Measuring Wave Length of Light Through Different Substances

[Elbraido]

Hi this is my first question and love these Forums.
I have a Independent research task due next term.
My idea was:
Seeing the change of wave length of light as it passes through different substances.
My question is:
How do i measure the wave length? Because I could use a spectrometer but my teacher told me that as the light leaves the substance such as glass it returns back to its orignal wavelength. This my biggest obstacle and I would find a solution really usefull.

Thanks

 

[Phylosopher]

I am not sure you can directly measure the wavelength of light! Maybe other members have a solution to this problem.

See, the problem is that, if you want to measure the wavelength, say with an electronic device. You are ought to measure energy. But the energy of light doesn't change, since the frequency doesn't change as well. Thus any method of measuring energy will be futile, and will require knowing the index of refraction of the material before hand (or the speed of light in the material).

If you are going to use the idea of refraction, you will think that you are measuring the wavelength directly. But you are actually measuring the index of refraction, and then applying it to the velocity of light in the medium and so finding the wavelength.

I am not sure there is a direct method to measure the wavelength. I would be interested if anyone have an idea.

It looks to me that index of refraction is somehow synonym in a physical way to the wavelength and the speed of light in general^^"

 

[Phylosopher]

Giving this extra thought.

Index of refraction is merely the permittivity and the permeability of the material, which itself defines the speed of light in the material, which itself related to the wavelength.

Are we having a chicken and egg problem!

 

[sophiecentaur]

Are we having a chicken and egg problem!

It's not as bad as that. Snell's Law tells you the proportional change in wavelength of the light if you just measure the angles of incidence and refraction.
There are a number of ways of finding the wavelength (in air) of the light you are using. Easy way would be to compare the colour of the light with the colours on an image of the optical spectrum with a rough wavelength scale on it (Google will deliver that). Harder, more fundamental way would be to use Interference with Young's Slits. If you have done this in your course, it will be familiar, else you need to Google it. Basically, when light (monochromatic, through a filter or from a laser is best) passes through two narrow slits, a fringe pattern is formed on a screen. Measuring the fringe width, the distance from the screen and the separation of the slits will allow you to deduce the wavelength of the light. This method allows you to deduce an 'invisible' distance by using three very 'visible' distances. A suitable pair of slits is usually available in Physics prep rooms. Alternative methods include using the lines on a CD as a cheap diffraction grating. But that's another layer of complexity.

 

[Phylosopher]

Harder, more fundamental way would be to use Interference with Young's Slits.

Oh, I didn't think about this!

Easy way would be to compare the colour of the light with the colours on an image of the optical spectrum with a rough wavelength scale on it (Google will deliver that).

I am skeptical, or maybe I didn't understood what you wrote correctly.

How would we compare? If we have a compartment that contains a certain medium, if we are outside the compartment, the color that we are going to see wouldn't have the same wavelength. So, a comparison will not tell us anything.

Furthermore, even if we are inside the medium. The apparatus that we use (The human eye) have a certain refraction index. So, I guess we will see the same colors that we see outside the medium anyway, without that much shift in the wavelength.

 

[sophiecentaur]

How would we compare? If we have a compartment that contains a certain medium, if we are outside the compartment, the color that we are going to see wouldn't have the same wavelength. So, a comparison will not tell us anything.

Of course you cannot 'see' the light that's inside the glass but (an aside) its 'colour' relates to its frequency and that wouldn't ever change. Snell's law would tell you the amount the wavelength changes in the glass. The change in direction of a beam is a direct consequence of the change in wavelength at the interface.

 

[spareine]

I have a Independent research task due next term. My idea was: Seeing the change of wave length of light as it passes through different substances.

I guess the simplest experiment is to place a convex lens on top of a flat surface and to look at Newton's rings, with different fluids between the two surfaces.

 

[berkeman]

My idea was:
Seeing the change of wave length of light as it passes through different substances.

Welcome to the PF.

What will be your source of monochromatic light? That will affect what methods you can choose from.

 

[Elbraido]

Welcome to the PF.

What will be your source of monochromatic light? That will affect what methods you can choose from.

I ll probably use some sort of laser that will have a lable that will tell me its frequency so its easier to calculate with maths.

 

[Elbraido]

I guess the simplest experiment is to place a convex lens on top of a flat surface and to look at Newton's rings, with different fluids between the two surfaces.

What would i do next?

 

[sophiecentaur]

What would i do next?

Look up Newton's Rings on Hyperphysics and there will be a description and a formula. (Google it there)

 

[ZapperZ]

OK, I am truly puzzled on why this is so difficult.

Here's the sequence of logic: wavelength in a medium -> speed of light in the medium -> index of refraction in the medium.

In other words, if I can find the index of refraction in the medium for the light wavelength or frequency that I'm using, then I can find the wavelength of light inside that medium. I will let you figure out the physics and math involved.

But to find the index of refraction inside a medium isn't that difficult by applying Snell's law. If you have a rectangular block, then do something like this:

https://www.physicsforums.com/threads/snells-law-experiment-with-glass-block.850964/#post-5336762

Find the entry point of the light beam, and find the exit point of the light beam, and you will have the angle that the light beam has been refracted inside the medium. Voila! You can easily find the index of refraction of the light beam inside the block!

Problem solved!

Zz.

 

[berkeman]

OK, I am truly puzzled on why this is so difficult.

I get the impression that he is supposed to do this without using Snell's law. But the OP hasn't really given many details...

 

[Elbraido]

OK, I am truly puzzled on why this is so difficult.

Here's the sequence of logic: wavelength in a medium -> speed of light in the medium -> index of refraction in the medium.

In other words, if I can find the index of refraction in the medium for the light wavelength or frequency that I'm using, then I can find the wavelength of light inside that medium. I will let you figure out the physics and math involved.

But to find the index of refraction inside a medium isn't that difficult by applying Snell's law. If you have a rectangular block, then do something like this:

https://www.physicsforums.com/threads/snells-law-experiment-with-glass-block.850964/#post-5336762

Find the entry point of the light beam, and find the exit point of the light beam, and you will have the angle that the light beam has been refracted inside the medium. Voila! You can easily find the index of refraction of the light beam inside the block!

Problem solved!

Zz.

Thanks for the help