Whether a material is opaque, translucent, or transparent depends on its absorption/transmission of light.
Work Step by Step
Opaque materials absorb incident light and do not transmit, meaning you cannot see through them. Transparent materials transmit light through them, which is why it's possible to see through them. Translucent materials are an intermediate, but while they transmit light through them, they scatter this light such that any objects that you look at through them are often not clearly resolved.