12.3 Young's double slit experiment

Question 1

Explain Young’s double slit experiment.

Answer 1

• Monochromatic light passes through two slits (distance s, slit seperation), interferes on a screen (distance D)
• Fringe spacing (w) = λD/s
• Central fringe (n = 0) is brightest
• Fringes get dimmer farther from centre.
• Larger λ or smaller s means wider fringes
• Proves light behaves as a wave.

Question 2

What does monochromatic mean?

Answer 2

A single-wavelength (pure colour)

Question 3

How does the interference pattern with single slit differ from double slit?

Answer 3

Central maximum very bright compared to the other fringes and also double the width of the fringes.

Question 4

Explain why it is necessary to use monochromatic light, along with single slit and a double slit to produce a stable interference pattern for light.

Answer 4

• Necessary to use monochromatic because different wavelengths interfere at difference positions, creating overlapping fringes that blur the pattern. Filtered light ensures that all waves interfere constructively/destructively at the same point.
• Necessary to use single slit followed by double slit because a double slit alone would produce light from two independent sources (not coherent, random interference). A single slit first ensures that the light reaching the double slit is ensured.
• Using double slit after single: splits the coherent light into synchronised waves that interfere resulting in stable alternating bright/dark fringes from constructive/destructive.