TwiddleLab / Physics / Light and Sight

Light Is One Slice of a Spectrum

Lesson 9 of 10 · 7 min · Free, no account needed

What you will learn

• Radio, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are one family: the electromagnetic spectrum. Only the wavelength differs.
• All of them travel at the speed of light, c ≈ 3 × 10⁸ m/s, and c = f × λ: a longer wave has a lower frequency.
• Your eyes see a tiny window, about 400 to 700 nm. Even light needs about 12.5 minutes to reach Mars.

Start the lesson

How the lesson goes

1. Wi-Fi, X-rays and the colour red are the same kind of thing. What makes them different?

   They are all waves. Guess how long a Wi-Fi wave is: drag the marker.

2. Slide the wavelength. Find radio waves, visible light and X-rays.

   The green marker is the real Wi-Fi wave. Only the wavelength changes from band to band.

3. Put each device on the band it uses.

   Long waves on the left, short on the right.

   Each device uses waves of a certain length: FM radio about 3 m, a microwave oven about 12 cm, a TV remote about 940 nm (infrared, invisible), a UV lamp about 365 to 400 nm, X-rays around 0.1 nm.

4. The electromagnetic spectrum.

   The electromagnetic spectrum: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Same kind of wave, different wavelength (λ).

   In empty space all of them move at the speed of light, c ≈ 3 × 10⁸ m/s (300 000 km every second).

   c = f × λ, like any wave: a long wave has a low frequency, a short wave a high one.

   Band edges are approximate: real bands overlap and are named by use as much as by length.

5. Which waves get through? Drag the wall and the hand into the beams.

   The boxes on the right show how much of each beam arrives.

6. Game: Mars rover. Drive to the rock.

   Every command travels at the speed of light, and still takes 12.5 minutes to reach Mars. Plan ahead: you have 4 sends.

7. Math: on this scale each tick is 10 times longer than the next. Tap the wave 1000 times longer than 1 mm.

   1 mm × 10 = 1 cm, × 10 = 10 cm, × 10 = 1 m. Three ticks is 10 × 10 × 10 = 1000 times longer.

8. Wi-Fi at 2.4 GHz: how long is its wave, in metres?

   Use λ = c / f with c = 300 000 000 m/s.

   λ = c / f = 3 × 10⁸ / 2.4 × 10⁹ = 0.125 m.

9. Your mission against the best plan. Replay both.

Previous lesson: Mixing Light vs Mixing Paint · Next lesson: Project: Laser Maze · Part of the course: Light and Sight

Lessons are interactive and need JavaScript. Turn it on to play them.