Properties of Light

Light has some interesting properties, many of which are used in
medicine.

1. The speed of light changes when it goes from one material to
another. The ratio of the speed of light in vacuum to its speed in a
given material is called the index of refraction. If a light beam
meets a new material at an angle other than perpendicular, it bends,
or is refracted. This property permits light to be focused and its the
reason we can read and see objects clearly.

2. Light behaves both as a wave and as a particle. As a wave it
produces interference and diffraction. As a particle a single
molecule can absorb it. When a light photon is absorbed its energy
is used in various ways. It can cause a chemical change in the
molecule that in turn can cause an electrical change. This is
basically what happens when a light photon is absorbed in one of
the sensitive cells of the retina. The chemical change in a particular
point of retina triggers an electrical signal to the brain to inform it
that a light photon has been absorbed at that point.

3. When light is absorbed its energy generally appears as heat. This
property is the basis for the use in medicine of IR light to heat
tissues.

4. Sometimes when a light photon is absorbed, a lower energy light
photon is emitted. This property is known as fluorescence.

5. Light is reflected to some extent from all surfaces.
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