P5 - Interference of waves

Question 1

What happens when waves meet a medium?

Answer 1

• All waves cause some kind of disturbance when in a medium, water waves disturb water particles, sound waves disturb air particles, electromagnetic waves disturb electric and magnetic fields.

Question 2

What happens when two waves meet?

Answer 2

• When two waves meet a point they both try to cause their own disturbance.

Question 3

What is constructive inference?

Answer 3

• Waves can disturb in the same direction and reinforce each other ( constructive interference ).

Question 4

What is destructive interference?

Answer 4

• Waves disturb each other in opposite directions and cancel each other out.

Question 5

What is the total amplitude of the waves?

Answer 5

The total amplitude of the waves at a point is the sum of the displacement ( take direction into account ).

Question 6

How do speakers work using interference?

Answer 6

• Two speakers play exact note at the same time.
• Depending on where you stand in front of them, you will hear a loud sound or almost nothing, at certain points the sound waves will be in phase, here you get constructive interference. The amplitude of the waves will be doubled, so you will hear a loud sound.
• These occur where the distance travelled by the waves from both speakers is the same or different by a whole number of wavelengths.

Question 7

What happens if you get out of phase waves in speakers?

Answer 7

• Sound waves will be exactly out of phase, here you get destructive interference, and the waves will cancel out. This means you will hear almost no sound.

Question 8

Where do out of phase points occur?

Answer 8

• Out of phase points occur where the difference in the distance travelled by the waves ( the path difference ) is 1/2 wavelength, 1 1/2 wavelengths, 2 1/2 wavelengths etc..

Question 9

What do you need need to get a stable interference pattern?

Answer 9

• To get a stable interference pattern you need to use coherent wave sources.

Question 10

What are coherent wave sources?

Answer 10

This means sources where:
• The waves are at the same frequency, and so wavelength.
• The waves are in phase, the troughs and crests of the wave line up.
• The waves have a similar amplitude.

Question 11

What are the coherent sources for light?

Answer 11

• For light, the coherent sources are monochromatic light ( this the type of light needed to make an interference pattern ).