The filament of a standard 100-W light bulb has a resistance of about 100 ohms.

Question

The filament of a standard 100-W light bulb has a resistance of about 100 ohms. For this laboratory the glass of the light bulb is unfrosted, so that the filament can be viewed directly. The light bulb is unfrosted, so that the filament can be viewed directly. The light bulb is connected to a variable AC voltage source (a Variac), which can deliver up to 140 volts AC. When the voltage is low, about 20 volts, the filament is a dull, red color. When the voltage is high, 100 V, how does the filament appear? Explain why. When an electron in an atom makes a transition between two energy levels, E_high and E_low, a photon is emitted. If the photon is in the red portion of the visible light spectrum, what is its wavelength' and what is the energy difference between E_high and E_low? (A figure in almost any physics text showing the spectrum of colors might be helpful. A useful web site is "What Wavelength Goes with a Color? " at:http://eosweb.larc.nasa.gov/EDDOCS/Wavelengths for Colors.html) lambda = nm E_high = E_low = eV What are the wavelength and the energy difference for a blue photon? lambda = nm E_high - E_low = eV

Explanation / Answer

At low voltages, the power dissipated through the filament is very less and it emit only low energy photons such as infrared and red colour photons.

When we increase the voltage, it starts emitting higher energy photons i.e. photons in complete visible spectrum as well as ultra violet radiation. When we see this radiation it appears as white in colour.

So at high voltages, it appears in white colour.

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