You can put a transparent glass in the sunlight and you will get a vague outline of the glass on the surface.   However when the glass is partially filled with water, there are a number of changes.

You can see how the top of the glass is outlined in the outline shadow on the counter. With the water, there are two prominent features. The first is there is a shadow from the surface of the water as seen in the next photo.   The shape of the shadow is dependent on the angle of the sun. These photos were taken just after 1 in mid December.

So what happened to the sun light on entering the water. It was refracted towards the bottom and when it exited the water was refracted out into a bright spot at the bottom of the glass, as seen in the next photo. The sunlight slowed when it entered the water and speeded up upon exiting.

Many similar refractions occur in the atmosphere and are often observed by observers. These include rainbows (which include reflection) and sun dogs.