Try and answering this.

The waves are refracted by the cornea, and again by the lens. Some of them are stopped by the iris, while other will pass through the pupil. They propagate through the vitreous humor and reach the retina, where they will be focused onto the photosensitive cells. Depending on their wavelength, they may excite cones (color sensitive) or rods (black and white). The excited cells create an action potential in the optic nerve, which is combined with the signals of other photocells into a pattern that is transmitted to the visual cortex of the brain.

Sounds more like biology than physics...

americans began to notice growing differences between the northern and southern sections of the country. there were more people living in cities in the north, which made it more urban. more people lived on farms in the south, which made it more rural. while many people were farmers in the northern states, there were now other ways to make a living, such as working in mechanical cotton gins factories manufacturing cloth, tools and other goods. people in the north supported high tariffs on goods imported from europe because the extra cost encouraged people to “buy american” instead. in the southern states, planters did not like tariffs because they sold most of their production to europe and bought their tools from there as well. southerners believed the protective tariff was something that only northern factory owners keep their profits high. so the southern politicians came up with an idea. they would opt out. they would simply ignore the protective tariff. it was ‘null and void’ in south carolina. the state would not collect the taxes on goods imported into south carolina, no matter what the president said they had to do. this decision led to the nullification crisis.