Chemical Effects The energy of light can chemically change the surfaces of materials absorbing it. Light changes the chemistry of the eye’s retina, so that the retina produces signals about sight. Green plants need light for photosynthesis, the chemical process by which they make food.

Photoelectric and Photoconductive Effects When certain materials absorb light, the light’s energy free electrons form atoms on the materials’ surface. In some devices, these freed electrons can flow through a circuit as electric current. Solar cells and other electric eyes operate by means of such photoelectric effects. Some materials called photoconductors become better conductors of electricity when light shines on them.

Measuring Light Scientists measure wavelenghts of light in variety of metric units. One common unit is nanometer, which equals a billionth of a meter, or 1/25,400,000 inch. The wavelenghts of light in the visible spectrum range from about 400 nanometers for deep violet to about 700 nanometers for deep red.

The frequency of any wave equals the ratio of the wave’s speed to its wavelength. Frequencies are measured in units called hertz.

The Brightness of Light Scientists use various units to measure the brightness of light source and the amount of energy in a beam light coming from that source. The amount of light produced by any light source is called the luminous intensity of the source. The standard unit used to measure luminous intensity is the candela.

For many years, the luminous intensity produced by a certain size candle made from oil of sperm whales served as the standard. The unit was called a candle. One candela is now defined as the amount of light given off by source emitting at a specific frequency (540,000,000,000,000 hertz) and at a specific intensity (1/683 watt per unit of area called a steradian)

We can measure a beam of light with a unit called the lumen. Luminous flux is the measure of the perceived power of light. Luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light. The intensity of light falling on a surface varies inversely (oppositely) with the square of the distance between the source and the surface.

That is, if the distance increases, the illumination decreases by the square of the distance. This relationship is called the inverse square law.

Speed Of Light The speed of light in empty space, where atoms do not relay its travel- is 186,282 miles (299,792 kilometers) per second. This speed is said to be invariant because it does not depend on the motion of the light’s source.