Our eyes are designed to detect electromagnetic radiation with a little less than the energy needed to break covalent bonds, from 40 kcal mole^-1 (dark red) to 70 kcal. mole^-1 (violet).

A cis-trans isomerization is a safe means of detecting light because no bonds are permanently broken, only rearranged. In contrast, ultraviolet light is harmful because it breaks carbon-carbon single bonds, and infrared radiation goes undetected because it has too little energy to set off the retinal trigger.