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
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.