# Thin film interference and external / internal reflection

• Cocoleia
In summary, the conversation revolved around finding the phase difference in a thin film interference problem. The relationship between external and internal reflection was discussed, with the equations DOPL = 2dncosΘt and internal reflection meaning going from high index to low index. The concept of polarization of light was also mentioned. The conversation then focused on finding the relative phase in reflection from a single layer, with the suggestion to draw a picture and continue the path of reflection to a point that makes sense for comparison. Lastly, the importance of accounting for the index of refraction when determining the relative phase was emphasized.
Cocoleia

## Homework Statement

I am working on a thin film interference problem where i have to find the phase difference. What is the relationship between external/internal reflection and the thin film interference

for external:
[/B]

for internal:

## The Attempt at a Solution

So far i have that the DOPL=2dncosΘt
d being the thickness of the film

A picture might be helpful, in particular are we looking at reflection from a single layer or transmission?

Internal means going from high index to low index: i.e. Leaving the film going into air. External is the opposite. The equations you posted show the magnitude of how much is reflected vs transmitted. The parallel and perpendicular symbols refer to the polarization of the light: parallel or perpendicular to the plane of incidence. None of that has anything to do with finding the relative phase.

Let's suppose we are talking about reflection from a single layer. Draw a picture with the two possible paths: reflection from the top of the film and reflection from the bottom. We won't worry about multiple reflections for now. in drawing the picture remember that the light refracts going into the film. In setting the end points of the paths to compare, remember you have to get to a point where they are both back in air, parallel and equal distance from the assumed eventual destination. That is to say don't just stop the top path at the point of reflection. It has to be continued forward to a point that makes sense for comparison to the second path. Finally, when determining the relative phase don't forget that the path in the film accumulates more phase per distance traveled by the index of refraction.

Draw a picture and post it.

## What is thin film interference?

Thin film interference is a phenomenon that occurs when light waves reflect and interfere with each other as they pass through a thin film material. This results in the appearance of bright and dark bands, known as interference fringes, on the surface of the film.

## What causes thin film interference?

Thin film interference is caused by the reflection and interference of light waves that pass through a thin film material. When light waves reflect off of the top and bottom surfaces of the film, they can either constructively or destructively interfere with each other, resulting in the formation of interference fringes.

## What is external reflection?

External reflection is the phenomenon that occurs when light waves reflect off the surface of a material, such as a mirror or glass. This reflection follows the law of reflection, where the angle of incidence is equal to the angle of reflection.

## What is internal reflection?

Internal reflection is the phenomenon that occurs when light waves travel through a material and reach a boundary with another material with a lower refractive index. Instead of passing through the boundary, the light waves are reflected back into the material, creating an internal reflection.

## How is thin film interference used in practical applications?

Thin film interference has many practical applications, such as in anti-reflective coatings for glasses and camera lenses, thin film solar cells, and optical filters. It is also used in devices such as interferometers, which are used to measure small changes in distance or refractive index. Additionally, the study of thin film interference is crucial for understanding the properties of various materials and developing new technologies.

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