Waves

Question 1

What does a progressive wave do?

Answer 1

Carries energy from one place to another without transferring any material.

Question 2

How is a wave created?

Answer 2

When something causes particles or fields to oscillate at a source.
The oscillations pass through the medium as the wave travels, carrying energy with it.

Question 3

What is the wave cycle?

Answer 3

The complete vibration of a wave.

Question 4

What is the displacement (x) of a wave + the unit?

Answer 4

How far a point on the wave moves from its undisturbed point.
Unit = metres

Question 5

What is the amplitude (A) of a wave + its unit?

Answer 5

The maximum magnitude of displacement.
Unit = metres

Question 6

What is the period (T) of a wave + its unit?

Answer 6

The time taken for a wave cycle to complete, or pass a given point.
Unit = seconds

Question 7

How do you investigate the effect of tension, length and mass/unit length on the resonant frequencies of a string?

Answer 7

• Investigating Length:
Keep the tension and mass/unit length the same but alternate the length by moving the vibration transducer to and away from the pulley. Find the first harmonic and record f against L.
• Investigating Tension:
Keep the length and mass/unit length the same but alternate the mass by adding/removing masses to change the tension. Find the first harmonic and record f against T.
• Investigating mass/unit length:
Keep the length and tension the same but alternate the string sample to change the mass/unit length. Find the first harmonic and record f against mass/unit length.

Question 8

How do you set up the apparatus to investigate the effect of tension, length or mass/unit length on the resonant frequency of a string?

Answer 8

1. Measure and record the mass and lengths of different string samples using a mass balance and ruler. Find the mass/unit length of each string sample using the formula M/L.
2. Set up the apparatus, recording the mass/unit length, the length of string and the tension using T = Mg.
3. Turn on the signal generator and vary the frequency until you find the 1st harmonic.

Question 9

What is a transverse wave?

Answer 9

A wave with oscillations (vibrations) perpendicular to the direction of energy transfer.

Question 10

What are 3 examples of transverse waves?

Answer 10

• All electromagnetic waves
• Ripples in water
• Waves on a string

Question 11

What are longitudinal waves?

Answer 11

Waves with oscillations (vibrations) parallel to the direction of energy transfer.

Question 12

What are 2 examples of longitudinal waves?

Answer 12

• Sound waves
• Some earthquake shock waves

Question 13

What is a polarised wave?

Answer 13

A wave that only oscillates in one direction.

Question 14

What waves can be polarised?

Answer 14

Transverse waves.

Question 15

How is polarisation evidence that EM waves are transverse?

Answer 15

• Malus discovered that light was polarised by reflection.
• Physicists at the time believed that light spread like sound (as a longitudinal wave), so struggled to explain polarisation.
• Young suggested that light was a transverse wave with vibrating electric and magnetic fields travelling at right angles to energy transfer, which explained why light could be polarised.

Question 16

What happens when you view partially-polarised reflected light at the correct angle through a polarising filter?

Answer 16

You can block out unwanted glare (polaroid glasses do this).

Question 17

In phase:

Answer 17

• Two points on a wave are in phase if they are at the same point on the wave cycle (there phase difference is a multiple of 360 degrees/2pi radians).
• Points in phase have the same displacement and velocity.

Question 18

Exactly out of phase:

Answer 18

Two points on a wave are exactly out of phase is there phase difference is an odd multiple of 180* or 1pi radian.

Question 19

Interfering constructively:

Answer 19

When a crest meets a crest (or trough:trough) the displacements combine to give the resultant displacement.

Question 20

Interfering destructively:

Answer 20

•When a crest meets a trough with equal displacement, the displacements cancel out.
• If the displacements of the crest and trough are non-equal, the destructive interference isn’t total. The displacements need to be almost equal for the interference to be noticeable.

Question 21

What is the principle of superposition?

Answer 21

When two or more waves meet, their resultant displacement is equal to the vector sum of the individual displacements.

Question 22

What is a stationary wave?

Answer 22

• The superposition of two progressive waves with the same frequency (wavelength) travelling in opposite directions.
• Progressive waves don’t transmit energy like progressive waves.

Question 23

What should you expect from resonant frequencies investigation?

Answer 23

• Length: The longer the string, the lower the resonant frequency.
• Mass/Unit length: The heavier the string, the lower the resonant frequency.
• Tension: The looser the string, the lower the resonant frequency.

Question 24

What is coherence?

Answer 24

Two sources that have the same wavelength and frequency, and a fixed phase difference between them.

Question 25

Do sources need to be coherent to get clear interference?

Answer 25

Yes.

Question 26

What is path difference?

Answer 26

How much longer the path one wave has travelled than the path the other wave has travelled.

Question 27

Whether you get constructive or destructive interference at a point depends on what?

Answer 27

• The path difference of the waves.
• You get constructive interference if the path difference is a whole number of wavelengths, the waves at this point are in phase and reinforce eachother.
• You get destructive interference if the path difference is not a whole no. of wavelengths. (1/2).

Question 28

What does the amount of diffraction of a wave depend on?

Answer 28

The wavelength of the wave with the size of the gap.
• If the gap is larger than the wavelength, diffraction goes unnoticed.
• If the gap is smaller than the wavelength, the waves are mostly reflected back.
• You get noticeable diffraction if the gap is several wavelengths wide.
• You get the most diffraction when the gap is the same size as the wavelength.

Question 29

Why can you hear someone through an open doorway when they aren’t in your line of sight, but you can’t see them?

Answer 29

• The size of the doorway and the wavelength of the sound waves are roughly equal in size, so lots of diffraction occurs so that you can hear them in the next room.
• However, you can’t see them because the wavelength of light is around a hundred million times smaller than the size of the doorway so the amount of diffraction is tiny.

Question 30

Investigating diffraction of light waves:

Answer 30

• Need a monochromatic, coherent light source e.g. lasers
• The aperture must be around the size of the wavelength of the light waves to create a diffraction pattern.
RESULT:
• A central bright fringe with bright and dark fringes alternating on either side.
• The dark and bright fringes are created by constructive and destructive interference.