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Physics optics question - angles

If you continue the ray, total internal reflection would occur.
I don't understand why total internal reflection would occur - I can see that the angle of incidence is 30 degrees and the critical angle of glass-air boundary is around 37 degrees.

Am I missing something?

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Reply 1

RKM21

Ok, this is really easy and you'll kick yourself when you find out. If you continue the ray, what happens? It hits the glass on the inside (the line under figure 3). At what angle will it hit that line? Form a triangle with the normal of that line. 180-90-30 = 60 degrees.

As 60 is greater than the critical angle, TIR occurs.

Reply 2

Menoetius1

Original post by RKM21

Sorry, I am still confused lol
Could you draw a diagram and show me what you mean?

Reply 3

RKM21

Attachment not found

Try that.

(edited 7 years ago)

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Reply 4

Menoetius1

Thanks this helped alot, but how do you know you need that angle? Why couldnt it be 30? (on the other side, 90-60 = 30)

Reply 5

M0nkey Thunder

Original post by Menoetius1

Thanks this helped alot, but how do you know you need that angle? Why couldnt it be 30? (on the other side, 90-60 = 30)

You measure angles from the normal

Reply 6

Menoetius1

Original post by M0nkey Thunder

You measure angles from the normal

Could you give me an example of what you mean? I am really lacking some basic knowledge lol

Reply 7

M0nkey Thunder

Original post by Menoetius1

Could you give me an example of what you mean? I am really lacking some basic knowledge lol

Well the angle at which the line enters, the angle of incidence, is measured from the normal- hence why the angle they stated first was 56 and not 34. If you look at the diagram the other user made, you can work out what the other angle of incidence is. The principle is that if the angle of incidence is greater than the critical angle, total internal reflection occurs.

You go from the normal, up until you reach the ray and that is your angle of incidence.

(edited 7 years ago)

Reply 8

OGGUS

you meant to use Sin, that's all I remember, I'll try to get back to you asap, when I find out how to do it.

Reply 9

Ishan_2000

The angle of incidence is always measured between the incident ray and the normal! The normal is a dotted line drawn perpendicular to the surface the ray of light hits, or in this case, will hit, so the surface will be the top surface.

So, you construct a right angled triangle, with 30 degrees as the angle given to you in the question, 90 degrees as the bottom right angle of the triangle, and then 60 degrees as the final angle of the triangle! This is the angle of incidence, and therefore total internal reflection occurs, as 60 degrees is greater than the critical angle of the block.

Simply continue the ray of light until it hits the block. When it makes contact with the block, that is the second point of the triangle you will construct, with the first point being where this same ray of light entered the block. Finish this off with a right angle. Done.