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Q & A: Total Internal Reflection of Light and Sound

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Most recent answer: 12/09/2013
Q:
Is there a case of talking about critical angle when light passes from a rarer to denser medium? And if so what will be angle of refraction ? 90 degrees? Is it possible since the refracted way bends towards the normal?
- Nivedhika (age 16)
Namakkal,Tamil Nadu,India
A:

Hi Nivedhika,

As light travels from a medium with low index of refraction to a medium with higher index of refraction, it bends towards the normal. In this case, there is always a solution to Snell's law, so you will never get total internal reflection, and there is no critical angle.

Total internal reflection appears only when a wave crosses an interface from low wave speed to high wave speed at an angle larger than the critical angle. In the case of light, this can happen, for example, when light rays try to escape from a swimming pool and into the air. So, you can sometimes see cool effects like this one (from )

 

This is possible only because light travels about 25% slower in water than it does in air, and the light has a critical angle of about 49 degrees. Interestingly, the speed of sound is over 4 times faster in water than it is in air. So, any sound wave underwater can escape into the air*, and total internal reflection occurs instead for waves entering the water from the air! Furthermore, since the speed of sound is so much different for air and water, the critical angle is only 13 degrees!

Cheers!

David Schmid

*I should point out that any wave encountering a surface has some losses due to imperfect transmission, which is why you can see your reflection in a window. So in all cases of refraction, a bit of the wave is reflected, and a bit is transmitted. In the case of total internal reflection, however, NONE of the wave is transmitted.


(published on 12/09/2013)

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