eyeball. 1  Photoreceptors are the cells in the retina that respond to light. Their distinguishing feature is the presence of large amounts of tightly packed membrane that contains the photopigment rhodopsin or a related molecule.
In chemical analysis,  matrix refers to the components of a sample other than the analyte of interest. The matrix can have a considerable effect on the way the analysis is conducted and the quality of the results obtained; such effects are called matrix effects. For example, the ionic strength of the solution can have an effect on the activity coefficients of the analytes.
Definitions. 1  Sampling is the process of selecting a representative group from the population under study. 2  The target population is the total group of individuals from which the sample might be drawn. 3  A sample is the group of people who take part in the investigation.