2.4 Superposition and Interference

Question 1

What is the definition of coherent?

Answer 1

Waves have the same frequency and a constant phase difference.

Question 2

What is the principal of superposition?

Answer 2

When 2 or more waves meet in a medium, the resultant displacement of the medium at any point in space is equal to the vector sum of the displacement of each individual wave at that point.

Question 3

How can constructive interface occur?

Answer 3

• Waves are in phase.
• Both have same sign displacement (++).

Question 4

How can destructive interface occur?

Answer 4

• Waves meet in antiphase.
• Have opposite sign displacement (+-)

Question 5

What are standing/stationary waves?

Answer 5

Waves in which no energy is transferred.

Question 6

How is a standing wave formed?

Answer 6

Standing waves are set up when two waves meet that have the same frequency, speed and amplitude and are travelling in opposite directions.

Question 7

What are nodes?

Answer 7

• Created where destructive interface happens.
• Points of 0 displacement and don’t move.

Question 8

What are antinodes?

Answer 8

• Created where constructive interface happens.
• Points with maximum displacement and move.

Question 9

How can sound be used to shatter glass?

Answer 9

If the string is forced to vibrate (is plucked) at a frequency that matches one of its natural frequencies, it will vibrate with a large amplitude of vibration (resonance). This is how sounds can cause glasses to shatter.

Question 10

What end of an air column is an antinode?

Answer 10

The open end of the column is vibrating with maximum amplitude and is therefore an antinode.

Question 11

What end of an air column is an node?

Answer 11

The amplitude of vibration at the closed end of the node is zero and is therefore a node.

Question 12

What is resonance?

Answer 12

A condition where an object vibrates at its natural frequency and the amplitude of vibration is maximum.

Question 13

What are the conditions for observable interface?

Answer 13

1. They are coherent.
2. They have a constant phase difference.
3. They have similar amplitudes.
4. Light waves only: must be monochromatic

Question 14

What are the key points of the double slit experiment?

Answer 14

1. To obtain the best results should be carried out in a dark room / low light conditions.
2. Slit separation 0.1 to 0.5mm.
3. Distance from double slits to screen 1m to 5m.
4. To find fringe separation, measure the distance across several slits and divide by the number of gaps.

Question 15

What is monochromatic light?

Answer 15

Monochromatic light is light of a single wavelength – i.e. one colour. (Remember that different colours of visible light have different frequencies/wavelengths).

Question 16

What is a diffraction grating?

Answer 16

A glass/perspex material with many parallel equally spaced opaque lines.

Question 17

What is diffraction?

Answer 17

Diffraction is the spreading out of waves when they pass through an aperture / round an obstacle.

Question 18

What are the effects of wavelength and slit separation on fringe separation

Answer 18

• Increasing the wavelength increases diffraction and hence fringes spread out more.
• Decreasing the gap width increases diffraction, hence fringes move further apart.

Question 19

What is Huygen’s Principle?

Answer 19

Huygen’s principle states that each point on a wave front may be regarded as a new source of secondary spherical ‘wavelets’ which spread out with the wave velocity. These wavelets superpose to create the wave fronts.

Question 20

How does gap size relate to the amount of diffraction?

Answer 20

When the gap is of a similar size or smaller than the wavelength, there is more diffraction.

Question 21

Can you explain your answers in terms of Huygens’ principle?

Answer 21

When more wavelets are able to pass through a gap (e.g. larger gaps) the superposition that happens creates a ‘straighter’ wave front with the curves at the edges.