We all know that light is made up of photons and when they hit an object, they get reflected towards your eye. The lens present in the eye focuses the photons on the retina. There are two types of photo-receptors on the retina
Cones are liable for colored vision and require bright, focused light, whereas rods are responsible for black and white visions, and are more sensitive to light than cones thus they become active in the presence of the low amount of photon. So when the photons hit the retina, they stimulate these receptors. These receptors then send a signal to the brain and construct the picture of the object.
Have you wondered how animals see in the dark?
Now, the night vision of animals can be explained by 2 factors. First, the spectral range of light that they can see. The light is made up of photons with different energies, for example, gamma rays are photons with the highest energy and shortwave are photons with the lowest energy. When we arrange these photons according to their energy, we get the spectrum. We can see a very small range of this spectrum and this small range is known as visible light but on the other hand, many animals have an eye that is sensitive towards a larger range than the human eye. This is why some of them can see the infrared and ultraviolet spectrum too, for example, bees can see ultraviolet light and fish can see infrared both of which are invisible to human eyes.
The infrared light is very useful when it comes to night vision, as you may already be familiar with the fact that we use infrared light in our night camera and glasses.
The second reason behind the night vision is their eye structure, these animals generally have large eyes and pupils, their pupil open wider and allow a maximum amount of light to enter in the dark. Their lens is larger and situated near the retina. This also allows a lot of light to register on the retina. They have a high rod to cone ratio for example cats have a 25:1 ratio of rods to cones, which is only 4:1 for humans. It means cats only need 1/8th amount of light to see in the dark compared to humans. But lack of cones also means that they don’t get a good colored vision. This is the reason why nocturnal animals are generally color-blind. The rods also have a photosensitive pigment called rhodopsin which plays a vital role in night vision.
You must have seen that many animals like dogs, cat’s eyes glow during the night, this is because they have a layer behind the retina called tapetum lucidum.