Human eyes cannot perceive colours at night because cones, responsible for colour perception, are only activated under intense light conditions.

Colourblind people see colours differently because they do not have all the necessary information from the L, M and S cones. A simile would be to try to obtain different paint colours by mixing only two primary colours: we would never be able to obtain the full range of existing colours.

María Violeta Gómez Vicente, PhD in biochemistry and professor at the Department of Optics, Pharmacology, and Anatomy of the University of Alacant writes about the phenomenon of heterochromia.

There are numerous causes for eye redness, and many of them are not serious. Begoña Domenech Amigot, professor of the Department of Optics, Pharmacology, and Anatomy of the University of Alacant, explains.

The sclera, known for its color as «the white part of the eye», is an opaque, resistant, and fibrous membrane that covers part of the eyeball, giving it shape and protecting its interior.