In this article we will discuss about the nature of light.
Lighting is one of the major uses of electrical energy. Sun is the biggest source of natural light. Artificial lighting may be electrical or non-electrical. Where there is no natural light, use of artificial light is made.
Artificial lighting produced electrically has advantages like cleanliness, ease of control, reliability, steady output and low cost. When light falls on a surface, the surface is illuminated and the process is called illumination i.e. light is the cause and illumination is the result of that light on surfaces on which it falls.
Light is a form of vibrant radiant energy which falls on a body and it becomes visible to the eye.
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The waves of light travel with a velocity of 3 x 108 metres/second.
v = f x λ
where v is the velocity of light in m/sec, f is the frequency in Hertz (Hz) and λ is the wavelength in metres.
When the light falls upon any surface, the phenomenon is known as illumination.
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Light Spectrum:
The colour of light depends upon the wavelength of radiation. It has been found that the spectrum of light consists of SEVEN different colours. These are – Red, Orange, Yellow, Green, Blue, Indigo and violet. The wavelength of Red light is 75 x 10–8 m and of violet is 40 x 10–8 m. Thus the spectrum has wavelength varying from 40 x 10–8 m to 75 x 10–8 m.
However prior to red light, there are infrared or heat waves with a wavelength of about 3 x 10–5 m to 75 x 10–8 m and beyond violet there are ultraviolet rays with wavelength between 3 x 10–8 to 40 x 10–8 m i.e. after 7500 A.U. The radiations are known as infrared and below 4000 A.U. they are called ultra-violet.
Angstrom Unit (A):
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A metre or even a centimeter is a very large unit for measuring the wavelengths. So a smaller unit, known as Angstrom unit (Å) is generally used.
1 Å = 108 cm = 10–10 m and 1 micron (μ) = 10–6 m etc.
A list of colour with their wavelength is given below:
So in terms of these units:
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Wave length of red light = 7500 Å
Wavelength of violet light = 4000 Å
Wave length of blue light = 5000 Å
Wave length of yellow light = 6500 Å
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Visible light can have a wavelength between 4000 Å and 7500 Å.
Other colours have wavelengths between 4000 Å to 7500 Å. The wavelength of infrared rays varies from 3 x 10–5 Å to 7500 Å and of ultraviolet rays from 300 Å to 4000 Å.
The human eye is most sensitive to light having wavelength of 5500 Å. This light has greenish yellow colour and is unsuitable for most of the practical purposes.
The sensitivity of the eye to light of different wavelength varies from person to person. This also depends upon the age of the person. The graph between relative sensitivity of eye and wavelength is shown in Fig. 2.1.