recall that wavelengths in the range 400–700 nm in free space are visible to the human eye

📚 Electromagnetic Spectrum: The Rainbow of Light

What is the Electromagnetic Spectrum?

The electromagnetic spectrum is a continuous range of all possible electromagnetic waves, sorted by their wavelength (or frequency). Think of it like a giant rainbow that stretches from the very long waves of radio to the incredibly short waves of gamma rays. 🌈

Why do we care about wavelengths?

Wavelength determines how a wave behaves and what it can do. For example, radio waves can travel long distances and go through walls, while ultraviolet light can cause sunburn. In this lesson we focus on the part of the spectrum that our eyes can see.

The Visible Window: 400–700 nm

The human eye can detect electromagnetic waves with wavelengths between \$400 \text{ nm}\$ and \$700 \text{ nm}\$. This range is called the visible spectrum. Each wavelength corresponds to a different colour that we perceive.

Colour Mapping

Wavelength (nm)ColourReal‑World Example
400–450VioletPurple flowers, violet lights
450–495BlueBlue sky, blue LEDs
495–570GreenLeaves, green traffic lights
570–590YellowSunset, yellow pencils
590–620OrangeOranges, orange traffic lights
620–700RedRed roses, red traffic lights

Remembering the Range

  1. Think of the visible spectrum as a rainbow strip on a ruler that starts at 400 nm (violet) and ends at 700 nm (red).

  2. Use the mnemonic “Very Big, Green, Yellow, Orange, Red” to recall the order of colours.

  3. When you see a colour, you can estimate its wavelength: red is longest, violet is shortest.

Quick Quiz

  • Which wavelength range corresponds to blue light? (Answer: 450–495 nm)

  • What colour is produced by light at 650 nm? (Answer: Red)

  • Why can we’t see ultraviolet light? (Answer: It’s shorter than 400 nm, outside the visible range)

Takeaway

Remember: \$400 \text{ nm} \le \lambda \le 700 \text{ nm}\$ is the sweet spot where our eyes turn light into the beautiful rainbow of colours we enjoy every day. 🌟