Main Question or Discussion Point
I was testing a small spherical mirror with sunlight and wondered about something. The size of the mirror is 2.5cm. The spot size of the reflected light grows over larger distances but it doesn't seem linear. For example, at a meter or less, the spot is very close to the mirror size but at a few times that it's about twice the size. I was expecting the image to be essentially the size of the mirror with virtually parallel light from the sun which I have seen with larger mirrors. I tried the opposite experiment where I attached the mirror backwards to a large planar mirror and looked at the size of the dark spot. That got smaller to about half size at larger distances. I wouldn't think it's a true wave effect such as diffraction. Is it? I wouldn't think it's due to the finite angular image of the sun. Is it? Another clue, a larger circular mirror of about 10cm diameter shows only a small enlargement of about 1cm around the spot size over larger distances suggesting my hunch it's the solar image not being a point. If it's due to either of these I would think it's the angular size of the sun since my primitive calculations seem to suggest a reasonable close correlation. But that should, be linear. Maybe it is diffraction from the edge. Any thoughts? Thanks.