Can humans see infrared rays

Answer 1

Humans can see infrared (ir) rays provided that a) the rays are sufficiently powerful, b) the background environment is sufficiently dark, and c) the wavelength of the infrared is sufficieitntly short.

The normal range of human vision is 400nm to 700nm. Nowadays many people own infrared laser pointers as well as infrared tactical flashlights and headlamps. The light from a 100mW 808nm laser pointer is clearly visible at night in a dark room. However, it appears dim, even though it is 20 times as powerful as a typical 5mW laser pointer. The light from a 1W 980nm laser pointer is completely invisible.

The light from a 5W 940nm LED flashlight is visible at night in a dark room, but only if the flashlight is pointed directly into the eyes, and even then it looks very, very dim.

It is not possible to see body heat with the naked eye, as the ir is not sufficiently intense.

The retina of the human eye has three types of cone cells for color vision, commonly known as red, green, and blue. Above 600nm the blue cones play no perceptable role in color vision. Above 700nm the sensitivity of the green cones tapers off more slowly than that of the red cones, resulting in a very gradual apparent color shift towards orange at higher wavelengths. This phenomenon is known as infrared color reversal.

At very low intensities, thermal energy within the human eye charges cone cells with enough energy to respond to stimulus from ir photons that would ordinarily be too weak to elicit a response. Due to the random nature of heat at the microscopic level, only a small percentage of cone cell are charge sufficiently for this to occur. A 1 degree C rise in body temperature increases sensitivity to all wavelength by an average of 2.5%.