Getting familiar with Thin Film Interference

1. THIN FILM INTERFERENCE
LO9
Source: https://figures.boundless.com/16755/full/oil-20slick.jpe

2. Produces colours of soap bubbles, oil films, peacock
feather and some type of butterflies.

3. Net reflection — (2) and (6). If they are in PHASE, constructive interference
will make net reflected ray relatively bright. Unless, destructive interference
with no reflection.
Source: Nelson Physics for Scientists and Engineers
depend
Extra distance travelled by light in coating

4. MINDMAP
Phase changes
𝜆n = 𝜆/n
index of refraction
Wavelength in
a medium
1. 180o (hard reflection): medium with
greater refraction index.
2. No change (soft): medium with smaller
refraction index.
2t= m𝜆n/2 (constructive interference)
——————————
t=(2m+1)𝜆n/4 (destructive interference)
m=0,1,2,…
thickness

5. CONCEPT CHECK ☑
• n1= 1.50 , n2= 2.00
• In all cases, determine what happened to reflected light when its
thickness is approaching zero.
Source: Boston University

6. SOLUTIONS
• Case A:
( ✔ ) eliminated by destructive
interference
( ) bright by constructive interference
• Case B
( ✔ ) eliminated by destructive
interference
( ) bright by constructive interference
In both cases, light experienced half
wavelength shift due to the inverted
wave from the bottom of the surface,
therefore create destructive
interference.

7. SOLUTIONS
• Case C:
( ) eliminated by destructive interference
( ✔ ) bright by constructive interference
• Case D:
( ) eliminated by destructive interference
(✔ ) bright by constructive interference
In case C, wave is inverted all the way
therefore no shift in wavelenght.
While in case D, neither wave is
inverted.

8. PROBLEM 1
An anti-reflective coat (n= 1.38) is used on a glass
surface.
a) Find the minimum thickness of the coating that can
be used for blue light (480 nm).
b) Find the next two possible thickness.

9. SOLUTION
• Visualize
• Anti-reflective coating = destructive interference
n=1.34
⟷t

10. SOLUTION
a) 𝜆n = 𝜆/n
𝜆n = 480/1.34 = 358 nm
2t= 𝜆n/2
t = 358/4 = 89.5 nm
b) Next two possible conditions,
2t= 3𝜆n/2 2t= 5𝜆n/2
t = 268.5 nm t= 447.5 nm
Note:We need extra phase
shift of 180o to create
constructive interference
therefore we need wall
thickness of 𝜆n/4, 3𝜆n/4, 5𝜆n/4,
and so on.