Light: Definition, luminous and non-luminous objects and sources
Definition: In complete darkness we cannot see anything. When you enter a dark room, nothing is visible, but as soon as a bulb or a candle is lit, the objects placed in the room become visible. Hence, light helps us to see the objects around us. Light is a form of energy that produces visual sensations in our eyes. We get light from various sources like sun, electric lamp, candle, oil lamp, burning wood etc.
Light can pass through transparent material like glass, water etc. but cannot pass through opaque material like wood, metal etc. Light always travels in a straight line. Light travels at the speed of 300 billion meters per second in the habitable zone. Light takes about 8 minutes to reach the Earth from the Sun.
Luminous and Nonluminous Objects
The substances which emit light are called luminous bodies. Luminous objects shine with their light. luminated bodies can be natural or man-made. Stars, sun, firefly etc. are natural luminous bodies. Electric lamp, oil lamp etc. are man-made luminous objects.
The substances which do not emit light are called Non-luminous objects. Unlit objects do not shine by themselves. Non-illuminated objects appear shiny because due to the reflection of light received from an illuminated object, they reflect the light falling on them, due to which the object appears shiny. Like moon, table, chair and books etc. do not emit light. Hence it is non-luminous substance.
Moon is also a non-luminous substance but it shines in the night because it shines by restricting the sunlight falling on it. Like the moon, we can also talk about the light of the earth. The light coming from the Sun is reflected from the surface of the Earth, which is called Earth's light. Earth's light can be seen from the Moon.
Sources of Light
The objects or substances which emit light are called sources of light. All luminous objects emit their own light, therefore all luminous objects are sources of light. Natural light objects such as the Sun, stars and fireflies are sources of natural light. Man-made luminous objects such as a burning candle, burning electric lamp, torch, burning wood, bulb, etc. are man-made sources of light.
Cold and Hot Sources of Light
The sources of light which emit light and energy are called hot sources of light and the sources of light which emit only light are called cold sources of light. Objects which emit light when heated strongly are called incandescent or incandescent objects. Incandescent objects are called very hot sources of light.
Electric bulb has a wire made of tungsten. This wire does not emit light at normal temperature. When electricity passes through the wire, it gets heated and starts emitting light, at this stage the wire remains in excited state. The luminance of a wire depends on its temperature. Sun and stars also emit light due to their extremely high temperature. Its temperature on the surface of the Sun is about 6000 degree Celsius. Such high temperatures in the Sun and other stars are due to the release of increasing amounts of energy during the nuclear fusion reaction. The color and brightness of stars depend on their temperature.
Tubelight is a cold source of light because it emits light by the process of fluorescence at room temperature. There is no electric wire in the tube light.
A tubelight is made of a long thin-walled glass tube that is internally coated with a fluorescent material. There is a small wire at each end of the tube. The tube is filled with a mixture of argon gas and mercury vapor at low pressure. When the tubelight is switched on, the stars get heated to a very low temperature. These electrons collide with mercury atoms and generate ultraviolet rays. The fluorescent material coated on the inner walls of the tube absorbs ultraviolet rays and emits light due to fluorescence.
Firefly burns-extinguishes due to photochemical reaction and emits light, therefore firefly will also be called a cold source of light.
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