So many things! Rainbows inside without a mirror or reflective surface, impossible. Testing for curvature was a big one. Another example is with moonlight - not only can you test the temperature of the moon light and compare it to the moon shade, you can also look at the moon on a cloudy night and see how it illuminates the clouds closest to it and not ones further away (but still in your vision)... if the moon really were 240k miles away, the light ‘reflecting off it’ from the sun would NOT be a narrow beam. You can test this yourself with a torch - when the torch is close to an object, it’s circle of light is small, but as you pull the torch away from the object, the area that is lit up increases. This effect does not change with different size lights. And furthermore when you shine light on a ball, it only reflects a point of light, not a whole hemispheres worth (unless it’s a concave or disc shape). I’m happy to attach some pics to illustrate if you require.