Determining the the index of refraction

[kubajed]

Homework Statement

I must determine the refractive index of the adhesive tape. Tools I can have is: laser, tape, carton, scissors etc.
I don't know how to do it.

Homework Equations

n=c/v
n=sin alfa/sin beta

The Attempt at a Solution

I tried a lot of configurations to observe something but without effect

 

[Bystander]

https://en.wikipedia.org/wiki/Abbe_refractometer

 

[kubajed]

I can't use any tools except that I wrote in first post.

 

[Merlin3189]

Perhaps you could tell us about the methods you have tried?

I've never done this and don't know the answer.
I'd have thought about other experiments to measure thin layers.
Since you have a laser, I'd also try to think of how refractive index might affect a laser beam.

 

[berkeman]

I can't use any tools except that I wrote in first post.

But using the link posted by @Bystander and related links at wikipedia like this one:

https://en.wikipedia.org/wiki/Refractive_index

What kinds of things could you try to do to get the laser passing through the tape to give you something useful...?

 

[kubajed]

I tried to get stripes on carton. But it doesn't work.

 

[berkeman]

I tried to get stripes on carton. But it doesn't work.

I don't understand what you mean by that. Can you give a more complete description of what you are trying to do?

And does this image help at all? (although the thin sample you have may require special tricks to get good numbers...)

https://upload.wikimedia.org/wikipe...fraction_photo.png/220px-Refraction_photo.png

 

[kubajed]

Stripes like using diffraction grating, but it was stupid. I know that outgoing beam is moved in comparison to incoming beam. But the problem is small thickness of tape.

 

[berkeman]

I know that outgoing beam is moved in comparison to incoming beam. But the problem is small thickness of tape.

So can you think of ways you could amplify the small displacement from the thin dielectric layer? How transparent is the tape?

Since you have a laser, I'd also try to think of how refractive index might affect a laser beam.

In addition to diffraction, what other property of light is affected by a change in the dielectric constant as the light beam transitions from one medium to another?

 

[berkeman]

So can you think of ways you could amplify the small displacement from the thin dielectric layer? How transparent is the tape?

Can you post a picture of the tape, or link to the product? There is another trick that you can use to get around the thinness of the tape sample...

Like: https://images-na.ssl-images-amazon.com/images/I/41wtuh6hckL._SY355_.jpg

 

[kubajed]

I can use a lot of connected layers but transparency is so low then.

 

[berkeman]

I can use a lot of connected layers but transparency is so low then.

That picture holds the key to the trick I was thinking of. Did you ever see the old Star Trek movie "The Wrath of Khan"?

 

[kubajed]

No, I didn't

 

[berkeman]

No, I didn't

Fair enough. I'm trying to think of a way to give you a hint for the trick, without just giving it straight away...

Here is the quote I was going to refer to to try to get you to think differently about the problem...

 

[kubajed]

Do you suggest to experiment with 3D?

 

[kubajed]

Could you tell me a little bit more?

 

[berkeman]

Do you suggest to experiment with 3D?

Not exactly. The point is to think of other geometries that you could use to take care of the problem of having such a thin sample.

 

[berkeman]

Back when I was in high school, I found a short book on problem solving (I wish I could remember the title). It gave lots of tips and tricks for solving all kinds of problems, as well as general techniques for ways to think about problems. One of the best tips was when you were presented with a difficult problem that had obstacles to its solution, think about ways to turn things around up upside-down or some other way (physically or conceptually) to see if those alternative views gave any clues to ways to get past the obstacles.

I believe that I came up with a good trick for you to use to do this experiment, and I used that tip from the book on problem solving to get to it. Hope that helps.

 

[sophiecentaur]

When you learn about refractive index, there is something else that can happen when a light ray passes from one medium to another, other than just bending the ray. Go about a page beyond the first page in the book.

 

[kubajed]

The laser beam is also reflected. It is it?

 

[sophiecentaur]

What are the conditions for that to happen? Does the beam need to travel 'into' the other medium? Does it need to be thick?

 

[kubajed]

It can only follow the bonduary of medium if angle is critical angle

 

[sophiecentaur]

So.. . . ? You don't need many more clues, do you?

 

[kubajed]

So I can measure this angle. Thanks.
But it will be very small?

 

[sophiecentaur]

It will depend on the index of the substance it's exiting from. Look up the value of various plastics and work out the critical angles from glass. (Check that mu for glass is higher!)

 

[sophiecentaur]

But it will be very small?

Why "small"? For substances of similar densities, C will be quite large. Calculations (above) will show you.

 

[kubajed]

I try to measure this angle, but for each angle (0-90deg) there is still a reflected and passing-through beam.

 

[sophiecentaur]

I try to measure this angle, but for each angle (0-90deg) there is still a reflected and passing-through beam.

Did you look up Total Internal Reflection? That is what I was referring to.

 

[kubajed]

Yes, I saw that. But there is no that angle

 

[sophiecentaur]

Yes, I saw that. But there is no that angle

It depends how you are doing your experiment. What is the basic requirement for TIR?? (Does it work both ways?
Clue: Look up Abbe Refractometer. It's an old fashioned instrument for measuring the mu of minute quantities of substances.

