Properties of Light

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.