text: The optical depth is a measure of the proportion of radiation absorbed or scattered along a path through a partially transparent medium ; the optical depth to a close-by object is zero ; as the distance to the object increases , so does the optical depth .
simplified: The fog between a person and an object that is immediately in front of them has an optical depth of zero . As the object moves away , the optical depth increases until it reaches a large value and the object is no longer visible .