How is indirect sun exposure to the retinas different from direct sun exposure to the retinas in terms of harmfulness?
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The danger to the eyes from the sun is in the form of electromagnetic radiation. That is, visible light, radio waves, x-rays, etc.
The problem isn’t necessary direct vs indirect, but rather it has to do with the *amount* of radiation hitting your eyes. If you look directly at the sun (direct exposure), *lots* of EM radiation floods your eyes which will most certainly cause damage. If you take and reflect that same EM radiation onto a low reflectivity surface, only a tiny amount of EM radiation (mostly visible light) will reflect back to your eyes, which won’t cause damage nearly as fast and is *significantly* safer to look at.
Take a normal (untreated) mirror as another example, they are pretty good at reflecting light, but don’t reflect thermal radiation very well. They also don’t reflect UV-C at all (gets absorbed by glass), UV-B is mostly absorbed (and partially reflected), and UV-A is almost fully reflected. So when you look at the sun in a mirror, there’s still a lot of UV rays hitting your eyes, but also *tons* of visible light. This is better than looking straight at the sun and getting all those harmful things, but still very bad for your eyes.
tl;dr – Indirect sun exposure is generally less harmful because not all surfaces reflect everything the sun sends out. If you have a perfect reflector, indirect sun exposure would be just as bad as direct sun exposure.
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