Why Can’t Humans See Infrared or Ultraviolet?
Point a TV remote at your phone’s camera and press a button. On many phones, a small flash appears on the screen. Look directly at the remote and you see nothing. The camera is picking up near-infrared light that your eyes miss. So why can’t we see infrared or ultraviolet? Ultraviolet is a different story. Most of it is absorbed before it reaches the retina.
What Human Eyes Can Detect
Human eyes typically respond to wavelengths from about 380 to 700 nanometre, although the boundary is not perfectly sharp. Violet sits near one end of this range and red near the other. Beyond violet is ultraviolet. Beyond red is infrared.
At the back of the eye, rods and cones contain visual pigments made from opsin proteins and a light-sensitive molecule called retinal. When one of these pigments absorbs a photon, retinal changes shape. That tiny shift is the first step in turning light into a nerve signal.
Human visual pigments are most sensitive to visible wavelengths. For most infrared wavelengths, a single photon carries too little energy to set off the same reaction.
Why Ultraviolet Is Filtered Out
The main barrier to ultraviolet is not the retina but the front of the eye. The cornea and natural lens absorb almost all UV before it reaches the photoreceptors. This filtering also protects the eye, because ultraviolet can damage living tissue.
Why Infrared Feels Like Heat
Objects around room temperature emit most of their thermal radiation in the infrared. That radiation can warm the skin, but our thermoreceptors respond to the change in temperature rather than forming an infrared image.
Thermal cameras use sensors built for those wavelengths, which is why they can show warm bodies even in darkness.
Pit vipers handle infrared differently. Heat-sensitive pits near the face help them locate warm prey. Their eyes are not doing that job.
Cameras can translate some of this hidden radiation into images we can see. So the remote was flashing all along. Your eyes just had no way to register it.
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