Young’s Double-Slit Experiment (SL IB Physics)

• Interference patterns depend on:
  • The coherent light incident on the slits
  • The distance between the slits and the screen where the interference pattern is observed
  • The separation between the slits

• In particular, it can be seen that:
  • The longer the wavelength λ, the further apart the bright fringes on the screen become
  • The further the screen is placed from the slits, the further apart the bright fringes on the screen become
  • The greater the separation between the slits, the closer the bright fringes on the screen become

Differences between the interference pattern of red and blue light. Red light has a longer wavelength than blue light, so the fringes are more spaced out

• Two slits are separated by a length d, and a screen is placed a distance D away

Light rays of wavelength λ incident on two slits a distance S apart interfere and form bright fringes on a screen placed a distance D away from the slits

• When a light ray of wavelength λ is incident on the two slits, the observations are:

1. A central bright fringe forms

• • This is where the two waves have travelled the same distance to the screen and their path difference is zero 

2. Bright fringes are formed on either side of the central fringe

• • This is where the path difference between the waves is equal to exactly one wavelength 
  • This is constructive interference 

3. Dark fringes are formed between the bright fringes

• • This is where the path difference between the waves is some number of wavelengths plus half a wavelength 
  • This is destructive interference 

• Further bright fringes will be located at each position on the screen where the path difference is exactly equal to nλ
  • For example: 2λ, 3λ, 4λ...
  • This is constructive interference

• Dark fringes are located in between the bright ones, where the path difference is exactly equal to
  • For example, 
  • This is destructive interference

Double-Slit Equation

• The separation s of successive fringes is given by:

• Where:
  • s = separation between successive fringes on the screen (m)
  • λ = wavelength of the waves incident on the slits (m)
  • D = distance between the screen and the slits (m)
  • d = separation between the slits (m)

• Note that s is the separation between two successive bright fringes or two successive dark ones
• The above equation shows that the separation between the fringes will increase if:
  • The wavelength of the incident light increases
  • The distance between the screen and the slits increases
  • The separation between the slits decreases

Dependence of the interference pattern on the separation between the slits. The further apart the slits, the closer together the bright fringes