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We can see light waves only if they travel directly at us and get in our
eyes. Usually, light must hit something solid and "light it up" (be
reflected into our eyes) before we can be sure any light is there. We
don't see light go through the dark, because there is nothing for it
light up yet.
None of the light is heading towards us and entering our eyes.
Sometimes there is dust in the air it reflects enough light into our
eyes for us to see a "light beam".
Actually, it is pretty lucky we cannot see light as it goes by. If we
could see all the light waves flying around, the whole room would look
very bright. And, like when someone shines a flashlight in our eyes, we
could never see well enough to get around!
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