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

What is coherence?

Answer 2

at points where the two waves are neither in phase nor in anti phase, the resultant amplitude is somewhere between the two extremes

Question 3

What are the properties of coherent waves?-

Answer 3

• same frequency
• a constant phase difference

Question 4

What. is path difference defined as?

Answer 4

• the difference in distance travelled by two waves from their sources to the point where they meet

Question 5

What is the condition for constructive intereference?

Answer 5

path difference of nλ

Question 6

What is the condition for destructive intereference?

Answer 6

(n+1/2)λ

Question 7

How is interference shown with sound waves?

Answer 7

• sound waves are longitudinal thus have compressions and rarefactions
• constructive interference occurs when the compression and rarefactions line up and the sound appears louder
• destructive interference occurs when the compression lines up with a rarefaction and vice verse thus the sound is quiter
• the 2 waves interfere causing areas of constructive and destructive interference

Question 8

How does interference occur in microwaves?

Answer 8

• 2 source interference for microwaves can be detected with a moveable microwave detector
• constructive interference occurs in regions where the detector picks up a maximum amplitude
• destructive interference occurs in regions where the detector picks up no signal

Question 9

what must occur with the waves when two source interference fringes to be observed?

Answer 9

• coherent
• monochromatic

Question 10

What does coherent mean about waves?

Answer 10

they have a constant phase difference

Question 11

What does Monochromatic waves mean?

Answer 11

single wavelength

Question 12

What does Youngs Double Slit Experiment demonstrate?

Answer 12

how light waves can produce an interference pattern

Question 13

How does Youngs double slit experiment occur?

Answer 13

• when a monochromatic light source is placed behind a single slit, the light is diffracted producing two light sources at the double slits A and B
• Since both light sources originate from the same primary source, they are coherent and will therefore create an observable intereference
• both diffracted light from the double slits create an interference pattern made up of bright and dark fringes

Question 14

intereference patterns in a monochromatic laser light with a white light source:

Answer 14

• each wavelength of light produces its own patter

Question 15

Why is the central fringe of a a white monochromatic light also white?

Answer 15

• centreal fringe is white at that position the path difference for all wavelengths present is zero therefore all the wavelengths will arrive in phase
• central fringe is thus same colours as the light source