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Question

If the sun gives off UV rays, does a flame give off a small amount of UV rays as well? Or does the sun give off UV rays because it burns a certain element or chemical compound?
Asked by: Samuel

Answer

The wavelength of light that is radiated from an object depends upon the temperature of the object. The sun is very hot so it emits radiation at many different wavelengths. The surface can be approximated as a blackbody at 6000 K. A normal orange flame may burn at 600 K or so. It isn't hot enough to radiate UV rays but it does radiate in the visible and infrared parts of the electromagnetic spectrum.
Answered by: Trevor Clair, Physics Undergrad, Canada


The question you have asked was actually one of the problems that led to the development of the quantum theory.

Let us consider how a body emits electromagnetic rays like UV rays. By the gain of energy ( at high temperatures or through some other method ), the atoms of a material get excited, that is, their electrons get promoted to higher energy levels depending upon the amount of energy they have received. Now, when these electrons make a transition back to some lower energy level, electromagnetic rays are emitted in the process which carry away the energy.

In the sun, the temperature is high enough to let the electrons acquire that amount of energy that is necessary for them to let out UV rays as they transit to some lower level. ( This is not due to some special material in the sun ). But in a candle, gaining that much amount of energy for an electron is very difficult because of relatively low temperature.

This problem is similar to the ultraviolet catastrophe, a paradox, that was as a result of classical explanation of radiation. According to this, even bodies at room temperature should emit high-energy rays like gamma-rays. This doesn't actually happen because of quantization of energy. Photons of high energy rays have a certain minimum amount of energy and so, these rays cannot be emitted unless the electron acquires that minimum energy.
Answered by: Vibhav Singh Chauhan, Physics Undergrad. Student, IIT-Delhi.



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