When we heat up a solid body above a certain temperature, it radiates light energy. Actually, when we gradually heat up a body, above room temperature, it radiates energy in form of electromagnetic wave. The nature of that electromagnetic wave depends on its wavelength. Again, a hot body can radiate the electromagnetic wave of different wavelengths.
Types of Electromagnetic Wave
Among those wavelengths of the radiated energy, there are some wavelengths which would be in our visible range. The electromagnetic waves with the wavelength within human visible range are lights. At the lower range of temperature, the body radiates mainly heat energy. When the temperature increases, the body begins to radiate light energy. This is because, at the lower range of temperature, the wavelength of radiated electromagnetic waves is larger. At this range of wavelength, the nature of the electromagnetic wave is heat. But when we increase the temperature father, the wavelengths of the radiated energy become shorter and cause light energy.
The color of visible light energy depends on its wavelengths. Again, with increasing the temperature of the body, the wavelength of the radiated energy becomes shorter. Hence, firstly, the hot body radiates red colored light. Then with an increase in temperature light becomes yellow and then finally white.
It is not like that, at a certain high temperature, a hot body radiates electromagnetic waves only with a certain wavelength. Rather it radiates electromagnetic waves of all possible wavelengths. But in higher temperature, the radiation of visible light dominates other forms of electromagnetic waves.
Even there will be some electromagnetic waves other than infrared waves of wavelengths beyond our visual range. These electromagnetic waves are called ultraviolet rays.
So, ultimately we can say, a hot body can radiate infrared rays in the form of invisible heat energy, visible light rays, and invisible ultraviolet rays.
Radiant efficiency is the ratio of energy radiated from a hot body in form of light to the total energy radiated from the body.
As we have already seen in our previous discussion in this article that, the quantity of different forms of radiated energy depends on the temperature of the body. So it is obvious that the radiant efficiency also depends on the temperature of the hot body. The radiant efficiency of a body is maximum when it radiates the whitest light. At that temperature of the body, radiation includes all visible wavelengths from extreme red to extreme violet. Now if we further increase the temperature of the body, radiation of ultraviolet rays starts dominating the light energy. The radiant efficiency begins to decrease.
It is found that maximum radiant efficiency occurs at 6,200°C. Although, the value of this maximum efficiency is only 20%.
It is needless to say, increasing the temperature of a body up to 6,200°C, is practically not possible. Practically the temperature of a hot body is much lower than that range. Hence, the radiant efficiency of practical light radiating bodies including incandescent lamps is always less than 20%.