A blacksmith might take a steel rod and put it in a flame when forging metal he can judge how hot the metal rod is by watching the changing colour of the steel in chemistry lessons you would have used Bunsen burners and noticed that you can get a blue flame from it whereas a tea candle burns with a yellowish flame as an object gets hotter two things happen to the light that it emits first it begins to emit more light at every wavelengths so that it gets brighter second it changes colour because the peak of the emission shifts to the blue end of the spectrum let me explain this using a graph with wavelength on the x-axis and the intensity of the wave on the y-axis take a star for example that has a surface temperature of 4000 Kelvin it emits radiation with all of these wavelengths ranging from infrared to visible light to ultraviolet but you can see that the peak intensity is around 700 nm in the red region of the visible light spectrum even though the star emits all these other wavelengths the fact that it emits red visible light at the highest intensity will make it appear red for a star that has a higher surface temperature such as 7000 Kelvin it emits more light so the graph must be higher to show that it emits all wavelengths at a higher intensity but if you look at it carefully we can spot that it no longer peaks in the red region the peak has shifted towards the left hand side so that the maximum intensity is now in the blue region this star appears BLUE comparing these two stars we begin to see a relationship between the object's temperature and the wavelength with the highest intensity so the peak wavelength is inversely proportional to temperature This law is know as the Wien's displacement law The constant of proportionality between them is called the Wien's constant which is 2.898 x 10^-3 meters Kelvin this law is only applicable to objects that can be considered as black bodies a black body is an object that absorbs all radiation incident upon it and then it re-radiates all energy our Sun for example can be approximated as a black body that has a surface temperature of 5800 Kelvin using Wien's displacement law let's work out its peak wavelength peak wavelength is equal to 2.898 x 10^-3 divided by 5800 K that gives us 5.02 x 10^-7 m or we can say it's 502 nm and that is green in the visible light spectrum so our Sun emits the most visible light in the green region but why doesn't it appear green to us? this is more to do with biology than physics our eyes have receptors for red, green, and blue colours even though the Sun's spectrum peaks at green it also emits in the red and blue regions our eyes have not evolved to distinguish colour of highest intensity from colours of lower intensity our brain is going to mix all three colours together forming white thanks for watching my video if you think you've learned something from it be sure to subscribe!