Why can't we see infrared light with our eyes?

In one line

The rule

The human eye can only see light in a certain range of wavelengths, called the visible spectrum (about 400–700 nanometres).

Infrared light has longer wavelengths than visible light, so our eyes cannot sense it.

Let's understand it simply

Imagine your eyes are like a camera with a filter. This filter only lets in certain colours — like red, green, and blue. Anything outside this range, like ultraviolet (UV) or infrared (IR), is blocked. Infrared is a type of light, but its waves are longer than red light. Our eyes just don't have the right parts to catch these longer waves, so we can't see them — just like you can't hear a dog whistle, even though it's making a sound.

How it works

1. Light enters your eye through the cornea and lens.
2. The lens focuses the light onto the retina at the back of your eye.
3. The retina has special cells (rods and cones) that detect light.
4. These cells only respond to light in the visible range (400–700 nm).
5. Infrared light has wavelengths longer than 700 nm, so the cells do not react to it.
6. Because the retina doesn't send signals for infrared, your brain never 'sees' it.

The story behind it

Scientists learned about the limits of human vision by studying light and how it behaves. In the early 1800s, William Herschel discovered infrared light while using a thermometer to measure heat beyond the red part of sunlight. He found that there was invisible light carrying heat — what we now call infrared.

Where you see it in real life

• TV remote controls: The remote sends infrared signals to your TV, but you can't see the light — only the result.
• Sunlight: Sunlight contains infrared, which you feel as warmth on your skin, but you can't see it.
• Night vision cameras: These cameras can 'see' infrared, so they work in the dark, unlike our eyes.
• Gas stoves: The heat you feel from a flame is partly infrared radiation, even if you can't see it.
• Mobile phone cameras: Sometimes, if you point a remote at your phone camera, you can see a faint light — that's the infrared, which the camera can detect!

Why it matters

• Safety: Not seeing infrared protects our eyes from some types of damage, as too much invisible light could harm us.
• Technology: Understanding what we can't see helps us build tools (like night vision goggles) to detect it.
• Biology: Some animals, like snakes, can see infrared, which helps them hunt in the dark — showing how vision is adapted to needs.
• Science: Knowing our limits helps scientists study the world in new ways, using special detectors for invisible light.

Common mistakes

• Thinking infrared is a colour we just 'miss' — it's not a colour for us at all.
• Believing our eyes can see all types of light — we can only see a small slice of the spectrum.
• Confusing heat and light — not all heat is visible, and not all light is hot.

Remember it

Our eyes are like a radio tuned to only one station — we can't pick up infrared because we're not built for it.