Double slit interference

Question 1

How is a coherent source of light made?

Answer 1

illuminated two closely spaced parallel slits

the two slits act as coherent sources of waves - they emit light waves with a constant phase difference and frequency

Question 2

What are Young’s fringes?

Answer 2

alternating bright and dark fringest form on the screen where the diffracted light from the double slit overlap

Question 3

What happens if the single slit is too wide?

Answer 3

each part of it produces a fringe pattern

which is displaced slighly from the pattern

due to adjacent parts of the single slit dark fringes become narrower

Question 4

How are the fringes formed?

Answer 4

• due by interference of light
• bright fringe:
  
  • light from one slit reinforces the light from the other slit
  • light waves arrive in phase with eachother
  • • dark fringe:
  • the light from one slit cancels the light from the other slit
  • the waves from the twp slits arrive 180 out of phase

Question 5

What is fringe seperation?

Answer 5

the distance from the centre of a bright fringe to the centre of the next bright fringe

Question 6

What is the meaning for each part of the equation w = λD / s?

Answer 6

D = distance from the slits to the screen

s = slit spacing

w = fringe separation

Question 7

How do the fringes become more widely spaced? w = λD / s

Answer 7

distance from the slits is increased

λ is increased

split spacing is reduced

Question 8

How is fringe seperation measured?

Answer 8

measure across several fringes from the centre of a dark fringe to another,

easier to locate than bright obtain w by d

iving you measuredment by the number of fringes you measured across

Question 9

Mark scheme : explain how the pattern of bright and dark patches is formed.

Answer 9

slits act as coherent sources

waves diffract as slits waves superimpose

bright patches: constructive interference / waves reinforce

dark patches : destructive interferance / waves out of phase

Question 10

What are the two requirements for light sources to be coherent?

Answer 10

same phase / constant phase difference

same wavelength / frequency

Question 11

Why must S be narrow?

Answer 11

• narrow slit gives wide diffraction
  
  • ensures that S1 and S2 are illuminated
• S acts as a point source
  
  • narrow single slit ensures it provides coherent sources of light at S1 and S2

Question 12

How are S1 and S2 act as coherent sources?

Answer 12

• both illuminated by the same source giving the same wavelength
• paths to S1 and S2 are of constant length,
  
  • giving constant phase different
  • or SS1 and SS2 so waves in phase

Question 13

what is coherent light

Answer 13

same freq, wavelength and same phase difference

Question 14

red laser replaced with a white light source

describe how the appearance of the maxima would change

Answer 14

• white central
• subsidary maxima consist of specrtrum of light
• maxima wider, minima narrower
• blue nearest to central maxima, red farthest