Psychophysics is a branch of psychology that studies the relationship between the physical properties of stimuli and our psychological perceptions of them. E.g., "color" is a psychological perception of specific wavelengths of the visible spectrum of light. Color does not exist without an eye to see it. Every sensory modality has similar interesting relationships with the physical world.
The experiential perception is NOT the same as the physical stimulus in many cases.
See: http://en.wikipedia.org/wiki/Psychophysics
No relationship. psychology is the study of the mind and physics is the study of matter and energy and how they interact.