--- title: "Light and Shadows" book: "Primary & Middle School Physics" subject: physics language: en chapter: 3 exercises: 9 source: https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows --- # Chapter 3 — Light and Shadows Switch off the lamp in a room with no window: everything disappears. Switch it on: the room comes back. Seeing needs [light](#def-g1-light-and-shadows-source). And wherever there is [light](#def-g1-light-and-shadows-source), there are also [shadows](#def-g1-light-and-shadows-shadow) — your faithful dark twin follows you on every sunny pavement. Let us find out where [light](#def-g1-light-and-shadows-source) comes from, and how [shadows](#def-g1-light-and-shadows-shadow) are made. ## 3.1 Where light comes from **Definition 3.1 (Light sources).** A *light source* is a thing that makes its own *light*: the Sun, a lamp, a candle flame, a firefly. Everything else — a book, a ball, the walls, you — makes no light of its own: we only see these things when light from a source falls on them. **Example 3.2 (Sources or not?).** The Sun, a torch, a burning match and a television screen all make [light](#def-g1-light-and-shadows-source): sources. The Moon, a mirror and a white wall can look very bright, but they make no [light](#def-g1-light-and-shadows-source) of their own — they only send back [light](#def-g1-light-and-shadows-source) that reaches them. Switch the lamp off, and the brightest mirror goes dark. **Example 3.3 (Try it: the dark room).** In the evening, close the shutters or curtains of a room, switch off every lamp, and wait while you count slowly to $20$. What do you see? At first, almost nothing — with no source, there is no [light](#def-g1-light-and-shadows-source) to see by. Then open the door a crack: one thin slice of [light](#def-g1-light-and-shadows-source) is enough to bring back tables, chairs and toys. **Remark 3.4 (Never look at the Sun).** The Sun is by far the strongest [light source](#def-g1-light-and-shadows-source) we ever meet. Never look straight at it, not even for a moment, and never through binoculars or a magnifying glass: its [light](#def-g1-light-and-shadows-source) is so strong that it can hurt your eyes for good. The Sun is to be enjoyed sideways — by the [light](#def-g1-light-and-shadows-source) it throws on everything else. ## 3.2 How shadows are made **Definition 3.5 (Shadow).** A *shadow* is the dark patch that appears behind a thing that blocks the [light](#def-g1-light-and-shadows-source). [Light](#def-g1-light-and-shadows-source) cannot go through your body, so behind you the ground gets no [light](#def-g1-light-and-shadows-source) from the Sun: that dark you-shaped patch is your shadow. ![The lamp shines, the ball blocks, the wall behind stays dark: lamp, blocker, shadow — always in that order, in a straight line.](https://one-course.com/images/onecourse/chapters/physics-1/g1-light-and-shadows/fig-450522eef3d7.svg) *The lamp shines, the ball blocks, the wall behind stays dark: lamp, blocker, [shadow](#def-g1-light-and-shadows-shadow) — always in that order, in a straight line.* **Method 3.6 (Making a sharp shadow).** 1. Make the room dark — [shadows](#def-g1-light-and-shadows-shadow) hide when [light](#def-g1-light-and-shadows-source) comes from everywhere; 2. switch on *one* lamp or torch; 3. hold a toy between the lamp and the wall; 4. look at the wall: there is the [shadow](#def-g1-light-and-shadows-shadow) , on the far side of the toy from the lamp. Move the toy closer to the lamp: the [shadow](#def-g1-light-and-shadows-shadow) grows huge. Move it close to the wall: the [shadow](#def-g1-light-and-shadows-shadow) shrinks to the toy’s true size. **Example 3.7 (Try it: shadow animals).** With the lamp of [Method 3.6](#met-g1-light-and-shadows-make), put your hands in the [light](#def-g1-light-and-shadows-source) instead of a toy: two fingers up make rabbit ears, both thumbs crossed make a flying bird. The wall shows the shape your hands cut out of the [light](#def-g1-light-and-shadows-source) — a [shadow](#def-g1-light-and-shadows-shadow) always keeps the shape of its blocker, seen from the lamp. **Remark 3.8 (A shadow is not a thing).** You cannot pick up a [shadow](#def-g1-light-and-shadows-shadow), fold it, or keep it in a box. A [shadow](#def-g1-light-and-shadows-shadow) is not made of anything: it is a *place where [light](#def-g1-light-and-shadows-source) is missing*. That is why it vanishes the moment the lamp goes out — there is nothing there to stay behind. ## 3.3 Shadows follow the light **Example 3.9 (Which side is the shadow on?).