While observing the world, light from the object is needed to be incident on eyes then interpreted by the brain to make that object visible which is known as illumination. There are two types of illumination which are listed below.
- Primary Illumination
- Secondary Illumination
Explanation
Primary Illumination
The illumination in which light coming from the primary source of light i.e. sun, stars, bulb, fire, flame e.t.c is directly incident on eyes and make these sources visible is known as primary illumination. Primary source of light are those sources of light that can generate their own light i.e they are luminous and hence no need of any other source of light to make them visible. This is the case of primary illumination. They are used in homes and other residential places to illuminate the surrounding.
Primary sources of light are luminous due to some phenomena that occurs in nature such as black-body radiation (in bulb), reverse photoelectric effect (in X-rays), nuclear reaction (in sun), chemical reaction (in flames i.e in fire or candle) e.t.c. They emit light due to chemical and nuclear reactions and also due to effect of current being passed through them.
Secondary Illumination
The illumination in which light coming from the primary source of light is reflected by the secondary source i.e an object, then incident on eyes and make secondary source visible is known as secondary illumination. Secondary source of light are those sources of light that cannot generate their own light i.e they are non-luminous and they always need light coming from the primary sources of light to make them visible. for example moons, planets, table, chair, cloths e.t.c.
Most of the sources that are seen around are secondary sources of light. Some of secondary sources can also be made luminous through some phenomena. For example, through black-body radiation, a simple iron rod can be made to glow if placed in extreme heat (like fire) for a certain period of time.