L12 Interference

Question 1

define interference

Answer 1

when 2 waves are in the same place at the same time and produce effects of two waves in accordance with the principle of superposition.

2 types: constructive, destructive

Question 2

state 2 conditions for interference

Answer 2

• constant PHASE DIFFerence between 2 waves

- same AMPlitude between 2 waves

Question 3

constructive vs destructive interference

Answer 3

(1) in phase, constant path length difference (mlamda), summation of 2 waves, resultant amp=summation amp
(2) out of phase, constant path length difference (m+1/2 lamda), cancellation of waves

Question 4

demonstration of interference in ripple tanks

Answer 4

consider 2 vibrators hooked together in a tank, the circular waves from each source overlap paths, interfering with eo.

nodes=positions where destructive interference occurs (crest meets trough)
points/antinodes=positions where constructive interference occurs (crest meets crest)

Question 5

demonstration of interference in young’s double slit

Answer 5

involves source slit that allows more and coherent light to pass through 2 slits, dark and bright fringes observed.

• dark fringes (destructive interference) phase diff half lamda
• bright fringes (constructive interference) phase difference lamda

Question 6

what is thin film interference

Answer 6

eg. oil slicks, soap bubbles in themselves are colourless but in thin layer/film produce colours

Question 7

thin film interference factors

Answer 7

1. interference of 2 reflected waves
   - incident light on transparent thin film produces 2 reflected beams from front and back surface each
   - phase change when reflected at denser medium, no phase change when index the same
   - light reflected exhibits colours from that of constructive interference
2. multiple reflections
3. thickness of film
   - too thick: no more multiple reflections, colours interfere constructively with eo=white colour
   - too thin: destructive interference= colours disappear, fringes =black

Question 8

application of thin film interference

Answer 8

1. newton rings
   - used to test precision of surfaces
   - thin air wedge*** produce TLI light reflected constructively(light rings)/destructively (dark rings) interfere with eo
2. anti-reflection coatings
   - when thickness of fluoride coating is a quarter the thickness of the wvl of the incident light, reflected light from the front and back surface destructively interfere
   - destructive interference=no light reflected back from the lens, increasing transmission of light through the lens and improving clarity of image
3. multi coating
   - needed to cut down reflections caused by different wvl of visible light
   - destructive interference only occurs middle range (green-yellow)
   - red and blue wvl reflected result in lens appearing purple