Can light escape the medium when total internal reflection occurs?

Click For Summary

Discussion Overview

The discussion revolves around the phenomenon of total internal reflection (TIR) in the context of light behavior in water. Participants explore questions regarding visibility of light sources from outside the medium, particularly focusing on scenarios involving a laser pointer and a light bulb submerged in water.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant questions whether a light source, such as a laser pointer, can be seen from outside the water when TIR occurs.
  • Another participant suggests that visibility may depend on the angle of observation, comparing the scenario to fish in a tank.
  • Some participants assert that beyond a certain angle, light reflects back into the water, while others challenge this by stating that objects do not completely disappear from view, but may appear distorted.
  • A participant proposes using a tube to limit the line of sight to the light bulb, questioning if this would affect visibility.
  • There is a discussion about the nature of light rays and how they behave at the critical angle, with some participants suggesting that rays can still reach the observer's eyes even if they are at a shallow angle.
  • One participant concludes that the tube's purpose was to minimize direction change due to refraction, acknowledging that light would still reflect off the water's surface.

Areas of Agreement / Disagreement

Participants express differing views on whether light sources can be seen from outside the medium under TIR conditions. There is no consensus on the visibility of objects or the impact of angles on perception, indicating ongoing debate and uncertainty.

Contextual Notes

Participants reference the behavior of light at various angles and the effects of refraction and reflection, but the discussion does not resolve the complexities involved in these interactions.

RubinLicht
Messages
131
Reaction score
8
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
 
Science news on Phys.org
RubinLicht said:
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.

RubinLicht said:
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?
 
Drakkith said:
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
 
RubinLicht said:
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.
 
Drakkith said:
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?
 
jbriggs444 said:
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.

refractionfigure4.jpg
 
RubinLicht said:
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.
 
Drakkith said:
How so? TIR happens when the rays try to exit the water, not the other way around.

refractionfigure4.jpg
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.
 
  • Like
Likes   Reactions: jbriggs444
  • #10
RubinLicht said:
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.

Hmmm. I see. I suppose that makes sense.
 
  • #11
Drakkith said:
Wouldn't change anything. The light would still reflect off of the surface of the water in the tube.
I have my answer now. The purpose of my tube was to "catch" the light right after it leaves the water in order to minimize the direction change caused by refraction. If that makes any sense. So once it gets past the critical angle, it should get a lot darker (but not completely since light reflects off of water and the walls and such :( )

Thanks everyone for the help
 

Similar threads

Undergrad Outer layer refractive index - total internal refection on waveguide
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Polarisation of Light: Reflection, Critical Angle & Total Internal Reflection
  • · Replies 8 ·
Replies
8
Views
3K
Undergrad Energy Loss During Total Internal Reflection
  • · Replies 14 ·
Replies
14
Views
4K
Graduate Multiple reflections and transmissions of light inside a cube
  • · Replies 2 ·
Replies
2
Views
3K
Graduate Total internal reflection for a long time
  • · Replies 4 ·
Replies
4
Views
2K
Graduate How Does Total Internal Reflection Occur in Salt Solutions?
  • · Replies 2 ·
Replies
2
Views
2K
High School Is it Total internal reflection, or something else?
  • · Replies 2 ·
Replies
2
Views
2K
High School Total Internal reflection is Glass Prism Question
  • · Replies 3 ·
Replies
3
Views
6K
Undergrad EM wave reflected or transmitted?
  • · Replies 8 ·
Replies
8
Views
3K
Undergrad Photonics Question - Laser and Total Internal Reflection
  • · Replies 6 ·
Replies
6
Views
3K