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Question 22: An incandescent bulb is connected to dimmer switch. When the bulb operates at full power, it appears white. But as it is dimmed it looks upon more and more red. Explain.


The brightness of light depends upon the intensity of the radiation. However, intensity of radiation is determined by the energy of the radiation. Therefore, light seems brighter if the radiant energy is greater.

Now power dissipation of current in a resistance is given by P = I2R. When the bulb operates at full, maximum current is flowing through the circuit of the bulb. Therefore, maximum power is dissipated in the filament of the bulb. This makes the filament hottest. This hot filament emits radiations of shorter wavelength as the Wien Displacement law suggests that wavelength is inversely proportional to the absolute temperature of the hot body.

When the bulb is slowly turned dimmed, the current through it decreases. Therefore, the power also decreases (P = I2R) which in turn decreases the temperature of the filament. Again applying Wien’s Law, the wavelength of the radiation increases.

So at full current, the filament is hotter than at reduced current. The hotter filament emits radiations of smaller wavelength, of more energy and brighter.

The less hot filament (dimmed current) emits radiations of longer wavelength, of less energy and less brighter.


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