5.4 Double slit interference

Question 1

What type of waves do the double slit need to emit in order to observe the interference of light?

Answer 1

» Coherent source of waves

Question 2

What are coherent sources of waves?

Answer 2

» Which means that they emit light waves with a constant phase difference
» And the same frequency and wavelength

Question 3

What can you observe in the double slit experiment?

Answer 3

» Young’s fringes
» Spaces of dark fringes and bright fringes

Question 4

What happens if the single slit which is used to illuminate the double slit becomes too wide?

Answer 4

» Dark fringes of the double slit pattern become narrower than the brige fringer
» Due to each part of the slit being displace slighting from the pattern

Question 5

How are the fringes formed?

Answer 5

» Due to the interference of light

Question 6

How is a bright fringe formed?

Answer 6

» Superposition of the 2 waves
» Diffraction from waves overlap
» Waves from each slit will arrive in phase
» Causing points which interfere construcitvely

Question 7

How is a dark fringe formed?

Answer 7

» Superposition of the 2 waves
» Diffraction from waves overlap
» Waves from each slit will arrive 180 degrees out of phase
» Causing points which interfere destructively

Question 8

What is the fringe seperation?

Answer 8

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

Question 9

What is the equation for fringe seperation?

Answer 9

» w = (wavelength x D)/s
» w = fringe seperation
» wavelength = wavelength
» D = The distance from the slit to the screen
» s = The slit spacing - distance between the centres of the slits

Question 10

How can you measure the fringe seperartion, w?

Answer 10

» Measure across several fringes from the centre of a dark fringe to the cetnre of another dark fringe
» Divide by the number of fringes you saw

Question 11

How can 2 loudspeakers demonstrate interference?

Answer 11

» Must be connected to the same signal generator
» As they are coherent source of sound waves
» You can detect poitns of cancellation and reinforcement as you move across in front of the speakers

Question 12

What is path difference?

Answer 12

» The difference in the distance travelled by two waves

Question 13

What is the purpose of putting a single slit before the double slit in the young’s experiment?

Answer 13

» To make sure the light has a fixed path difference
» Also acts as a single source, diffraciton light to both slits

Question 14

What is the purpose of the filter in the young’s experiment?

Answer 14

» To make light monochromatic

Question 15

What is the consequence of having the path difference has a whole number?

Answer 15

» Having a difference of a whole number of wavelengths, such as M x lambda
» Means the 2 sources of light interfere constructively

Question 16

What is the consequence of having the path difference has a whole number and a half?

Answer 16

» Having the difference as a whole number and a half of wavelength such as (n+1/2) x lambda
» Means the waves interfere destructively

Question 17

How can the appartus be changed to obtain an accurate value of the wavelength?

Answer 17

» Increase D
» Measure across more than 2 maxima
» Therefore reduces percentage error

Question 18

If a microwave receiver was placed at the points of dark fringes, explain why the signal strength falls to a minimum?

Answer 18

» Path difference for the 2 waves
» Give rise to phase difference
» Destructive interference occurs

Question 19

How can you find the frequency of a wave by having the path difference?

Answer 19

» Path difference = 2 x wavelength
» Find wavelength
» Use C/wavelength = frequency

Question 20

Why at some points about the fringes, the minimum intensity is not zero?

Answer 20

» Intensity decreases with distance
» One wave may travel further than the other wave
» Amplitudes do not cancel out

Question 21

What is the importance of the double slit experiment?

Answer 21

» Suggests that the nature of light is a wave rather than a particle

Question 22

Explain what happens when a laster emitting visible light has a shorter wavelength, how will this affect the spacing of the maxima on the screen?

2 Marks

Answer 22

» Shorter wavelength means less diffraction due to w being directly proportional to wavelength
» Therefore the spacing of the maxima would decrease as the maxima would become closer together