---
title: "Light Travels in Straight Lines"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 28
exercises: 11
source: https://one-course.com/books/physics/1/en/chapter/28-light-travels-in-straight-lines
---

# Chapter 28 — Light Travels in Straight Lines

Sunbeams slant through the morning mist between the trees — straight as stretched strings, every one. A torch in a dusty attic throws a straight-edged blade of [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source). You can hear a friend calling from around the corner, but you cannot *see* them there. All these are one law wearing different clothes — the oldest law of [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source).

## 28.1 The law of the straight line

**Proposition 28.1 (Light travels in straight lines).**

In [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) (as in water, glass, or empty space), [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) travels in straight lines. It does not curve around corners, does not drift like smoke, does not bend to find you behind a wall. Where a straight line from the source cannot reach, no [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) from that source arrives.

**Definition 28.2 (Light ray).**

To draw [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source), physicists use the *light ray*: a straight line with an arrow, showing one thread of [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) and the way it runs. A torch beam is drawn as a fan of rays; a thin laser beam as a single ray. Like the [force](https://one-course.com/books/physics/1/en/chapter/12-pushes-and-pulls-forces#def-g2-pushes-and-pulls-force) arrow, the ray is a little drawing that carries a law inside it: [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)’s roads are ruler-straight.

![A torch beam drawn the physicist’s way: straight rays, arrowed to show which way the light runs.](https://one-course.com/images/onecourse/chapters/physics-1/g4-light-straight-lines/fig-3be61d1aeb6b.svg)

*A torch beam drawn the physicist’s way: straight rays, arrowed to show which way the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) runs.*

**Example 28.3 (Where the law shows itself).**

Once you know the law, you see it everywhere: the straight sunbeams through mist or blinds; the sharp straight edge of a torch beam in dusty [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air); the laser pointer’s dead-straight thread; and every lighthouse sweeping straight blades of [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) across the night sea. Bent [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) is never seen — fountains and forests may curve, [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) will not.

![Nature’s own proof: sunbeams crossing misty air are perfectly straight — light does not bend around the trees.](https://one-course.com/images/onecourse/chapters/physics-1/g4-light-straight-lines/img-a0977fdf3117.jpg)

*Nature’s own proof: sunbeams crossing misty [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) are perfectly straight — [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) does not bend around the trees.*

## 28.2 Proving it with three cards

**Method 28.4 (The three-card experiment).**

1. Punch a small hole in the middle of three stiff cards, at the same height;
2. stand the cards upright in a row, a hand-span apart, and a lit candle (or torch) beyond the last one;
3. move your head until you see the flame through all three holes — possible only when the three holes and the flame stand on one straight line (check: a taut string through the holes reaches the flame);
4. now slide the middle card sideways, just a finger’s width.

The flame vanishes. [Light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) that could have zigzagged — through hole one, a jog sideways, through the shifted hole, and on to your eye — simply does not: no straight line, no [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source).

![The three-card experiment: the flame is visible exactly when flame, holes and eye line up straight. Shift one card, and the view dies.](https://one-course.com/images/onecourse/chapters/physics-1/g4-light-straight-lines/fig-bdc0908f231c.svg)

*The three-card experiment: the flame is visible exactly when flame, holes and eye line up straight. Shift one card, and the view dies.*

**Example 28.5 (Shadows explained for good).**

Now the [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow) of your first school year gives up its secret. Rays run straight from the lamp; the ball stops the ones that hit it; the rays just missing its edge run straight on. Behind the ball lies exactly the space that no straight ray can reach — the [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow), with its sharp edge drawn by the last rays to slip past. Straight rays, sharp [shadows](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow): crooked [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) would smear every [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow) into fog.

## 28.3 Seeing is receiving light

**Proposition 28.6 (How seeing works).**

You see a thing when [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) from it travels in a straight line into your eye — its own [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) if it is a source, bounced sunlight or lamplight otherwise. Nothing leaves your eye to grab the world: seeing is *receiving*, catching rays like a net catches balls.

**Example 28.7 (Around the corner).**

Your friend calls from around the corner of the school: you hear them, but cannot see them. [Sound](https://one-course.com/books/physics/1/en/chapter/19-sound-around-us#def-g3-sound-around-us-sound) spreads like pond rings and washes around the corner’s edge; [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) holds its straight roads, and no straight line joins your friend’s face to your eye through the brick. To see around corners, you must *fold the road* — give the ray a hard, shiny surface to bounce off. That trick is called the mirror, and it owns the next year’s first [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) chapter.

**Remark 28.8 (The champion runner).**

[Sound](https://one-course.com/books/physics/1/en/chapter/19-sound-around-us#def-g3-sound-around-us-sound), we measured, jogs at $340\,\mathrm{m}$ each second. [Light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) on its straight roads is in another league entirely: it crosses the whole sky faster than any counting could catch — which is why the flash arrives “at once” and the thunder trudges in seconds behind. How fast exactly? The number is so outrageous that we save it for a later year — with the tale of the clever tricks it took to [measure](https://one-course.com/books/physics/1/en/chapter/7-measuring-length#def-g2-measuring-length-measure) it at all.

