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### Course: High school physics - NGSS>Unit 6

Lesson 3: Emission of electromagnetic radiation

There are many different processes and phenomena that emit electromagnetic radiation. Humans have taken advantage of many of these processes to develop technologies that use electromagnetic radiation. Created by Sal Khan.

## Want to join the conversation?

• A few questions ...

1. If matter at room temperature normally only seems to emit IR rays, then how cold would matter have to be to emit EM waves in the radio wave and microwave frequencies?

2. How hot would matter have to be to emit EM waves in the X-ray and gamma ray frequencies?

3. Since the sun emits EM waves in all frequencies, does this mean it has some hot parts to emit the higher energy waves, and some colder parts to emit the lower energy and frequency waves? Or is it in fact that all matter at a certain temperature will emit EM radiation of the highest energy and frequency possible for that temperature, PLUS all of the EM waves with a lower frequency than that maximum frequency?

4. Since all EM waves carry at least some energy, do all of them increase the thermal energy of matter when they are absorbed by it and their energy is taken away by the matter?

5. When an object absorbs visible light, it gains a certain amount of energy. It then emits IR radiation, which has a lower frequency and therefore less energy than visible light. So how has energy been conserved here? Does the object maybe emit more total photons of IR radiation than it absorbed photons of visible light, since each photon of IR radiation carries less joules of energy?

6. Why does the trend happen to be that absorbed visible light is emitted as IR radiation anyway? Why, for instance, is absorbed visible light not just emitted as visible light again?

Looking back, I realise this was more than a few questions, but I'm really curious about this stuff. Thanks so much!
Due to law of conservation of energy, energy cannot be gained. Therefore, in cases where light is absorbed, much of the time, energy has gone to other purposes and has thus been converted, and therefore the re emitted light will be of lower energy. However, note that much of the time, we are not actually absorbing visible light, rather we are reflecting it. The visible light that bounces off a human and the thermal energy that can be captured in the infrared spectrum are completely separate.
(1 vote)
• So I have been researching absorption and emission spectra, and it seems to be that Emission spectra are all the (distinct) wavelengths that an object can(or will?) emit, while the Absorption spectra is all the wavelengths that it gets energy from, that it can absorb. Please verify or clarify, thank you.
(1 vote)
• Emission spectra occur when an atom "emits" a photon when one of its electrons decays back to one of the lower orbitals, whereas an absorption spectra shows at what wavelength the light has been absorbed by the atom (ie: the electron used the photon to get to a higher energy orbital). This is why in an emission spectra, you only see the wavelengths of light that are emitted (the rest is black), whereas in an absorption spectra, you really do not see that specific wavelength because it has been absorbed by the atom, which corresponds to the black lines.