What is the difference between infrared and ultraviolet light?

In one line

The rule

Infrared light has longer wavelengths than visible red light, while ultraviolet light has shorter wavelengths than visible violet light.

• Infrared wavelength: about 700 nanometres (nm) to 1 millimetre (mm)
• Ultraviolet wavelength: about 10 nm to 400 nm

Let's understand it simply

Imagine a rainbow. You see all the colours from red to violet. But the rainbow does not show all the light that exists. There is more light just outside what your eyes can see.

On the left of red, there is infrared light. You cannot see it, but you can feel it as heat. For example, when you stand near a hot tawa (pan), you feel warmth on your hand — that is mostly infrared.

On the right of violet, there is ultraviolet light. You cannot see this either, but it can burn your skin. That is why you get sunburn if you stay too long in the sun — ultraviolet rays are responsible.

How it works

1. Light travels as waves. Each wave has a length, called wavelength (distance from one wave peak to the next).
2. Visible light is the part of the spectrum your eyes can see (about 400–700 nm).
3. Infrared has longer wavelengths than red light (more than 700 nm). It carries less energy than visible light.
4. Ultraviolet has shorter wavelengths than violet light (less than 400 nm). It carries more energy than visible light.
5. Our eyes cannot detect either, but our skin and some devices can sense them.

The story behind it

Infrared light was discovered by William Herschel in 1800. He used a thermometer and sunlight passing through a prism. He noticed it was hottest just beyond the red part — he called this "infrared" (meaning "below red").

Ultraviolet light was discovered soon after, in 1801, by Johann Ritter. He saw that silver chloride (a chemical) darkened faster just beyond the violet part of the spectrum. He called this "ultraviolet" (meaning "beyond violet").

Both discoveries showed that there is more to light than what we see.

Where you see it in real life

• Infrared remote controls: The red light from your TV remote is actually infrared. It sends signals to the TV.
• Thermal cameras: These cameras "see" heat, not light. Firefighters use them to find people in smoke.
• Pressure cookers and tawa: The heat you feel from the side, even if you don’t touch it, is infrared.
• Sunburn: When you get sunburnt after playing cricket in the sun, it is ultraviolet rays damaging your skin.
• Sterilizing water: Some water purifiers use ultraviolet light to kill germs.
• Fluorescent materials: Some white clothes or posters glow under special UV lamps (used in parties or banks to check notes).

Why it matters

• Infrared is used in night vision, TV remotes, heaters, and medical scans.
• Ultraviolet is used to kill germs, detect fake notes, and make some materials glow.
• Health: Too much ultraviolet can damage your skin and eyes, but some is needed for vitamin D.
• Science: Understanding these helps in astronomy, weather forecasting, and even studying plants.

Worked example

Let's compare their wavelengths:

Given: Visible red light wavelength ≈ 700 nm
       Visible violet light wavelength ≈ 400 nm
Step 1: Infrared is longer than red → >700 nm
Step 2: Ultraviolet is shorter than violet → <400 nm
Step 3: So, infrared: 700 nm – 1 mm; ultraviolet: 10 nm – 400 nm

Common mistakes

• Thinking both are dangerous: Infrared is mostly safe and just feels warm. Ultraviolet can harm skin and eyes.
• Believing both are used for heating: Only infrared is used for heating; ultraviolet is used for killing germs.
• Assuming both are the same as visible light: Both are invisible to our eyes, but have very different effects.
• Mixing up their positions: Infrared is beyond red (longer wavelength), ultraviolet is beyond violet (shorter wavelength).

Remember it

Infrared is "below red" (warmth you feel), ultraviolet is "beyond violet" (causes sunburn).