interference

Question 1

principal of superposition

Answer 1

when two/more low energy waves are superposed on the same medium, the waves travel independently through each other, & the resultant displacement at each position is the algebraic sum of the displacement due to each wave

Question 2

define Newton’s ring

Answer 2

• a phenomenon in which an interference pattern is created by the reflection of light between two surfaces, a spherical surface and an adjacent flat surface
• in optometry, newton’s rings are used in an equipment for patients w keratoconus

Question 3

condition for interference to occur

Answer 3

both waves must have nearly the same frequency and amplitude

Question 4

Young’s double-slit experiment

Answer 4

• w monochromatic light, series of bright & dark fringes are formed
• bright fringes
  > constructive interference
  > m lambda phase difference
• dark fringes
  > destructive interference
  >(m+1/2) lambda phase difference
• fixed wavelength
  > fringes wider when slits are closer(easier to see)
  > fringes narrower when slots are further(harder to see)
• fixed slit separation
  > fringes wider for longer wavelengths
  > red fringes spreads out further than blue
  > interference fringes are coloured for white light illuminated

Question 5

what is constructive interference?

Answer 5

• two waves have to be in phase(crest aligned)
• summation of both waves
  > resultant amplitude equals to sum of two component amplitudes
• light is brighter(double the brightness)

Question 6

what is destructive interference?

Answer 6

• two waves have to be out of phase(by 1/2 lambda)
• amplitude of waves cancel out each other
• no light ( dark because light cancel each other)

Question 7

what is thin film interference?

Answer 7

Result from multiple reflection process
- A single ray will reflect multiple times in a thin film
> Results in phase change at reflection
> Form colour fringes
Phase changes at reflection
- Ray reflected from front surface of thin film
> Light in lower refractive index medium before reflection undergoes phase changes
- Ray reflected from the back surface of thin film
> Light in higher refractive index medium before reflection has no phase change
Due to multiple reflection process and varying phase changes
- Minimum (dark) fringe in reflected light (Destructive interference)
- Maximum (bright) fringes in transmitted light (Constructive interference)
Thickness of thin film
- Affects distance travel of the different waves
- May be adjusted to give constructive (m lambda) or destructive [(m+1/2) lambda] interference on either surface
- When film is too thick, the phenomenon of multiple reflection will not occur

Question 8

AR coating

Answer 8

• AR coating are thin films deposited on lens to
  > Reduce the amount of reflection by destructive interference
  > Increase the transmission of the lens by constructive interference Conditions for interference to produce observable interference
• Magnesium fluoride (MgFl) widely used to create the thin film of AR coating (n = 1.38)
• With thin film on the lens, phase change will result in undesirable effects
  > Constructive interference from reflected rays
  > Destructive interference from the transmitted rays
• Thickness of thin film have to be adjusted to result in the desirable effects
  > Destructive interference from reflected rays
  > Constructive interference from transmitted rays
  > Achievable when thickness of thin film is ¼λ
• Visible light consist of different colours and thus different wavelengths
  > Single layer of AR coating is not sufficient
  > Multilayered thin films are used to solve the problem
  > Increases range of incident angles over which the coating will be anti-reflective
• higher refractive index lens has more AR benefits as there is more reflection for higher refractive index uncoated lenses