Reflection of light off film, wavelength and thickness?

In summary, the question is about the thinnest film of MgF2 on glass with a refractive index of 1.39 that can produce strong reflection for light with a wavelength of 680 nm. The solution involves using the ratio of refractive indices and wavelengths to find the original wavelength. Further assistance can be found in textbooks or online resources on thin film interference.
  • #1
mithraz87
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0
What is the thinnest film of MgF2 (n = 1.39) on glass that produces a strong reflection for light with a wavelength of 680 nm?

This is the exact question I am faced with. I didn't really know where to begin so I tried using the ratio of n-air/wavelength to n-film/(new wavelength) to try and find the original wavelength. I found that, and really didn't know where to go then. I have looked around, and I just can't seem to find anything. Any help would be greatly appreciated.
 
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  • #2
Hi mithraz87,

mithraz87 said:
What is the thinnest film of MgF2 (n = 1.39) on glass that produces a strong reflection for light with a wavelength of 680 nm?

This is the exact question I am faced with. I didn't really know where to begin so I tried using the ratio of n-air/wavelength to n-film/(new wavelength) to try and find the original wavelength. I found that, and really didn't know where to go then. I have looked around, and I just can't seem to find anything. Any help would be greatly appreciated.

From your post, I was wondering if you asking what type of problem this is? This problem is dealing with the interference by thin film; I would think that your textbook has at least a section devoted to the topic with some examples.

You can also look at http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/thinfilm.html#c1

Does that help? If you are still stuck, post what you have tried.
 
  • #3


I would approach this question by first understanding the basic principles of reflection and thin film interference. When light strikes a film, part of it is reflected off the surface while the rest is transmitted through the film. The reflected light waves can interfere with each other, leading to constructive or destructive interference depending on the thickness of the film and the wavelength of light.

In this case, we are dealing with a thin film of MgF2 on glass. The key information we have is the refractive index of MgF2 (n = 1.39) and the wavelength of light (680 nm). We also know that for strong reflection, we need constructive interference, which means the thickness of the film must be an integer multiple of half the wavelength of light.

To determine the thinnest film that would produce strong reflection at 680 nm, we can use the equation for thin film interference:

2nt = mλ

Where:
n = refractive index of the film
t = thickness of the film
m = order of interference (integer)
λ = wavelength of light

Since we want the thinnest film, we can assume m = 1. Rearranging the equation, we get:

t = (mλ)/(2n)

Substituting the values, we get:

t = (1 x 680 nm)/(2 x 1.39)

t = 245.3 nm

Therefore, the thinnest film of MgF2 on glass that would produce strong reflection at 680 nm is 245.3 nm thick.

Note: This calculation assumes that the film is surrounded by air. If the film is on a glass substrate, we would need to account for the refractive index of glass as well.
 

1. How does the thickness of a film affect the reflection of light?

The thickness of a film directly affects the amount of light that is reflected off of its surface. Thicker films will reflect more light, while thinner films will reflect less light. This is due to the interference of light waves as they travel through the film and interact with each other.

2. What is the relationship between the wavelength of light and its reflection off of film?

The wavelength of light plays a significant role in the reflection off of film. When the thickness of a film is close to the wavelength of the incident light, constructive interference occurs, resulting in a strong reflection. However, when the thickness is much smaller than the wavelength, destructive interference occurs, leading to a weaker reflection.

3. Can different types of film have an impact on the reflection of light?

Yes, the material and properties of the film can affect the reflection of light. For example, a film with a higher refractive index will result in a stronger reflection than a film with a lower refractive index. Additionally, the surface texture and smoothness of the film can also affect the reflection of light.

4. How can the reflection of light off film be manipulated?

The reflection of light off of film can be manipulated by changing the thickness of the film, the angle of incidence of the light, and the type of film used. By carefully controlling these variables, scientists and engineers can design films with specific reflective properties for various applications, such as anti-reflective coatings or mirrors.

5. Is the reflection of light off film the same as the reflection off of a mirror?

No, the reflection of light off of film and a mirror are different phenomena. Mirrors have a smooth, flat surface that results in specular reflection, where the angle of incidence is equal to the angle of reflection. Film, on the other hand, has a textured surface that can lead to diffuse reflection, where light is scattered in various directions. Additionally, mirrors have a high reflectivity, while film can have varying levels of reflectivity depending on its thickness and properties.

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