Light is electromagnetic radiation. What that means is that light is a type of energy that is formed by the oscillations of particles. Below is an image that illustrates how light particles move in waves. The “visible” portion of the diagram is what humans can see, which is only the teeny tiniest portion of this spectrum.
Notice that there are very short wavelengths to the left and longer to the right. Also notice there is a rainbow in the visible portion. The wavelengths that are visible to humans roughly correspond to the spectrum of colors.
There are two parameters of light.
Wavelengths correspond to the distance of the wave. The figure on the right represents two cycles of the wave.
Amplitude or intensity specifies how much light there is.
Intensity is measured in photons, which is the smallest amount of light that our eyes respond to. We need about 5-7 photons to detect that light is there.
The Eye (overview)
The function of the eye is to determine how much light there is and where it is coming from.
As an overview, we have two lenses in the eye. The cornea, or the outer portion and the lens, or the inner portion. If you touch your eye you would be touching your cornea.
Why do we need both of these lenses?
If our pupil, or the hole in the front of the eye that lets light in, is small (think pinhole camera) it wouldn’t let enough light in. So the pupil can change size. But the large amount of light that is allowed in needs to be focused. The cornea is the transparent covering of the eye, which works to bend light on one portion of your eye and it serves to focus about 70% of the light.
Our second lens (crystalline lens) focuses light based on distance (also known as accommodation or the changing curvature of the lens with distance).
When something is further away the lens is relaxed and less curved, like this.
When an object is close the lens must focus the light and is more curved, like this.