Simplify the following text for a Grade 7 native English-speaking person.
Original: Optical depth , or optical thickness is a measure of transparency , and is defined as the negative logarithm of the fraction of radiation ( e.g. , light ) that is not scattered or absorbed on a path .
Simplified: Optical depth is a measure of transparency . It is defined as the fraction of radiation ( or light ) that is scattered or absorbed on a path .