Fringe formation

Question 1

Use Huygens’ wave theory of light to explain the formation of these fringes by the double slits. You may be awarded marks for the quality of written communication provided in your answer.

Answer 1

• waves/ wavelets are emitted by each slit (1) • each slit diffracts light (1) • the two slits are coherent emitters / sources of light waves (1) • bright fringes formed where light from one slit reinforces light from the other slit (1) • path difference to a bright fringe = whole number of wavelengths (1)

Question 2

In an experiment to demonstrate the wave nature of light, a parallel beam of monochromatic light was directed at two closely spaced slits, as shown in Figure 1. A pattern of bright and dark fringes due to this light passing through the slits was seen on the screen. Explain why this fringe pattern was formed:

Answer 2

• light, passing through each slit, is diffracted (1) • diffracted light from one slit overlaps with (diffracted) light from the other slit (1) • bright fringes formed where light waves from each slit reinforce (or in phase) (or interfere constructively) (1) • dark fringes formed where light waves (from the two slits) cancel (1) (or out of phase by 180°) (1) • path difference = whole number of wavelengths for a bright fringe [or whole number + ½ wavelength for a dark fringe]

Question 3

Discuss why this fringe pattern cannot be explained using Newton’s corpuscular theory of light:

Answer 3

• corpuscles passing through a slit form a bright fringe (1) • two slits produce only two bright fringes according to corpuscular theory (1) • more than two fringes are observed (1) • dark fringes (or cancellation) cannot happen with corpuscles (1)

Question 4

Young’s experiment = evidence as

Answer 4

interference is a wave property

Question 5

Newton’s double slit prediciton

Answer 5

Newton said light consisted of particles which wouldn’t diffract so pass straight through so only 2 bright fringes seen dark fringes can’t happen with corpuscles

Question 6

Explain Young’s experiment

Answer 6

Young used a light source behind a narrow slit. In front of this was a screen with two narrow slits in it. Light spreading out by diffraction from the slits was equivalent to two coherent point sources. When light from one slit to overlap with light from the other forming bright fringes when they reinforced (interfere constructively when in phase/path difference is n*lambda) and dark fringes when they cancelled (interfere destructively when out of phase/path difference is lambda*(n+0.5)). This formed a fringe pattern of bright and dark fringes on the screen.

Question 7

Why was Young’s experiment evidence for Wave-theory

Answer 7

Young proved Huygens right with his double-slit experiments by proving that light could both diffract (through the narrow slits) and interfere (to form the interference pattern on the screen) – proving Huygens theory was correct. The alternating band of bright and dark fringes was predicted by Huygen but not Newton who thought that corpuscles passing through a slit would form a bright fringe and so two slits would produce only two bright fringes and so as there was multiple bright dark fringes and dark fringes which Newton couldn’t explain thus proving Huygens theory correct

Question 8

Use the wave theory of light to explain the formation of interference fringes

Answer 8

Wavelets emitted are diffracted when passing through slits (which act as coherent emitters). Bright fringes formed when light from one slit reinforces another (when path difference = nlambda) and darl fromhes when light waves cancel (when path difference = lambda(n+0.5)