I did an experiment in which small canisters (image attached) containing water, were wrapped around with colour papers of different colours were kept under the Sun (image attached)

enter image description here enter image description here.

The different colours were red, green, blue, orange, yellow, black and white? The water in the canister with black paper heated up the most and the one with white paper heated up the least.

Here is my doubt from the experiment. An object is red because it absorbs all colours except red. Likewise, an object is blue because it absorbs all colours except blue. We know that red has the least energy whereas blue has comparatively higher energy.

Let's say white light's energy is 100 units and blue and red lights have energies say, 30 and 10 respectively. Then, the red object absorbs 90 units of energy (white light minus reflected red) and blue absorbs 70 units of energy (white light minus reflected blue). Going by this methodology, the water in the red canister must be hotter than that of the blue canister (since more energy absorbed = more heat)

But it was the other way around. The blue canister had hotter water. Can someone please help me with why this happened? Let me know if my way of approaching this experiment is right. Or should I try again?

NOTE: I used a proper red and blue colour paper (not a shade or a lighter version of the colours) The canisters were kept without their lids.


1 Answer 1


For conducting an analysis as you want you must fold the colour within the actual solar power spectrum. An accurate analysis would involve the knowledge of the actual reflectance spectra of the coloured wraps, and of course matching thermal property and fit to the canister.

Note that the solar power impinging to your canister isn't constant all along the colours and, in addition to this, what we call colour has different bandwidths.

You can see that something looking blue absorbs more energy from the sun than a red counterpart by looking at the solar irradiance (power versus colour, basically):

enter image description here

Taken from https://bit.ly/2LyxMhP


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