Many believe that humans’ best friends can only see in various shades of grey. While it is true that their colour range is limited, dogs can still appreciate sunsets, rainbows and forests. They just see everything a little differently. The question is why?

Human and dog eyes work very similarly. Light rays enter the eyeball through the pupil, after being refracted by the cornea, pass through the crystalline lens and hit the retina at the back of the eye. The lenses help both of us see objects sharply, since light rays are focused and controlled. Yet the magic happens in the retina.

The retina is covered with two different types of light sensitive receptors, namely cones and rods. Cones are important for seeing colours and can be divided into red, green and blue for humans. When light hits the cones, they send signals to the brain. The brain does most of the work making a crystal clear 3D picture. Dogs, however, miss the cones that detect red. This makes their world look a little more brownish or yellowish when compared to ours. They have trouble telling purple and blue, or red and brown apart.

This phenomenon also happens in humans and is labelled as colour blindness. The condition means that people have difficulty distinguishing between certain colours.

The second receptor type is called rods. These just tell light and dark apart. They are handy when there isn’t much light around and to help detect different sizes and shapes. For even better night vision, dogs also have a tapetum, a mirror-like structure in the back of the eye. The tapetum reflects incoming light so that it hits the retina again. That mirror is why a dog’s eyes glow weirdly in the dark.