Basic Theory of Light
Light is nothing but electromagnetic radiation that creates visual sensation on human eyes. Although all electromagnetic radiations cannot create sensation on our eyes but within some specific range of radiation it can be observed by human eyes and this is called light. So definition of light can be written as the electromagnetic radiations which create visual sensation to human eyes are called light. The radiations with wavelength ranging from 380nm to 700nm are only sensed by normal human eyes. So, the radiations with wavelength within this range are light. Light energies are of different colors from violet to red. The radiation with minimum visual wavelength i.e. around 380nm, is violet light and the radiation with maximum visual wavelength i.e. around 720nm, is colored red.
When sunlight strikes on a face of transparent prism, the sunlight is deviated from its incident direction and separated in different colored rays. The rays change their direction after refraction in the prism, according to their angle of deviation. This angle of deviation is further dependent upon the wavelength of the light rays. The pattern of the sunlight after refracted through transparent prism is known as spectrum of that sunlight. From this prism experiment it is found that the color at the middle of the spectrum is greenish yellow.
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Artificial Sources of LightAlthough sunlight is the main natural source of light on our planet, there are many artificial processes of producing light such as incandescence, luminescence, fluorescence, and phosphorescence.
Incandescence: The process of producing visual light by heating an object is referred as incandescence. There are some objects which can produce visible light energy when is made hot to a high temperature. They lead to a continuous spectrum of radiation.
For example, when we heat up a iron piece to sufficiently high temperature, the piece becomes red hot and radiates reddish light from its surface. Incandescent lamp is a very popular source of artificial light, where a high resistive filament is made hot by electric current and that ultimately radiate light to the surroundings.
Luminescence: It is the emission of light without the effect of heat. It can be considered as one kind of cold-body radiation. Light may be produced from any of the following processes such as chemical reaction, effect of electricity, subatomic motions, or stress on a crystal.
There are some substances which can produce visual radiation when they absorb invisible radiation of some specific wavelengths. That means these substances absorb radiation of one wavelength and radiate radiation of other wavelengths. For example, when fluorescent substance is struck by ultra violet rays, it radiates visible rays.
Phosphorescence: There are some substances which can absorb light energy when these are exposed to the light, and can radiate this energy later as glow. For example, when calcium sulphide coated surface is kept in light, and then it is brought to dark, the surface will glow as tiny light source.
Human Eye VisionHuman acquires more than 80% of information through eyes. An eye comprises three main parts, iris, lens and retina. Iris regulates the light to come into the eye by expanding and contracting. Eye lens focuses under the control of ciliary muscles forms image on to the retina. There is a screen in back of the lens called retina. The retina is equipped with lots of light sensitive or optical nerves that communicate with brain. There are two types of nerves available on the retina. These are cones and rods. Rods are more sensitive in dim light. The central portion of the retina is called fovea. This is the most sensitive spot of retina. Normally rod cells are concentrated away from the central fovea of the retina. The cone nerves are mainly concentrated at the fovea. The cones are mainly responsible for bright vision with distinct color sensitivity. Due to these two types of nerves there are two types of vision.
- Photopic vision
- Scotopic vision.