Maybe if there is a medium between the emitter and receiver that scatters light, like an atmosphere. Or if you don't have line of sight because there is an occ

Maybe if there is a medium between the emitter and receiver that scatters light, like an atmosphere. Or if you don't have line of sight because there is an occluding object like a planet or star in the way. Or you have line of sight but it's really close to a super bright object so you can't see anything.