Why does the inner wall of a smooth glass seemingly have rings when viewed from above?

Question

I was looking at my glass and I found these rings. Are they reflection of the upper circle of the glass?

Answer 1

More likely the rings are reflections of the bottom of the glass. Light that entered through the sides and that was reflected at the bottom, travels up the wall, and bounces repeatedly.

An experiment I did with one of my drinking glasses: rings appear in the drinking glass if a beam of light is aimed at the bottom of the glass. Then, if the bottom is lowered into water, the rings disappear again. This means the rings are due to internal reflections in the glass, because the water allows the light to escape from the glass.

Answer 2

I believe this is a side effect of manufacturing process.

In high-volume manufacturing glass is formed into target shape in the mould at high temperature, where glass is still soft. It takes just few seconds.

After glass formed into target shape in a mould - it has to cool down slowly to avoid excessive stress which might crack the glass which will take hours. Doing so inside the mould would be expensive as mould is needed for further copies of the glass every few seconds.

So glass is slowly cooling down outside of the mould, while still being soft and not supported by mould walls. Due to gravity it curl up a little, and only then solidify completely.

This effect is likely very sensitive to process parameters (i.e. slightly higher temperature after mould - and you get these rings). Few of my glasses do not have this - probably they were moulded at slightly lower temperature.