Thin films: reflection between glass plates

In summary: C). In summary, when considering the interference of light in a set up with two parallel glass plates separated by an air film, the condition for maximum intensity is given by 2nd=mnλ, where d is the thickness of the air film, n is the refractive index of the glass, and m is an integer.
  • #1
vetgirl1990
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Homework Statement


Two parallel glass plates of index of refraction n
are separated by an air film of thickness d. Light of wavelength λ
in air, normally incident on the plates, is intensified
on reflection when, for some integer m

a) 2d=mλ
b) 2d=mλ/n
c) 2d=mnλ
d) 2d=(m+1/2)λ
e) 2d=mλ/2

Homework Equations


If only one ray is π shifted: 2nd=mλ (min), 2nd=(m+1/2)λ (max)

If both rays π shifted: 2nd=mλ (max), 2nd=(m+1/2)λ (min)

The Attempt at a Solution


I've attached a diagram of two different for how I picture light should be transmitted in this problem. I'm not sure which one would be a correct approach.

Going with the first diagram,
This would result in a net π shift, so it's conditions would be 2nd=mλ (min) and 2nd=(m+1/2)λ (max).

I chose answer (B) 2d=mλ/n, since the question asks for "intensity increase" so I am assuming it is asking about the condition for a bright fringe; and also because it's one of the only options that takes the index of refraction (n) into account.

I would just like to verify that my reasoning is correct.
 

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  • #2
vetgirl1990 said:

Homework Statement


Two parallel glass plates of index of refraction n
are separated by an air film of thickness d. Light of wavelength λ
in air, normally incident on the plates, is intensified
on reflection when, for some integer m

a) 2d=mλ
b) 2d=mλ/n
c) 2d=mnλ
d) 2d=(m+1/2)λ
e) 2d=mλ/2

Homework Equations


If only one ray is π shifted: 2nd=mλ (min), 2nd=(m+1/2)λ (max)

If both rays π shifted: 2nd=mλ (max), 2nd=(m+1/2)λ (min)

The Attempt at a Solution


I've attached a diagram of two different for how I picture light should be transmitted in this problem. I'm not sure which one would be a correct approach.

Going with the first diagram,
This would result in a net π shift, so it's conditions would be 2nd=mλ (min) and 2nd=(m+1/2)λ (max).

I chose answer (B) 2d=mλ/n, since the question asks for "intensity increase" so I am assuming it is asking about the condition for a bright fringe; and also because it's one of the only options that takes the index of refraction (n) into account.

I would just like to verify that my reasoning is correct.

You are wrong . The light interference happens because of multiple reflection inside the thin air film between the thick glass plates. d is the thickness of the air film. n is the refractive index of the glass. You can consider the set-up that the air film is surrounded by glass and the light arrives to the glass-air boundary from glass, and reflected from the other side of the air film, from glass.

The
 

FAQ: Thin films: reflection between glass plates

1. What are thin films?

Thin films are a type of material that has a thickness of only a few nanometers to micrometers. They are often used in various applications, such as coatings, electronic devices, and optical components.

2. How are thin films created?

Thin films can be created through various methods such as chemical vapor deposition, sputtering, and spin coating. These methods involve depositing a thin layer of material onto a substrate, such as glass plates.

3. What is the purpose of using thin films between glass plates?

The purpose of using thin films between glass plates is to create a reflective surface. When light passes through the glass plates, it reflects off the thin film, creating interference patterns that can be used for various applications, such as anti-reflective coatings or optical filters.

4. How does the thickness of the thin film affect the reflection between glass plates?

The thickness of the thin film determines the wavelength of light that is reflected. When the thickness is equal to a specific wavelength of light, constructive interference occurs, resulting in a bright reflection. If the thickness does not match a specific wavelength, destructive interference occurs, resulting in little to no reflection.

5. What are some practical applications of thin films between glass plates?

Thin films between glass plates have various practical applications, including anti-reflective coatings on eyeglasses, touchscreens, and solar panels. They are also used in optical filters, such as in cameras and telescopes, to selectively filter out certain wavelengths of light.

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