3.3 - Interference

Question 1

When does interference occur?

Answer 1

When waves overlap and their resultant displacement is the sum of the displacement of each wave.

Question 2

When are waves coherent?

Answer 2

When they have the same frequency and constant phase difference.

Question 3

What is coherence used for?

Answer 3

The production of an observable/ hearable interference pattern.

Question 4

What is an example of a coherent light?

Answer 4

A laser light.

Question 5

What is an example of an incoherent light?

Answer 5

Filament lamps - incoherent light waves.

Question 6

What is path difference?

Answer 6

The difference in distance travelled by 2 waves from their sources to the point where they meet.

Question 7

What must the path difference be for constructive and destructive interference?

Answer 7

Constructive - nλ
Destructive - (n + 1/2)λ

Question 8

What is the path difference for constructive and destructive interference?

Answer 8

Constructive - when the path difference is an integral number of wavelengths.
Destructive - path difference is 1/2λ path difference.

Question 9

How can interference and diffraction of lasers be demonstrated?

Answer 9

With the Young’s double slit experiment.
Constructive interference is shown as bright fringes.
Destructive interference is shown as dark fringes.

Question 10

What is the safety issue with lasers?

Answer 10

• Produce very high energy beam of light, can cause permanent eye damage or blindness.

Question 11

What are the 6 precautions taken with lasers?

Answer 11

• Never look directly at a laser of its reflection
• Don’t shine the laser towards a person
• Don’t allow a laser beam to reflect from shiny surfaces into someone’s eyes
• Wear laser safety goggles
• Place laser warning sign
• Stand behind laser

Question 12

How can constructive and destructive interference be demonstrated with sound waves? (used for noise cancelling headphones).

Answer 12

2 speakers.
- Constructive interference occurs when compression and rarefactions line up and the sound appears louder
- Destructive interference occurs when the compression lines up with a rarefaction and vice versa. The sound is quieter.

Question 13

How can constructive and destructive interference be demonstrated with microwaves?

Answer 13

• 2 slits in metal plates
• microwave transmitter directed at slits
• microwave detector picks up interference (con - max amplitude, des - no signal)

Question 14

What is monochromatic light?

Answer 14

Light with one singular wavelength.

Question 15

How are the interference pattern points numbered?

Answer 15

n = 3
n = 2
n = 1
n = 0
n = 1
n = 2
n = 3

Question 16

What does Young’s Double Slit Experiment show?

Answer 16

How light waves can produce an interference pattern.

Question 17

What is the fringe space equation?

Answer 17

W = λD/S (all measured in metres)
W =fringe width
λ = wavelength
D = distance between double slit to the screen
S = distance between the 2 slits

Question 18

What would the interference pattern of a white laser look like (double slit)?

Answer 18

Central fringe = white
the 1st and 2nd orders will be the whole colour spectrum.

Question 19

What is the independent + dependent + control variables in the Young’s Double Slit Experiment?

Answer 19

Independent - Fringe width, w
Dependent - Distance between the slits and the screen, D
Control -