Note 5 Temperature

When a piece of metal is heated, and as it gets hotter and hotter the colour changes. We often say 'red hot' for something very hot.
The colour of the temperature we see comes to us as a light wave.
The light from stars comes to us in the colour of the stars temperature. You can see red stars and blue stars, hot and hotter.
Now think of a rainbow.
If all the colours were mixed together we would get white light.
White light that we see is a combination of the colours of the light spectrum.
The waves in the spectrum are of a different wave length to each other. For instance the red end of the spectrum has longer waves than the others that is why as the sun goes 'down' the last colour to go is red.
White is the hottest colour.
So look at a rainbow next time and discern the colours. You will not see white but you might be lucky and see all the different colours. I say lucky because atmospheric conditions cause you to see a rainbow and the conditions are not always exactly the same each time. That's Nature for you.
Back now to the 'light spectrum' it's worth knowing that the light we see and its colours resides as part of a much broader band which includes ultra violet, x rays, infrared etc. You have probably come across this in your school lessons? Light is also measured in, 'degrees kelvin'. For instance the temperature of daylight is around 5500k
How does this sit with taking a picture, well, the colour of the light influences the outcome of the image.
Your image can go from warm to cold depending on how you set you 'white colour balance. (WCB). Or at what time of day you expose.
Flash has a different colour temperature. Tungsten light has a different colour temperature. And so on.
Depending on the look you wish to put in your image as regards a 'warm' image or a 'cold' image you can set that within most digital cameras.
A word on film, a particular type of film or more so a particular brand will carry a certain colour temperature. This in some cases can be changed at the developing stage but with difficulty, that of knowledge and chemicals.
If you take an image on an adjustable setting digital camera you can set your own 'colour balance'. If you shoot in RAW files then you can change it in the software and as many times as you like producing images with different light balances.
If you are just starting out in photography with an adjustable setting digital camera, you might be better to just shoot in RAW and set your white balance to auto and adjust in the software. Knowing a bit about the science of the colour temperature of light makes the job easier.