In high school physics, the teacher showed that light is transmitted through two polarizing filters with parallel alignment, but not when the two filters had alignments offset by 90 degrees. 

Recently, I saw a demonstration where the second filter was rotated 90 degrees to shut off the transmission of light...and then a third filter was added to the string in an orientation equal to the first filter, and the transmission of light was restored.

Why?

I am exploring the properties of waves when traveling thorough a transparent medium. I observed that when white light is shone through a prism the blue light refracts more than the red light in the spectrum. This is due to the interaction between the blue light and the molecules within that medium. 

I wish to know if anyone can give me more details the nature of this interaction.

A more through explanation as to why wavelength of the light effects the speed through a medium would be appreciated and any relevant laws or theories as this is a mystery to me.

I always assumed they were pinhole lenses, but then I realized I really don't know (and don't want to break apart a perfectly good phone).

I walked into my study this afternoon at 2 and saw an odd reflection on the West wall. As my eyes grew accustomed to the darkened room I realised it was no simple relection but an inverted image (IMG_1987) of our garage outside to the East (IMG_1990).

The light was entering the room through a triangular gap in the curtains where a curtain hook had broken (IMG_1989).

It seems a large and irregular gap to create such a clear image. How large can a 'pinhole' be and still create a clear image? What determines this size?