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Name: David A.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 3/22/2004


Question:
Is the critical angle for light going from air to water the same as the critical angle for light going from water to air?


Replies:
Yes it is the same angle both ways, but think: you can only get Total Internal Reflection going from water to air. The ray at critical angle is always in the water, and the same ray's segment in air is always parallel to the surface. Regardless of whether the light is going from water to air, or air to water. A fan-spray of light is always wide in the air and narrow in the water or in glass.

coming from inside the water:

Critical angle is the incidence angle beyond which the light ray cannot exit. The ray would be totally reflected instead. At exactly the critical angle in water, the exit ray in air is parallel to the water surface.

coming from the air:

Critical angle is the angle in the water made by a ray of grazing incidence in the air. Every ray possible in a 180-degree hemisphere in air, is squeezed into a critical-angle cone in the water. There is no angle in air which can make an angle in water greater than "critical angle".

You could play games with this. If the bottom of your swimming pool is painted black, and you put a trash can lid over your head under water, you may be unable to see any of the sky or sunlight. It could be surprisingly dark under that simple disc. Maybe that is why carp under water-lily pads hide so well.

People who design LED lamps are often hoping they can find a way to make some of the trapped light hit the clear plastic bulb's surface at a better angle, so it all gets out from inside and shines bright. Even tougher for light to get out of the LED chip itself, because semiconductors have very high index of refraction, 2.4-3.5 typically. If the light generated in the chip is buzzing around at all random angles (true for LED's, but not for lasers), then perhaps less than 25% can get out of a smooth chip into air on the first try.

Jim Swenson


No. The path of light is reversible. As a result there is no critical angle (total reflection) for the interface from air to water.

Greg Bradburn



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