How do human eyes feel light?

Human visual organs are mainly composed of eyes, optic nerve and related parts of the brain. The structure of the human eye is shown in the figure. Eyes are like cameras, and pupils are like the aperture of cameras. The iris around the pupil acts as an aperture. It can adjust the pupil size according to the intensity of light and control the amount of light entering the eyes. The lens is like the lens of a camera, focusing the external scene on the retina. It is the photoreceptor cells on the retina that play a photosensitive role. It transmits the information it feels to the visual center of the brain through the optic nerve, forming a visual perception of light and shade, color, shape, dynamics, distance, etc., in order to obtain external information.

There are two kinds of photoreceptor cells in the retina. There are dense cone cells in the central part facing the pupil, and rod cells are mainly around the central part. There are about 8 million cone cells, and there are three kinds of them, which respond to the light in the red, green and blue regions of the spectrum. There are about1.200 million rod-shaped cells, which have high sensitivity and can feel weak light.

In a bright environment, when the brightness exceeds several cd/m2 (Candeira /m2), the rod-shaped cells lose their activity, mainly because the cone-shaped cells play a major photosensitive role. Vision at this time is called bright vision. The characteristic of bright vision is that it can distinguish the color of vision and accurately distinguish the details of objects. At this time, the eyes are most sensitive to the green light with the wavelength of 555nm. With the gradual decrease and increase of wavelength, the sensitivity gradually decreases, until 380nm and 780nm, the sensitivity drops to zero, that is, no light sensation is produced. When the brightness is less than a few percent of cd/m2 in a very dark environment, the cone-shaped cells lose their activity, while the rod-shaped cells recover their photosensitive function. The vision at this time is called dark vision. Its characteristic is that it can only distinguish between light and shade, without color sense, and its ability to distinguish the details of objects is greatly reduced. Dark vision is most sensitive to radiation with a wavelength of 507 nm. Between the above two situations, people are in a dark but not very dark situation, and when the twilight is boundless, people's vision is called intermediate vision. You can distinguish colors when the brightness is high, and only have a sense of light and shade when the brightness is low.