We observe that as the current through the filament is increased, the filament gets hotter and brighter. Also the color of light changes from red to orange to yellow. It only gets till yellow-white, but never any bluer. Even when the temperature reaches 2000 degree centigrade in a black body radiator, using a power of 40 KW, the color never moves to blue end of the spectrum. Why is blue so hard to make? When we consider further up the spectrum beyond blue, the ultraviolet range of spectrum, is hardly produced at all, not even by extremely hot objects like the Sun (which mostly emits light in the visible region of spectrum), at a temperature of around 5500 degree centigrade. Why is it so hard to make, even at such high temperature levels?
-
$\begingroup$ Have a look at Wien displacement law: en.wikipedia.org/wiki/Wien%27s_displacement_law. The Sun is way too cold for the light to appear blue, the peak is in the green region, explaining in part why we see it as yellow/white. $\endgroup$– untreated_paramediensis_karnikCommented Sep 25, 2020 at 12:06
2 Answers
The filament is radiating according to Planck's law and thus the temperature of the filament is what decides the dominating "colour" of the light (if you want to call it that). With increased power, the temperature of the filament gets higher and we are moving our radiation peak towards the shorter end of the visible spectrum.
The temperature of the filament simply doesn't get hot enough to move the radiation peak towards the blue end of the spectrum. The filament would break before this.
Short wavelength (blue) light is hard to make because it is hard to make. It requires a large amount of power. shifting the peak wavelength an equal nanometer interval requires exponentially increasing quantities of power as the wavelength gets shorter.
Filaments can't do it, the necessary power melts them. you do get vaguely tinted "blue" light from very massive stars, basically, type O and B. but we cannot really see their full spectrum because much of it is outside the visual limit and much of it is filtered by the Earth's atmosphere. and speaking of, the sky has a blackbody correlated color temperature of around 17,000K, though not due to temperature but to scattering. Outside artificial lights, "blue" depends on a combination of visual, terrestrial, and physical (quantum) limitations.
-
$\begingroup$ what do you mean by "the sky has a blackbody correlated color temperature of around 17,000K, though not due to temperature but to scattering"? $\endgroup$ Commented Sep 30, 2020 at 20:41
-
$\begingroup$ use your favorite search engine and search "correlated color temperature" and "rayleigh scattering". $\endgroup$– drollereCommented Sep 30, 2020 at 20:50