Lesson 2: What is Light?

Light is a form of energy that is visible. You can see it. The sun is a natural light source. Fire is another. Light spreads out or radiates from the sun, in all directions. Radiation is a transfer of energy that does not require matter. Energy that travels in such a way is called radiant energy.

Less than percent of the sun’s energy hits earth! A light bulb would be an example of an artificial light source Artificial light is manufactured by humans. It radiates in all directions from the source as well. All sources of light require an energy source. i.e. electric battery for a flashlight, or nuclear fusion for the sun.

When light is absorbed by a surface, it can convert into several different forms. i.e. heat, electricity, chemical energy. A darker substance absorbs more light energy usually converting it to heat energy. i.e. black sweater on a sunny day. Observatories are located away from cities to avoid the light pollution put off by cities. Mayall telescope at the Kitt Peak Observatory in Arizona, USA. High altitude location, over 2000m, and clear desert skies reduce interference to incoming light.

Light can be directed by putting a reflective surface on one side of the light source Some objects will emit visible light when they are heated to extreme temperatures. This is called incandescence. Incandesence In this process the transfer of energy goes from: Electrical Energy  Thermal Energy  Visible light Because of this energy transfer incandescence can be recognized as being very inefficient.

Fluorescence is the change from invisible ultraviolet light being absorbed by particles and released or reflected as visible light Problems arise because the phosphor coating and the mercury vapour are both toxic making disposal difficult. Fluorescence is much more efficient than incandescence. This is evident by how little heat is produced. Electrical energy  Energy absorbed by mercury particles  Ultraviolet light energy  Energy absorbed by phosphor particles  Visible light energy

Phosphorescent light sources are similar to fluorescent light sources in that they absorb energy and later release it as visible light. When the energy source is removed the object can continue to glow. This is known as phosphorescence. When a chemical reaction produces light it is called chemiluminescence. When the chemicals combine, energy is released in the form of light energy. Chemiluminescent Sources

Bioluminescence is light produced by a chemical reaction that occurs inside the body of a living organism. It is most common in the deepest parts of the ocean where there is no available light. It is done in order to attract prey or mates. i.e. fireflies. Objects that produce visible light are called luminous, where as objects that do not produce visible light are called non- luminous.

Electricity costs about 8¢/kW·h (cents per kilowatt hour) A watt is a unit of power and is represented with a W A kilowatt is 1000 watts A kilowatt-hour is 1000 watts operating for 1 hour. Because the idea of light being a form of energy does not explain how shadows result, the ray model was introduced. A ray is a straight line that represents the path of a beam of light. The light travels in all directions, however the drawing of a few rays can be used to predict shadows.

Objects that allow light to pass through without obstruction are called transparent. These objects appear clear. When a substance allows light to pass through, but not enough to make a defined image, it is called translucent. When an object totally blocks the light not allowing any to pass through, it is known as opaque. They cast shadows.