 

[kubajed]

These:

• the light is in the more dense medium and approaching the less dense medium.
• the angle of incidence is greater than the so-called critical angle.

 

[sophiecentaur]

SO which two media are you using with your experiment?

 

[kubajed]

The tape and air. I can't use any others media except plastic of cm ruler.

 

[berkeman]

The laser beam is also reflected. It is it?

Yes, and what does the magnitude of the reflection versus the magnitude of the transmitted beam depend on? (Also watch out for multiple reflections if you use this technique...)

 

[sophiecentaur]

The tape and air. I can't use any others media except plastic of cm ruler.

Are you sure that nothing else is available? School labs always have glass blocks.
But you could perhaps stick the tape on a flat surface and see if your laser beam can be ducted inside the tape ( entry via a cut end of the tape. - like in an optical fibre.)
Light would escape from the surface near the cut end but it would be cut off at greater distances, past the Critical Angle due to TIR. If this doesn't make sense,look up optical fibres and "light pipes".

 

[sophiecentaur]

Yes, and what does the magnitude of the reflection versus the magnitude of the transmitted beam depend on? (Also watch out for multiple reflections if you use this technique...)

He has no equipment for that sort of measurement. Real table top exercise.

 

[kubajed]

I can put the tape on white carton.

 

[sophiecentaur]

I can put the tape on white carton.

Something black may be better, if poss. You would not want to see any other light if poss. You need to play with the equipment and see how the light behaves. If you could sandwich the end of the tape between two narrow 'jaws' you could restrict the entry of light to what you need.

 

[kubajed]

I asked my teacher and I can use only white carton. Can you paint it?

 

[sophiecentaur]

I had a very casual go with a (too powerful) green laser and some sticky tape. I couldn't see any effect when shining the laser into one edge. No obvious angle at which light stopped emerging from the face of the tape.
The problem could be that the tape is just not optically flat enough. Then there's the side with adhesive on.
A good clean cut might help, to keep the angle of incidence the same all over the way in.
Sticky tape is such a rubbish substance to be analysing. You have my sympathy. I wonder if your teacher has a method that actually works , haha!

 

[kubajed]

You aim the laser on the cutted edge? So in your opinion it is impossible in this conditions?

 

[sophiecentaur]

You aim the laser on the cutted edge? (YES as with an optical fibre) So in your opinion it is impossible in this conditions?

No. That's going too far. I am saying that I had a problem in an initial attempt. I just held it all, unsteadily in my hand and made no special preparation.
Lessons I learned:
Cut the end of the tape carefully (razor blade / scalpel) to get a flat end.
Mask off the light source so that it only gets through the end of the tape and not round the sides.
Fix it all down on the bench.
Perhaps clean all the adhesive off the surface.
Use an appropriate strength of laser (yours will be 'legal').
Given time and motivation, I think I would make progress with the method. The main thing, I think, is to restrict light getting in, so you can be sure that what you are seeing is between the faces of the tape.
The method would work fine if you could use a block of high density glass (with a high refractive index) and clamp the clean surface of the tape to the block. Then you could shine the beam through the glass and easily see when you get TIR. It wouldn't be unreasonable to approach the teacher about this and give your reasons. He may have anpther method in mind but I can't think what it could be, with such a thin sample.
He / she may consider it OK to make up a thick block with many layers of tape and to measure angles i and r but the dispersion at each layer of the 'sandwich' is bound to spread the beam more and more, introducing a lot of uncertainty in the angle measurement.
PS Giving you a plastic cup may imply a suggestion that you use Water as the dense medium (Sneaky! lol), in which case you could immerse the tape (stuck to a flat surface) and find the critical angle for Water / plastic when the beam starts to be reflected off the tape surface. This will only work if the mui of the plastic is less than the mu of water. Worth having a go, perhaps.

 

[ehild]

Homework Statement

I must determine the refractive index of the adhesive tape. Tools I can have is: laser, tape, carton, scissors etc.

What are those etc-s?
You need to measure something anyway. What are you allowed to measure? Lengths? angles?
If you have a polarized laser, you can find the Brewster angle of the tape, with p-polarized light.

 

[kubajed]

I can use: tape, laser (unpolarized), white carton, plasticine, ruler, measuring tape, scissors. And only these. Has someone another idea?

 

[sophiecentaur]

I can use: tape, laser (unpolarized), white carton, plasticine, ruler, measuring tape, scissors. And only these. Has someone another idea?

If they stop you using water, too, they may as well tell you not to use electricity! lol
I still like the Critical Angle idea with the tape under water. Have you tried it?
PS This sort of investigation really needs lots of hands-on time. No amount of talking about it and reading what PF has to say, will give you the progress that actual practical messing about can. You don't even need the laser to do a lot of this at home.

 

[sophiecentaur]

BTW I just went to the trouble of pointing a laser beam into the periphery of a roll of adhesive tape. The beam is definitely bent towards the normal but there is a curve involved so the actual angle is a bit uncertain. Squashing the roll in a vise could produce parallel layers. There is also the problem of the laminated structure and the fact that there is glue in between layers. But it could be a start.