I work in a chemistry laboratory. We are currently trying to understand photochemistry. In our studies, we typically use LEDs of different wavelengths. For example, 365 nm, 405 nm and 450 nm. When I shine the LEDs on surfaces such as a bench or skin and try to measure the temperature of the surface, the temperature goes up as the wavelength increases. My understanding was that IR light is associated with thermal heating. Could someone explain how in the series of LEDs I am using, longer wavelengths tend to generate more heat compared to shorter ones?
I think all 3 LEDs are of similar intensity.
[Physics] How does visible light generate heat
photonsthermodynamicsvisible-light
Best Answer
Given the same intensity or the same power density, the heating potential of all three LED's should be the same, since all energy radiated by the LED's will eventually turn into heat - it's just a matter of where the light radiated by the LED's will be absorbed.
There is no universal law saying that the absorbance for infrared light should be greater than the absorbance for visible light, so the outcome of experiments like yours, will depend on the absorption coefficients of irradiated objects with respect to specific wavelengths.
As an example, the graphs below (copied from this site) show the absorption coefficient spectrum for different skin components.
We can see that the absorption coefficient of water is increasing with the wavelength, while the absorption coefficient of melanin is decreasing.
So, what you've observed in the lab may say something about the light absorbance properties of the objects you've tested, but is not an indication that light with a longer wavelength produces more heat.