5.4 Double slit interference

Question 1

Define coherent

Answer 1

2 sources od waves are coherent if they emit waves with constant phase difference

Question 2

Define Youngs fringes

Answer 2

parallel bright and dark fringes observed when light from a narrow slit passes through 2 closely spaced slits

Question 3

Describe the fringe patterns on Youngs modulus

Answer 3

The fringes are evenly spaces with decreasing intensity going from the central maximum outwards.

Question 4

What would happen to the fringe pattern if the slits are too wide?

Answer 4

dark fringes of the double slit patten become narrower than the bright fringes and contrast is lost between the dark and bright fringes.

(due to slightly displaced fringe pattern from adjacent parts of the single slit)

Question 5

What forms fringes?

Answer 5

The interference of light

Question 6

Explain how a bright and dark fringe is formed

Answer 6

Bright, the light from one slit reinforces the light from the other slit (light waves from each slit arrive in phase with each other)

Dark, the light from one slit cancels the light from the other slit. (light waves from the 2 slits arrive 180 degrees out of phase)

Question 7

How would you measure fringe separation?

Answer 7

Measure across several fringes from the centre of a dark fringe to the centre of another dark fringe. Divide you measurement by the number of fringes you measured across to get W.

Question 8

Explain how 2 loudspeakers can be used to demonstrate interference

Answer 8

As you stand in-between the 2 speakers, the sound waves from both speakers meet and interfere with each other creating an interference pattern.

From the in-between the waves from both speakers in in-phase and undergo constructive interference combining into one wave with the same wavelength but larger amplitude.

Moving close to either speaker you hear nothing as the waves arrive to you half a wavelength out of phase and undergo destructive interference. Amplitude is reduced/at 0.

and so on..

Question 9

What happens if you change the tone to increase the wavelength of the 2 waves from speakers?

Answer 9

The interference pattern changes.

When the wavelength increases, theres fewer points of constructive and destructive interference ( theres more space between the loud and quiet zones)

Likewise, when the wavelength decreases, there are more regions of interference.