Electric bulb
The glass becomes hot when an electric bulb is kept switched on. It is not so in the case of a tube light. Why?
K. Ananthanarayanan
Kanyakumari, Tamil Nadu
Answer 1: When a current is passed through the filament of an electric bulb, the filament gets heated to very high temperature (about 3000 degrees centigrade) when it emits light according to the laws of black body radiation.
Note that, in the electric bulb the power dissipated gets converted to both heat and light. In fact, only a small fraction of the power is available to us as light and the rest appears as heat, which heats up the glass bulb.
However, in a tube light, the electrons emitted by the filaments at the ends of the tube excite the gas atoms/molecules inside the tube through an electric discharge process. These excited molecules and atoms radiate UV radiation which cause fluorescence of the white phosphor coated on the inside wall of the glass tube.
The light emitted is not due to black body radiation. Actually, the colour of light obtained from a tube light would correspond to a colour temperature of about 6,500 degrees Kelvin. This only represents the emission colour.
It may be pointed out here that this process of fluorescent light emission does not require heat as input; therefore there is no heating of the surrounding glass tube.
Dr. H. K. Sahu
Senior Scientific Officer
Indira Gandhi Centre for Atomic Research
Kalpakkam, Tamil Nadu
Answer 2: The mechanisms of light produced by these two kinds of lamps are entirely different. The electric-bulb produces light by the process called incandescence while the tube light produces light called fluorescence.
Incandescence is the glow generated when heating a material, in this case the tungsten filament, which, being an electrical resistor would naturally generate lot of heat when electric current passes through it, by the process called Joule heating. It is common to experience heat in the vicinity of a 1000 W bulb or so and very little heating in lamps with lower power ratings.
The fluorescence lamp generates light by converting the invisible ultra-violet (UV) light generated through an electric discharge process along of the length of the tube to visible light. The UV to visible light conversion called fluorescence is accomplished with the help of a white coating made of fine particles of an inorganic phosphor/fluorescent material. Curiously we can see all tube-lights are given colour-temperature, usually marked/inscribed daylight 6,500 K which only represents its emission colour rather than temperature. So no heating is involved in the generation of light from a fluorescent lamp in contrast to that of an electric bulb.
R. Jagannathan
Centre for Electrochemical Research Institute
Karaikudi, Tamil Nadu
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