelling waves carry energy through from a medium without an overall movement of the medium.

e. g.
- Sound waves (cause vibrations, use up energy)
- X-rays (can ionize atoms, knocking electrons out of orbit)

Question 3

In what direction does the vibration travel in;

• Longitudinal waves
• Transverse waves

Answer 3

• In longitudinal waves, the direction of vibration is parallel to the wave direction.

(Long; travels along the wave i.e. parallel)

• In transverse waves, the direction of vibration runs perpendicular to the wave direction.

(Trans; different i.e. perpendicular)

Question 4

How can one demonstrate the different wave types (longitudinal/transverse)?

Answer 4

By using a slinky. Yeah.

Question 5

Define ‘wavelength’ in terms of a wave graph.

Answer 5

The length (m) of a full cycle, i.e. crest to crest.

Question 6

Define ‘amplitude’ in terms of a wave graph.

Answer 6

This is the maximum displacement (m) from the mean position of the wave.

This can be taken as the distance between the base (mean) line and the troughs/crests.

Question 7

What types of waves;

• need a medium to travel through
• do not need a medium?

Answer 7

Mechanical waves require a medium.

Electromagnetic (EM) waves do not require a medium.

Question 8

Define ‘frequency’ in terms of wave graphs.

Answer 8

The number of vibrations per second (Hz) a wave emits.

Question 9

Define ‘period’ in terms of wave graphs.

Answer 9

The time taken for a cycle to occur (s).

T = 1/f

Question 10

Name five phenomena that waves exhibit.

Answer 10

• Reflection (light rays reflect off a mirror, sound echoes off walls).
• Refraction (light rays hit glass at an angle, slow down/sound waves refract over a lake).
• Diffraction (light going out a door).
• Interference (used in riot control with speakers).
• Polarisation (transverse waves only, polaroid sunglasses for eclipse viewing).

Question 11

Distinguish between the types of interference.

Answer 11

• Constructive interference, when waves combine to form a resultant amplitude greater than original amplitudes.
• Destructive, when waves combine to form an amplitude smaller than original amplitudes.

Question 12

What happens when two waves meet;

• ‘in-phase’
• ‘out of phase’ 180 deg. ?

Answer 12

• They combine to form a larger amplitude than the two originals waves.
• They will cancel each other out and there will be zero amplitude (no volume).

Question 13

What are stationary waves?

Answer 13

Waves of the same amplitude and frequency in a confined space that constructively and destructively interfere.

Question 14

What is the doppler effect?

Give an example of an occurrence and an application of the effect.

Answer 14

It is the apparent change in frequency caused by the relative motion of source and observer.

• Occurrence: Ambulance siren high pitch towards, low driving away.
• Application: Calculating the velocity/direction of movement of stars using red/blue shift (doppler effect occurring for light)