Can light escape the medium when total internal reflection occurs?

In summary: Ill try to clarify further.In summary, the light will only be visible if it is incident on a surface at a smaller angle than the critical angle.
  • #1
RubinLicht
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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
 
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  • #2
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?
 
  • #3
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
 
  • #4
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.
 
  • #5
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.
 
  • #6
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?
 
  • #7
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
 
  • #8
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.
 
  • #9
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.
 
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  • #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
 

1. What is total internal reflection?

Total internal reflection is a phenomenon that occurs when a light ray traveling in a medium reaches the interface of a more optically dense medium at an angle larger than the critical angle. Instead of passing through the interface, the ray is reflected back into the original medium.

2. What is the critical angle?

The critical angle is the angle of incidence at which total internal reflection occurs. It is the angle at which the refracted ray would have an angle of refraction of 90 degrees, meaning it would travel along the interface between the two mediums.

3. What are some real-world applications of total internal reflection?

Total internal reflection has many practical applications, such as in fiber optics communication, where light is transmitted through thin glass fibers using total internal reflection. It is also used in prisms and mirrors to redirect light, in optical devices like microscopes and telescopes, and in diamond cutting where the critical angle is used to guide the cutting tool.

4. What factors affect total internal reflection?

The two main factors that affect total internal reflection are the angle of incidence and the difference in refractive indices between the two mediums. A larger angle of incidence and a larger difference in refractive indices result in a smaller critical angle, making total internal reflection more likely to occur.

5. Can total internal reflection occur in all mediums?

No, total internal reflection can only occur in a medium with a higher refractive index than the surrounding medium. For example, it cannot occur in air because its refractive index is lower than that of water. It also cannot occur in a medium where the angle of incidence is less than the critical angle.

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