Question

why glasses are transparent

Answer 1

When light falls onto a material it can behave in a number of ways:

It can be absorbed in the material (and its energy is converted to heat in the material)
It can be reflected from the surface
It can be transmitted through the material

Most materials exhibit a combination of those behaviours to greater or lesser extent.

If light is transmitted into a material (such as glass) then it emerges out the other side and hence you can see through the material. MOst materials which transmit light do not transmit all wavelengths (colours) of light equally, and you see only the colours that are transmitted when it emerges. Thus coloured glass has absorbed all the wavelengths except the colour that you see the glass to be.

IF you are asking WHY some materials transmit light then I'm afraid I can't help you.