Science Buddies: "Ask an Expert"

When light travles through different mediums does the frequency or wavelength change? If so are there some equations or method i could use to determine the frequency and wavelength? I'm using a laser for my experiment. Could i use a double slit?

Hi dbrown_318,

Two excellent resources that describe the issues you raise are:

http://www.physics.utoledo.edu/~lsa/_co ... action.htm
http://www.physics247.com/physics-homew ... vision.php

They include relevant equations. What is mentioned most is speed of light through the media, not frequency or wavelength.

double-slit does not appear to be necessary, but if anyone has another opinion, please pipe in. Thanks.

dbrown_318 wrote:When light travles through different mediums does the frequency or wavelength change? If so are there some equations or method i could use to determine the frequency and wavelength? I'm using a laser for my experiment. Could i use a double slit?

The color doesn't usually change unless you are going through an unusual medium (such as a frequency doubling crystal or some other 'exotic' nonlinear optical material).

There are a couple of experiments that use laser pointers and the principles of refraction in the physics section:

https://www.sciencebuddies.org/mentorin ... ?from=Home

https://www.sciencebuddies.org/mentorin ... ?from=Home

https://www.sciencebuddies.org/mentorin ... ?from=Home

What type of laser do you have?

Louise

There is also the concept of reflection to consider, and absorbtion.

If I shine a light on a black wall, the light does not pass through the medium at all. If I shine it on "mirror sunglasses", some of it reflects and some is passed through, and some is absorbed (if you leave it in the sun for a while it gets hot. That heat is absorbed light, not reflected and not transmitted)

Lasers are very useful for these kinds of studies because they are typically a narrow band of wavelengths and are visible to humans (unlike sunlight which has the whole spectrum, plus infrared and ultraviolet light).

Without lasers usually some kind of polarizing or refracting crystal is used to separate the light. With a laser, you can probably learn something about wavelength and frequency by mapping those to a color chart, and observing changes in color. It might be possible to use polarizing film designed to let certain wavelengths through, and see if it becomes "opaque" to the laser after it traveles through a medium.

In essence you need to find out what physical change, visible to you as experimenter, will happen if wavelength or frequency changes. Once you have figured that out, the rest of the experiment is relatively easy (shine the laser through different media, and see if the light changes in a way you can detect)

Just adding a few clarifications...

bradleyshanrock-solberg wrote: Lasers are very useful for these kinds of studies because they are typically a narrow band of wavelengths and are visible to humans (unlike sunlight which has the whole spectrum, plus infrared and ultraviolet light).

Lasers don't have to be visible to humans, and many are not. Quite a few are in the IR, and then may be frequency doubled to make them visible. This is actually the technology of a green laser pointer, which lases at ~1060 nm and then is put through a frequency doubling crystal to make 530 nm light. Also, they don't have to be visible to be useful, though obviously the narrow band does have a lot of utility for certain applications.

With a laser, you can probably learn something about wavelength and frequency by mapping those to a color chart, and observing changes in color.

Since a laser is narrow band, it is highly unlikely that you would see any color changes. Absorption will make the light less bright, but all the original components of the laser frequencies will be there (limited by the sensitivity of your detector... you could completely attentuate one component to the limit of detection). This is where white light (light from the sun) is more useful. Absorption can cause obvious color changes since you change the distrubtion of a whole bunch of frequencies. You cannot tell if a mixture of 532 nm and 531 nm light (green and green) is changing its ratio, but you could tell if a mixture of 700 nm and 532 nm (red and green) were changing its mixture (say by a material that absorbs all red light). Usually people who study absorption use white light sources (where white light = broad frequency bandwidth) coupled to a spectrograph or monochromator so they can look at absorption frequency by frequency.

It might be possible to use polarizing film designed to let certain wavelengths through, and see if it becomes "opaque" to the laser after it traveles through a medium.

Again, with a narrow band source, it is unlikley that you would find a "film" that could separate the frequencies of a laser. [A grating or a prism probably won't separate the frequencies sufficiently to be useful at home either] Secondly, since all the light of the laser is polarized the same way, all light from a laser source would interact similarly with the polarizer. Polarizing sunglasses work because the different sources of light outside have different polarization, so they are filtered differently through your glasses.

and see if the light changes in a way you can detect)

This is really the key point. The experiments I suggested are all doable with common equipment. With more sophiscated equipement, you can do more experiments.

Anyway, dbrown_318, let us know what you are thinking, and what kind of equipment you have and we will try to help. Lasers are cool, and I totally understand why you want to do a project with them!

Louise

dbrown_318 wrote:When light travles through different mediums does the frequency or wavelength change?

The relationships among wavelength, frequency, and refractive index of different mediums is summarized very clearly and concisely on this web page:
http://www.rpi.edu/dept/phys/Dept2/APPh ... node7.html

key points:
"As light moves from air into water, it not only slows, but the wavelength changes."
"The frequency of the waves does not change as the light moves from air to water."

ChrisG wrote:

dbrown_318 wrote:When light travles through different mediums does the frequency or wavelength change?

The relationships among wavelength, frequency, and refractive index of different mediums is summarized very clearly and concisely on this web page:
http://www.rpi.edu/dept/phys/Dept2/APPh ... node7.html

key points:
"As light moves from air into water, it not only slows, but the wavelength changes."
"The frequency of the waves does not change as the light moves from air to water."

Nice page Chris. I couldn't get the animation to work though. Just remember dbrown_318, as soon as the beam exits the material, you are right back to your original color (the air color). So, you would not be able to tell if a your laser beam had passed through a glass of water (for example) by looking at its color. So, no permanant color change to measure.

I'm thinking that an experiment on either Raman or Fluorescence might be interesting to do, and most similar to your interests in changing colors of light, if the refractive indices projects are not interesting to you.

The wikipedia article on Raman Scattering is pretty good. Raman was able to observe the effect using sunlight and colored filters, so I think could do this.
http://en.wikipedia.org/wiki/Raman_scattering
http://www.uky.edu/~holler/raman.html

The article in the wikipedia has a few mistakes, but is generally okay:
http://en.wikipedia.org/wiki/Fluorescence

Developing a question that you could easily do at home with these effects might be difficult. Both Raman and Fluorescence are widely used in research (in the field of spectroscopy), but without access to a lot of equipment, standard research questions are not easy for you to address. Most questions I can think of would be purely observations (i.e. 'what common household objects are fluorescent') and not really hypothesis driven research.

Louise