Determining the the index of refraction

[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.