Understanding Phase Shift Upon Reflection: A Closer Look at Fresnel's Equations

In summary, when light reflects off of a surface it undergoes a phase change. The phase change is dependent on the difference between the distances between the reflecting surface and the input surface.
  • #1
dimensionless
462
1
Let's say I have light at normal incidence. Under what circumstances is there a phase shift? Under what circumstances is there no phase shift? My best guess is that there is normally a phase shift of 180 degrees. The exception is when n_incident > n_reflected, but I don't really know.

To elaborate more, let's say I have monochormatic light normaly incident on a film. Why do I get a maximum when the film thickness is [tex]d = \frac{\lambda_0}{4n}[/tex] and a minimum when the film thickness is [tex]d = \frac{2\lambda_0}{n}[/tex].
 
Last edited:
Physics news on Phys.org
  • #2
dimensionless said:
Let's say I have light at normal incidence. Under what circumstances is there a phase shift? Under what circumstances is there no phase shift? My best guess is that there is normally a phase shift of 180 degrees. The exception is when n_incident > n_reflected, but I don't really know.
Right. When light goes from one medium ([itex]n_1[/itex]) to another ([itex]n_2[/itex]), the reflected light at that interface undergoes a phase change as follows:
if [itex]n_1 < n_2[/itex]: 180 degree phase change
if [itex]n_1 > n_2[/itex]: no phase change​

To elaborate more, let's say I have monochormatic light normaly incident on a film. Why do I get a maximum when the film thickness is [tex]d = \frac{\lambda_0}{4n}[/tex] and a minimum when the film thickness is [tex]d = \frac{2\lambda_0}{n}[/tex].
Looks like you are talking about a situation, like a soap film in air, where [itex]n_1 < n_2 > n_1[/itex]. There are two reflections: the first has phase change; the second does not. So if the optical path length through the film is 1/2 [itex]\lambda[/itex] (your first example), then the total phase difference between the reflections is zero and you get maximum constructive interference. Similarly, if the optical path length is an integral number of wavelengths (as in your second example), the net phase difference is 180 degrees: maximum destructive interference.
 
  • #3
I have a question that is somewhat related. Regarding the phase shift upon reflection, how do you show that it is 180 degrees when n1 < n2 and 0 degrees when n1 > n2 using Fresnel's equations? Something along the lines of an informal proof.
 

1. What is phase shift upon reflection?

Phase shift upon reflection is the change in the phase of a wave when it is reflected off a surface. The phase shift can be either positive or negative, depending on the properties of the surface and the angle of incidence of the wave.

2. What causes phase shift upon reflection?

The main cause of phase shift upon reflection is the change in the medium that the wave is traveling through. When a wave hits a surface, it changes from one medium to another, which can result in a change in the speed of the wave. This change in speed causes a change in the wavelength and thus a phase shift.

3. How does the angle of incidence affect phase shift upon reflection?

The angle of incidence of the wave has a significant impact on the phase shift upon reflection. As the angle of incidence increases, the amount of phase shift also increases. This is because the change in the medium that the wave is reflecting off becomes more significant at larger angles of incidence.

4. Is phase shift upon reflection always the same for all types of waves?

No, the amount of phase shift upon reflection can vary depending on the type of wave. For example, electromagnetic waves experience a different phase shift than mechanical waves like sound waves. Additionally, the properties of the reflecting surface can also affect the amount of phase shift.

5. How is phase shift upon reflection used in practical applications?

Phase shift upon reflection is utilized in various technologies, such as radar and sonar systems. These systems use the phase shift of reflected waves to determine the distance and location of objects. It is also used in optics to manipulate light and create interference patterns in devices like mirrors and lenses.

Similar threads

Replies
2
Views
1K
Replies
8
Views
1K
Replies
2
Views
2K
  • Advanced Physics Homework Help
Replies
1
Views
2K
  • Optics
Replies
4
Views
1K
  • Introductory Physics Homework Help
Replies
2
Views
2K
Replies
4
Views
8K
  • Introductory Physics Homework Help
Replies
7
Views
5K
  • Introductory Physics Homework Help
Replies
6
Views
3K
Replies
12
Views
1K
Back
Top