Learning objectives: describe the electromagnetic spectrum and how it is related to vision, the electromagnetic spectrum is a range of energy expressed in different wavelengths. Humans can only see a small portion of the spectrum; wavelengths 400 to 700 nanometers in length. Discuss the causes and treatments for each one. They have two parts, long proteins called opsin, and smaller light-sensitive ones called retinal: when the two combine, the molecule absorbs light. This is called isomerization and causes a chain reaction that activates thousands of charged molecules to create electrical signals in receptors. This activates the receptors: describe differences between the distribution of the rods and the cones, rods and cones are dispersed in different ratios in different areas of the eye, the fovea only has cones. If you are looking right at an object, you cannot see it: retinitis pigmentosa is genetic, although not everyone will get it, first it attacks peripheral rods, which results in poor peripheral vision.