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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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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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