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Is this XKCD https://what-if.xkcd.com/145/ saying that a surface is only heated by the component of the rays that are perpendicular to the surface?

Conservation of étendue:

enter image description here

DKNguyen
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  • @StephenG-HelpUkraine That actually doesn't help since Bill Clark's answer in that is basically what I am asking but you can read in the comments for that answer that there is some contestation about it. – DKNguyen Jul 02 '22 at 13:59

1 Answers1

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The claim is

... you can't smoosh light beams together without also making them less parallel, which means you can't aim them at a faraway spot.

I don't see where it says "a surface is only heated by the component of the rays that are perpendicular to the surface".

The diagram you show is a counter to the previous one, where the light all comes out in one direction

enter image description here

Conservation of étendue says you can't do this.

BioPhysicist
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  • I guess I don't see how the "making them less parallel" part is relevant unless that is the case. Because if it's not the case then all the rays still end up in a smaller area regardless to heat the spot. – DKNguyen Jul 02 '22 at 04:12