# Total internal reflection

• B

## Main Question or Discussion Point

So I just very recentlyearned about total internal reflection, which basically states that when the incident angle is large enough, the light will reflect back into the medium. I have a few questions:
Say we have a set up with a tank of water and a laser pointer in it
1) does this mean that if you try to look at the light source from outside the medium, would you be able to see the light source?
2) if the answer is no, then what if you put a light bulb in the water so that the light spreads out in all directions rather than just in one line. Would you only be able to see the light from the light bulb in a circular region above the water where the incident angle is less than the critic angle?

My bad If the questions don't make sense, my knowledge of the subject is hardly rigorous considering I learned it from a prep book for Sat subject tests

Related Other Physics Topics News on Phys.org
Drakkith
Staff Emeritus
1) does this mean that if you try to look at the light source from outside the medium, would you be able to see the light source?
That's right. Just remember never to look directly into the path of a laser beam, even if you think it you're safe.

2) if the answer is no, then what if you put a light bulb in the water so that the light spreads out in all directions rather than just in one line. Would you only be able to see the light from the light bulb in a circular region above the water where the incident angle is less than the critic angle?
Think about fish inside of fish tanks. Is there any angle from which a fish would suddenly seem to disappear?

Think about fish inside of fish tanks. Is there any angle from which a fish would suddenly seem to disappear?
I've never had one, but Id assume there is such an Angle

Drakkith
Staff Emeritus
I've never had one, but Id assume there is such an Angle
Oh. Well, you're right. Beyond a certain angle the light is simply reflected back down into the water. Take a look at something under the water next time you get in a pool. You should be able to notice it disappear as you move away and the viewing angle increases.

jbriggs444
Homework Helper
2019 Award
Oh. Well, you're right. Beyond a certain angle the light is simply reflected back down into the water. Take a look at something under the water next time you get in a pool. You should be able to notice it disappear as you move away and the viewing angle increases.
This is not correct. The image of objects in the water will become more distorted as the viewing angle approaches the horizontal, but it never disappears from view. Trace some rays. There is no forbidden angle on the air side. There are forbidden angles on the water side.

I see what you mean. What if you used like a tube of some sort to limit your line of sight only directly toward the light bulb, and have the tube end just above the water?

Drakkith
Staff Emeritus
This is not correct. The image of objects in the water will become more distorted as the viewing angle approaches the horizontal, but it never disappears from view. Trace some rays. There is no forbidden angle on the air side. There are forbidden angles on the water side.
How so? TIR happens when the rays try to exit the water, not the other way around.

Drakkith
Staff Emeritus
I see what you mean. What if you used like a tube of some sort to limit your line of sight only directly toward the light bulb, and have the tube end just above the water?
Wouldn't change anything. The light would still reflect off of the surface of the water in the tube.

How so? TIR happens when the rays try to exit the water, not the other way around.

I think he means that you can't see the light bulb if you stare directly at where the light bulb is. however, if you extend all the refracted waves, it's clear that one of the light rays from the source will still reach your eyes, it'll just be quite horizontal.

Drakkith
Staff Emeritus