A First Course in Physics · Ch. XXII, §§ 587–593 · Chapter 22: Invisible Radiations
69. Invisible light: infra-red and ultra-violet
Goal: Your student can show that the sun gives out invisible rays beyond both ends of the spectrum, explain that good absorbers are good radiators, and explain fluorescence.
You'll need
- The book, open to these pages
- A glass prism (or a mirror in a pan of water), a sheet of white paper, and three thermometers with bulbs blackened with a marker
- A TV remote control and a phone camera
- Tonic water (contains quinine) and an ultraviolet "black light" flashlight if you have one
- A black and a white piece of cloth or paper
- A notebook and pencil
What's in the book
The radiometer; infra-red rays beyond the red, reflected and focused like light; water absorbs them but some liquids pass them; ultra-violet rays beyond the violet, found with photographic plates; the sun's spectrum is far longer than what we see; bodies glow red at about 525° C and white at 800–1200° C; good absorbers are good radiators; fluorescence. Seven questions follow.
The lesson, step by step
Hook question
SayIs there light that our eyes cannot see?
Read together
Read §§ 587–593. The book's conclusion:
The sun's spectrum is therefore at least ten times as long, measured by wave lengths, as is the part which we see.
Try it: Herschel's experiment
In bright sun, use the prism to throw a spectrum onto white paper in a box (shade the paper). Put one blackened thermometer in the blue, one in the red, and one just beyond the red where you see no color. After 5 minutes, the one beyond the red often reads highest: invisible infra-red rays (§ 588). Keep a third thermometer in the shade for comparison.
Try it: see the invisible
Point a TV remote at a phone camera and press a button: the camera shows a flashing light your eyes cannot see. The remote sends infra-red.
If you have a black light, shine it on tonic water in a dark room: it glows blue (the quinine fluoresces, as § 593 describes). Don't look into the black light's bulb.
Explain the idea
Light is only a small band of a much wider family of waves. Longer than red: infra-red, which we feel as heat. Shorter than violet: ultra-violet, which causes sunburn and makes some things fluoresce. Dull black surfaces absorb and radiate well; shiny surfaces reflect and radiate poorly (§ 592). Set the black and white cloths in the sun and feel which grows warmer.
What we know now
Herschel discovered infra-red in 1800 with thermometers and a prism, much as you just did. Today we know these are electromagnetic waves (no ether is needed), and infra-red cameras, TV remotes and sunscreen (which blocks ultra-violet) all depend on them.
Say it back
Your student explains how a greenhouse (the book's "hotbed") stays warm (question 4).
Questions
Work questions 1–7 on book page 465.
Afterwards
Wear sunscreen and sunglasses in bright sun: both block ultra-violet. Find which things in your house have shiny surfaces to keep heat in (a thermos, an emergency blanket).
Tip
Never look at the Sun, even through a prism, and don't stare into an ultraviolet lamp.
Answers to the book's problems
1. By radiation. 2. The thin air absorbs little of the sun's radiation and so is warmed very little; the ground absorbs it and warms the air near it. 3. The blue and violet. 4. Glass lets in the sun's light, which warms the soil; the warm soil sends out long infra-red rays, which the glass will not let out. 5. A white hat. 6. In a tarnished one; a dull surface is a better radiator. 7. The white-hot iron.
Worked out for this site from the scan. If a number in your copy differs, trust the book.