Light and Heat

It is obvious to say that hot objects emit (give off) both visible light (a glow) and heat (that we can feel but not actually see). This is true of bonfires, light-bulbs and even our Sun. The connection between heat and light is taken completely for granted in everyday life: the glow of a hot object warns us that if we pick it up with our bare hands then we will get burned. But there is much more to this connection than a simple biological warning sign.

All hot objects emit infra-red radiation, which is invisible to the human eye but can be seen using suitable cameras. To see how the world seen in infra-red differs from the world seen with the human eye using visible light only, visit the Cool Cosmos Interactive Gallery website, where you can choose a type of image then adjust a slider to go from one type of viewing to the other. There is also a gallery of infra-red animal pictures on the Infra-Red Zoo website (which is another part of Cool Cosmos).

The details of how thermal energy in an object is converted into electromagnetic radiation are beyond our course but you are expected to know that the frequency of the radiation increases as the temperature of the object increases. This can also be stated in terms of wavelength: as the temperature of the object increases, the wavelength of the emitted radiation decreases.

Whichever way you think of this connection, you should be able to predict that a moderately hot object will emit mostly invisible IR radiation but a much hotter object will also emit a lot of visible light. You should also be able to predict that an object that glows yellow must be hotter than an object that is glowing red. Note that this is not about brightness: it is about colour (frequency or wavelength).

If you think back to the Astronomy module, you should know that the Sun (a main sequence star) currently has a surface temperature of about 5500 K. How do we know that? (Nobody has been to the surface of the Sun with a thermometer.) The answer is, we deduce its temperature from the spectrum of electromagnetic radiation that it emits – both visible and invisible. Similarly, you should recall that when the Sun stops fusing hydrogen and enters its next phase, it will become a Red Giant. As the name suggests, it will appear red to the human eye and that means its surface temperature will have dropped to around 3000 K.

This is a hugely important areas of physics that you may wish to research (just for interest… remember that the details are not in your course). If you want to know more then search for Wein’s Law and Blackbody Radiation then The Ultra-Violet Catastrophe, leading to The Photo-Electric Effect. This research will take you on a journey from the comfortable world of classical physics into the weird realm of quantum behaviour.

There is some very accessible GCSE-level information on BBC Bitesize (https://www.bbc.co.uk/bitesize/guides/zshgqhv/revision/1) and Wikipedia is good for this research too: start with the examples that are listed on its Wein’s Law page (https://en.wikipedia.org/wiki/Wien’s_displacement_law).

If you want to tax yourself and dig deeper, then a detailed but nicely paced explanation is given in sections 1.1 and 1.3 in The Dawn of Quantum Theory, which is part of an excellent free eBook by McQuarrie and Simon. Alternatively, take a look at the various boxes on the Hyperphysics website (http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html) but be warned that these are mostly just diagrams and bare facts that you will need to interpret yourself.