## 28.4 Exercises

**Exercise 28.1 ★.**

State the law of this chapter. Name two sightings of it outdoors.

**Solution of Exercise 28.1.**

[Light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) travels in straight lines — it never curves around corners. Sightings: straight sunbeams through mist or blinds, the sharp blade of a torch in dusty [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air), a laser’s dead-straight thread, lighthouse beams.

**Exercise 28.2 ★.**

What is a [light ray](#def-g4-light-straight-lines-ray), and what two things does its drawing show?

**Solution of Exercise 28.2.**

A straight arrowed line drawn to stand for one thread of [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source): it shows the road the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) takes (ruler-straight) and the direction it runs.

**Exercise 28.3 ★.**

In the three-card experiment, when exactly can you see the flame? What single move makes it vanish, and why?

**Solution of Exercise 28.3.**

Exactly when flame, all three holes and your eye stand on one straight line. Sliding the middle card a finger’s width kills the view: [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) will not jog sideways — no straight line, no [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source).

**Exercise 28.4 ★.**

How does the straight-line law explain the sharp edge of a [shadow](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-shadow)?

**Solution of Exercise 28.4.**

Rays run straight from the lamp; the blocker stops some, and the rays just missing its edge run straight on. The boundary between reached and unreached ground is drawn by straight lines — hence the crisp edge.

**Exercise 28.5 ★.**

You see this book right now. Tell the journey of the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) that makes that possible, from a lamp to your eye.

**Solution of Exercise 28.5.**

[Light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) leaves the lamp, runs straight to the page, bounces off the paper, and runs straight from the page into your eye — which receives it. Two straight legs with one bounce between.

**Exercise 28.6 ★.**

Why can you hear a friend around the corner but not see them? What does each traveler — [sound](https://one-course.com/books/physics/1/en/chapter/19-sound-around-us#def-g3-sound-around-us-sound) and [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) — do at the corner?

**Solution of Exercise 28.6.**

[Sound](https://one-course.com/books/physics/1/en/chapter/19-sound-around-us#def-g3-sound-around-us-sound) spreads like pond rings and washes around the corner’s edge to your ear. [Light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) keeps to straight roads, and no straight line joins the friend’s face to your eye through brick — so the voice arrives and the sight does not.

**Exercise 28.7 ★.**

A keyhole, a lit room beyond, and your eye: from where in the dark corridor can you see the lamp through the keyhole? Answer with a straight line.

**Solution of Exercise 28.7.**

Only from the spots where a straight line through the keyhole reaches the lamp — eye, keyhole and lamp aligned. Step aside and the line breaks on the door.

**Exercise 28.8 ★.**

Which arrives first from a distant firework, the bang or the flash? Which law and which measurement from this year explain the gap?

**Solution of Exercise 28.8.**

The flash: [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source)’s trip counts as instant, while the bang jogs over at $340\,\mathrm{m}$ per second — three silent seconds per kilometre of distance.

**Exercise 28.9 ★.**

“When I look at a [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star), something shoots out of my eye to touch it.” Correct this old belief with [Proposition 28.6](#prop-g4-light-straight-lines-seeing) — what travels, and which way?

**Solution of Exercise 28.9.**

Nothing leaves the eye. The [star](https://one-course.com/books/physics/1/en/chapter/26-the-solar-system#def-g4-solar-system-star)’s own [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) travels the straight road *to* you, and seeing is catching it — the eye is a net, not a lantern.

**Exercise 28.10 ★★.**

Design, with a drawing of rays, a way to watch the corridor from inside your room using the mirror trick teased in [Example 28.7](#ex-g4-light-straight-lines-corner): where must the shiny surface stand at the doorway, and what does it do to the ray’s road? (Submarines use exactly this, stacked twice, to see above the waves.)

**Solution of Exercise 28.10.**

Stand the shiny surface at the doorway, tilted at a slant (half a right angle) to the corridor. A ray running along the corridor hits the tilted mirror and its road folds: it turns the corner and runs straight through the doorway into the room to your eye. Two such folds, one above the other, make the submarine’s periscope.

**Exercise 28.11 ★★.**

On a misty morning, sunbeams fan out between the trees in visibly straight lines — yet your friend says: “We see the beams, so [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) must be visible stuff hanging in the [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air).” What are you both actually seeing when a “beam” shows in mist or dust, and what would the beam look like in perfectly clean [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air)?

**Solution of Exercise 28.11.**

You are not seeing the [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) in flight — you are seeing the countless tiny droplets and dust specks that the beam strikes on its way, each one bouncing a little [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) to your eye. In perfectly clean [air](https://one-course.com/books/physics/1/en/chapter/10-air-around-us#def-g2-air-around-us-air) there is nothing to strike, and the beam is invisible from the side: only the bright spot where it lands betrays it.