The physical stimulus for vision is light, a form of electromagnetic energy. The electromagnetic radiation to which human vision is sensitive to is a very small part of the entire electromagnetic spectrum. Light is composed of particles called photons and they travel in waves.¹
The three physical characteristics of wavelength, amplitude and purity are interpreted as colour, brightness and saturation respectively. Wavelengths of 400 nm appear as violet and 700 nm appears as red. Higher amplitude waves are perceived as brighter. The less pure a light source is, the more washed out it will appear.¹
The back of the eye contains three different cell layers: Photoreceptor layer, Bipolar cell layer and Ganglion cell layer. The photoreceptor layer is at the very back and contains two types of photoreceptors: rods and cones. Rods cannot perceive colour but work best in dim-lit conditions. This is why in the dark people tend to see in shades of grey.
The ventral dorsal stream (green) and the central stream (purple).²
The cones allow us to view colour and contain three different specific photopigments that respond to specific wavelengths of light: those sensitive to long (red) wavelengths, medium (green) wavelengths and short (blue) wavelengths.¹ Photopigment is most sensitive to medium wavelengths but has slightly different sensitivity than green cones.
The photopigments split into two when the appropriate wavelength of light hits the photoreceptor, sending a message to the bipolar cell layer, then to the ganglion cells which send the information through the optic nerve to the brain.¹ It travels to the thalamus and is then relayed to the primary visual cortex where it is finally interpreted for conscious use.
References:
1. Psychology: the Science of Behaviour - Neil R. Carlson
2. Image Credit: Wikimedia Commons