Module 4 Interference

Question 1

Principal of superposition

Answer 1

when two waves of similar type meet at a point, the sum of their displacements is equal to the displacement of the resultant wave

Question 2

What is a pulse?

Answer 2

A fragment of a wave

Question 3

Define path difference

Answer 3

difference in the distance from one source to the detector compared to another source to the detector

Question 4

Formula for path difference

Answer 4

S1P – S2P in meters (SP being length)

Question 5

What is phase difference?

Answer 5

the angle by which a wave lags or leads another

Question 6

What is the phase difference of waves in phase?

Answer 6

0 or 2π radians
(0 or 360 degrees)

Question 7

What is the phase difference of two waves in anti-phase

Answer 7

π or 3π radians
(180 or 540 degrees)

Question 8

Definition of coherent waves

Answer 8

when two waves of the same type are emitted with a constant phase difference, same frequency, wavelength, plane polarised in the same plane and similar amplitude (same magnitude)

Question 9

Definition of interference

Answer 9

Superposition of two progressive waves from coherent sources to produce a resultant wave with displacement equal to the vector sum of individual displacement of the two waves

Question 10

How is intensity increased if amplitude is doubled

Answer 10

intensity is increased by multiple of 4

Question 11

What is constructive interference?

Answer 11

when two waves meet in phase their amplitude/displacement adds up

Path difference is a whole number of wavelengths

Question 12

What is destructive interference?

Answer 12

When two waves meet exactly out of phase (in antiphase) the displacements cancel out and produce a wave of zero displacement and amplitude

Path difference is a whole number of wavelengths plus half wavelength

Question 13

Explain how you would measure the wavelength of a monochromatic laser using two source interference?

Answer 13

1. place the laser in front of a double slit board facing a screen
2. measure the distance between the centre of the slits using a traveling microscope by using a vernier scale - this is a
3. measure the distance between multiple fringes on the screen (say 8) then divide by the number of gaps (7) - this is x
4. measure the distance between the double slit and screen - this is D
   use the formula λ = ax/D

Question 14

How to observe the interference of sound waves?

Answer 14

• Attach a speaker to a signal generator
• place speaker behind a double slit board with slits approx 10cm wide
• Measure distance between centre of slits on the board - this is a
• Move a microphone parallel to the slit board at a given distance and measure where the maxima and minima are (loud and quiet) - this is x/2
  This is interference

Question 15

YOUNGS DOUBLE SLIT EXPERIMENT WITH WHITE LIGHT

Answer 15

Central fringe is white (path difference is the zero for all wavelengths in white light)
Each constructive interference position is a spectrum from blue to red (blue is closest to the straight through position so at a smaller angle than red)

Question 16

When investigating the interference of visible light, what is a suitable value for a (slit separation)?

Answer 16

<1mm

Question 17

Why when carrying out Young’s double slit experiment, does the value for a (the slit) need to be approx = to the wavelength. of light being used?

Answer 17

so light diffracts so that interference can be observed

Question 18

When carrying out Young’s double slit experiment, why must multiple fringes be measured to find the value of x?

Answer 18

to reduce percentage error due to random error (unc on x)

you are measuring a larger distance for several fringes so error/measurement x 100 is lower ie percentage uncertainty lower

Question 19

Describe how you would observe two source interference using a microwave generator

Answer 19

• Attach two transmitter cones to a microwave generator
• Move a receiver probe parallel to the transmitter cones at a set distance
• Receiver will measure alternating patterns of strong and weak signals

Question 20

How are fringes labelled?

Answer 20

as orders - zero order, first order, second order, etc. labelled from the straight through position
(order number is n in diffraction formula)

Question 21

What is a word to describe when waves are at 0º to each other?

Answer 21

in phase

Question 22

What do you use to measure the distance a in an the young’s double slit experiment?

Answer 22

a travelling microscope with a vernier scale

Question 23

What can be said about the relationship between wavelength and distance between maxima and minima?

Answer 23

shorter wavelength the shorter the path difference between maxima and minima

for maxima, path difference = whole wavelengths

for minima, path difference=half wavelength + whole wavelength