And eyes are highly sensitive: a dark-adapted rod cell can fire off a signal to the brain on absorbing a single particle of light, or photon, the smallest quantum unit of an electromagnetic wave.

• 同时眼睛的灵敏度也相当高，可在黑暗中发挥作用的杆细胞只要吸收到一个光子，也就是电磁波的最小量子单位，就会传送讯号给大脑。