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I remember seeing an experiment where Chris Bishop from the Royal Institution activates a mixture of hydrogen gas and chlorine gas using blue light. However, the blue light was in fact from white light passing through a blue filter. Since blue light is only a component of the white light's spectrum, how does the energy level increase with the blue light? I.e., why wouldn't the white light create enough energy to activate the mixture of gases?

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    $\begingroup$ "Taking something away" should not normally make that which is left more energetic. I wonder if you remember all the details of the experiment you saw correctly. Do you have any reference / link? $\endgroup$
    – Floris
    Sep 27, 2017 at 18:13
  • $\begingroup$ I found a very old (1863) reference to the "power of blue rays" for initiating a reaction between hydrogen gas and chlorine gas, where"red rays" had no such effect. But that's different than your proposed experiment in which removing the red light makes the blue light better able to start a chemical reaction... $\endgroup$
    – Floris
    Sep 27, 2017 at 18:17
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    $\begingroup$ @Floris youtube.com/watch?v=uFQdcKJUijQ around 22:04 $\endgroup$ Sep 27, 2017 at 18:19
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    $\begingroup$ Does he actually shine the unfiltered white light on to the gas? It doesn't seem so. $\endgroup$
    – Steeven
    Sep 27, 2017 at 18:35

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The experiment showed that red, yellow and green light did not provide the energy needed. It was the absence of blue photons that meant the activation energy was not being supplied.

Once you admit blue photons, an individual photon carries enough energy to provide the activation energy, and start the reaction. Since the reaction is exothermal, once you have started, it will "run away".

This is not unlike the early experiments regarding the photoelectric effect: intense light of insufficient energy per photon (long wavelengths) does not free electrons from the cathode; but a low intensity of blue light does.

This is a good demonstration of the fact that light is quantized - that it's the energy of the individual photons that matters, more than the total energy of the entire beam of light.

Incidentally - where it looks like he is shining white light on the sample (although he says "red light"), you are actually seeing the effect of saturation of the sensor. The projected beam clearly shows this.

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  • $\begingroup$ So, white light by itself would have detonated the charge? $\endgroup$ Sep 27, 2017 at 18:47
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    $\begingroup$ Yes I think it would. $\endgroup$
    – Floris
    Sep 27, 2017 at 18:48

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