| Incandescent light bulb - it is an illuminating instrument and a
source of artificial light. An electrical current passes through a
thin filament, heating it until it produces light.
Working principle:
In the light bulb, heating effect of the filament (glowing filament)
takes place during the passage of electrical current through it.
Temperature of the tungsten wire suddenly increases after the power is
on. The wire emits electromagnetic emissions according to the Planck's
rule. Planck's function has a maximum, position of which on the length
scale depends on temperature. This maximum slides with the temperature
increase in the direction of shorter wave length (Wien's displacement
law). To get a visible emission it is necessary that the temperature
should be several thousand degrees, in an ideal way 6000 K
(Temperature of the surface of the Sun). The lower is the temperature
the lower will be the part of visible light and the more it appears as
red emission.
A part of the consumed electrical energy by the lamp filament modifies
into light, part of it goes to thermal conductivity and convection.
Only small part of it lies in the visible light field, basic part
falls with in the infrared radiation. To increase the coefficient of
efficiency of the bulb and to get maximum (white) light it is
necessary to increase the temperature of the tungsten wire, which in
its turn is bound to the properties of the tungsten wire material -
melting temperature. Ideal temperature of 6000 K is unachievable, as
during this temperature any material melts, damages and stops current
flow. In the modern bulbs for incandescence, materials with maximum
melting temperature are used - Tungsten (3410°C) and very rarely,
Osmium (3045°C).
At practically achievable temperatures like 2300-2900°C illumination
is far from the white light and not the day light. Due to this reason
incandescent bulb releases light, which appears to be "yellow-red" in
color, than the day light. For the characteristics of the quality of
light so called color temperature is used.
In common air at such temperatures tungsten would have immediately
turned into oxide. Because of this tungsten wire is protected in a
glass bulb, filled with inert gases (commonly Argon). First bulbs were
made with vacuum flasks. However in vacuum under high temperatures
tungsten evaporates quickly, making wire thinner and darkening glass
flask as precipitates on it. Later bulbs are being filled with
chemical inert gases. At present vacuum flasks are used for bulbs of
lower energy. |
