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Refraction of light
directly. But if we look through glass at an angle, we can no longer see through it, and it behaves
as though it were a mirror. The largest angle at which the glass still appears transparent is called the critical angle.
For a surface to behave like a mirror, it must be very smooth.
If it is rough, like the surface
of a wooden table, the light will be scattered in many directions, and we will not be able to see
a reflection (see: Scattering of light). By polishing a table, some of the depressions in the wood are filled with polish, and the surface becomes smoother. As a result,
it becomes more mirror-like. Wax does the same job on car paint.
If an object does not absorb one kind of light wave in preference to others, all of the light is reflected, and it appears white in white light, blue in blue light and so on. If very little light is absorbed, the object will appear transparent, as is the case for glass. If an object absorbs all light uniformly, it will appear black. Depending on how much light is absorbed, the object will appear as darker or lighter shades of grey.
For something to look truly red, less than 10% of the red light must be absorbed, but over 90% of all other kinds of light. If the red light is absorbed only slightly less than the other waves, the object will still look a greyish-red. Greyish- blues and greens result in the
same way. (See also: Absorption of light.)
Sound can be reflected in the same way as light. In this case the surface does not have to be shiny or obviously reflective, just smooth and hard.
Just as light can be brought to a focus by a curved mirror, sound can be brought to a focus
by a room with curved walls. All whispering galleries have curved walls, allowing a person to whisper in one part of the gallery and be heard on the opposite side.
Ray of light
Refraction of light
When light passes from one transparent substance to another, it is often bent. That is called refraction. Refraction can be seen by placing a straw in a glass of water. The straw appears to bend
 Refraction of light – Refraction makes the straw appear to be bent as it enters the water. Refraction through glass makes the light be bent twice. It emerges from the block in the same direction as it enters, but displaced to one side.
Glass block
Ray of light
 Refraction of light – Refraction makes the position of a fish in the water be estimated incorrectly.
Apparent position
Real position
Due to refraction, the fish appears to be at a shallower depth that it actually is.
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