This process of capturing a photon and transforming it into a physiological response is known as phototransduction.
Some opsins allows vision in just a short stretch of wavelength. This is equivalent to seeing in only one colour. Two opsins allow vision in two colours, and is usual in mammals. Four opsins allows vision in full colour, and is usual in teleost fish, reptiles and birds. In mammals, only Old World monkeys, apes and humans have trichromacy, full colour vision.
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