Interference and Diffraction of Light

[cdhotfire]

I didnt go to school today, and so missed day of class, now i all lost someone help.

White light is incident normal to the surface of the film as shown below. It is observed that at a point where the light is incident on the film, light reflected from the surface appears green (lamda = 525 nm)


|
|--Light
|Air $n_{a}=1.00$
-------------------------
Film $n_{f}=1.38$
-------------------------

Glass $n_{s}=1.50$
-------------------------
Air $n_{a}=1.00$

i. What is the frequency of the green light in air?
ii. What is the frequency of the green light in the film?
iii. What is the wavelenght of the green light in the film?
iv. Calculate the minimum thickness of film that would produce this green reflection.


I know that the light will reflect off when it hits the film, and again when it hits the glass. Thats about all i can see here. [itex]f=\frac{v}{lamda}[/atex] i have the lamda outside but that's it. Can anyone help i don't want to get behind.

 

[xanthym]

I didnt go to school today, and so missed day of class, now i all lost someone help.

White light is incident normal to the surface of the film as shown below. It is observed that at a point where the light is incident on the film, light reflected from the surface appears green (lamda = 525 nm)


|
|--Light
|Air $n_{a}=1.00$
-------------------------
Film $n_{f}=1.38$
-------------------------

Glass $n_{s}=1.50$
-------------------------
Air $n_{a}=1.00$

i. What is the frequency of the green light in air?
ii. What is the frequency of the green light in the film?
iii. What is the wavelenght of the green light in the film?
iv. Calculate the minimum thickness of film that would produce this green reflection.


I know that the light will reflect off when it hits the film, and again when it hits the glass. Thats about all i can see here. [itex]f=\frac{v}{lamda}[/atex] i have the lamda outside but that's it. Can anyone help i don't want to get behind.

ITEM #1:
What is the frequency of the green light in air?
v = f*λ
::: ⇒ f = v/λ
::: ⇒ f = (3*10⁸ m/sec)/(525*10^(-9) m)
::: ⇒ f = (5.714*10¹⁴ Hz)

ITEM #2:
ii. What is the frequency of the green light in the film?
In the film, velocity & wavelength change. Frequency does not. Frequency is same answer prev item:
::: ⇒ f = (5.714*10¹⁴ Hz)

ITEM #3:
iii. What is the wavelength of the green light in the film?
λ_film = λ_air*n_air/n_film
::: ⇒ λ_film = (525 nm)*(1)/(1.38)
::: ⇒ λ_film = (380.4 nm)

ITEM #4:
Calculate the minimum thickness of film that would produce this green reflection.
The green reflection is caused by CONSTRUCTIVE interference between the ray reflected from the film's top surface with that reflected from the film's bottom surface. Both these rays undergo 180 deg phase shifts from the incident rays at their respective interfaces, so NO net phase shift results from the reflections themselves. However, a phase difference can occur from the added path length traveled by the ray in the film:
{Path Length of Ray in Film} = 2*{Film Thickness}
The minimum film thickness for CONSTRUCTIVE interference occurs when the film ray's path length is exactly {(1.0)*λ_film}. Placing this value into the above equation and solving for minimum film thickness:
{Path Length of Ray in Film} = {(1.0)*λ_film} = 2*{Minimum Film Thickness}
::: ⇒ 2*{Minimum Film Thickness} = (380.4 nm)
::: ⇒ {Minimum Film Thickness} = (190.2 nm) = (0.1902 um) = (1.902*10^(-7) m)


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[cdhotfire]

thank you very much, would of helped if it was yesturday, i figured it out later on, but many thanks for helping.