Since transparent objects allow light to pass through, how can they be visible ?
I'd say for two main reasons:
- probably, a perfectly transparent object does not exist in nature;
- most importantly, transparent objects - as any other (finite) object - have boundaries, interfaces.
There’s two main reasons you can see the object and both are due to the boundary. Sometimes you see the effects of refraction but mostly you see the partial reflection. Both very interesting subjects.