** In the morning, the Sun is on one side of you and your [shadow](#def-g1-light-and-shadows-shadow) lies on the *other* side. Face the Sun: your [shadow](#def-g1-light-and-shadows-shadow) hides behind you. Turn your back to the Sun: your [shadow](#def-g1-light-and-shadows-shadow) stretches out in front. The [shadow](#def-g1-light-and-shadows-shadow) always sits exactly opposite the source. ![The same stick, morning and midday. The shadow ends exactly where the straight ray grazing the stick’s tip meets the ground: a low Sun stretches it long, a high Sun pulls it short.](https://one-course.com/images/onecourse/chapters/physics-1/g1-light-and-shadows/fig-1178ed2ac07f.svg) *The same stick, morning and midday. The [shadow](#def-g1-light-and-shadows-shadow) ends exactly where the straight ray grazing the stick’s tip meets the ground: a low Sun stretches it long, a high Sun pulls it short.* ![A low evening Sun, a long shadow: the lower the Sun hangs, the farther every shadow stretches.](https://one-course.com/images/onecourse/chapters/physics-1/g1-light-and-shadows/img-2e050bc85025.jpg) *A low evening Sun, a long [shadow](#def-g1-light-and-shadows-shadow): the lower the Sun hangs, the farther every [shadow](#def-g1-light-and-shadows-shadow) stretches.* **Example 3.10 (Try it: chalk around a shadow).** On a sunny morning, stand still in the playground while a friend draws a chalk line around your [shadow](#def-g1-light-and-shadows-shadow). Come back after lunch and stand on the same spot: your [shadow](#def-g1-light-and-shadows-shadow) has moved out of its chalk line and changed its length! The Sun changed its place in the sky — and the [shadow](#def-g1-light-and-shadows-shadow) followed. The next chapter follows the Sun through its whole day. ## 3.4 Exercises **Exercise 3.1 ★.** Sort into [light sources](#def-g1-light-and-shadows-source) and non-sources: a candle flame; the Moon; a firefly; a mirror; a torch; a white wall. **Solution of Exercise 3.1.** Sources: the candle flame, the firefly, the torch. Non-sources: the Moon, the mirror, the white wall — they only send back [light](#def-g1-light-and-shadows-source) that falls on them. **Exercise 3.2 ★.** Why can you see nothing at all in a closed room with no window and no lamp? What is missing? **Solution of Exercise 3.2.** [Light](#def-g1-light-and-shadows-source) is missing. There is no source in the room, so no [light](#def-g1-light-and-shadows-source) falls on the things, and things without [light](#def-g1-light-and-shadows-source) on them cannot be seen. **Exercise 3.3 ★.** Name the three things needed to make a [shadow](#def-g1-light-and-shadows-shadow) on the wall, and put them in the right order along a line. **Solution of Exercise 3.3.** A [light source](#def-g1-light-and-shadows-source), a blocker, and a wall (or screen, or ground) — in that order along a straight line: the source shines, the blocker blocks, the wall shows the dark patch. **Exercise 3.4 ★.** The Sun is on your left. On which side is your [shadow](#def-g1-light-and-shadows-shadow)? You turn right around — where is it now? **Solution of Exercise 3.4.** The [shadow](#def-g1-light-and-shadows-shadow) is on your right — opposite the Sun. After turning right around, the Sun is on your right, so the [shadow](#def-g1-light-and-shadows-shadow) lies on your left: always on the far side from the Sun. **Exercise 3.5 ★.** Using [Method 3.6](#met-g1-light-and-shadows-make), make the [shadow](#def-g1-light-and-shadows-shadow) of the same toy first huge, then small. What did you move, and which way? **Solution of Exercise 3.5.** Moving the toy *toward the lamp* makes the [shadow](#def-g1-light-and-shadows-shadow) huge; moving it *toward the wall* shrinks the [shadow](#def-g1-light-and-shadows-shadow) back to the toy’s size. **Exercise 3.6 ★.** Can a [shadow](#def-g1-light-and-shadows-shadow) be picked up and put in a pocket? Explain what a [shadow](#def-g1-light-and-shadows-shadow) really is, using [Remark 3.8](#rem-g1-light-and-shadows-notathing). **Solution of Exercise 3.6.** No. A [shadow](#def-g1-light-and-shadows-shadow) is not a thing made of anything — it is a place where [light](#def-g1-light-and-shadows-source) is missing because a blocker keeps the [light](#def-g1-light-and-shadows-source) out. With nothing there, there is nothing to pick up. **Exercise 3.7 ★.** Why must you never look straight at the Sun, even though looking at a lamp only dazzles you a little? **Solution of Exercise 3.7.** The Sun is a far stronger source than any lamp: its [light](#def-g1-light-and-shadows-source) is so strong it can hurt the eyes for good, not just dazzle them for a moment. **Exercise 3.8 ★.** At midday the Sun hangs high; in the late afternoon it hangs low. When is your [shadow](#def-g1-light-and-shadows-shadow) longer? Draw the two [shadows](#def-g1-light-and-shadows-shadow) of a tree, one for each moment. **Solution of Exercise 3.8.** In the late afternoon, when the Sun hangs low. The drawing should show a short [shadow](#def-g1-light-and-shadows-shadow) at the foot of the tree at midday and a long stretched [shadow](#def-g1-light-and-shadows-shadow) in the afternoon, both pointing away from the Sun. **Exercise 3.9 ★★.** Mia says: “My [shadow](#def-g1-light-and-shadows-shadow) was in front of me, and after I turned around it was behind me — so the [shadow](#def-g1-light-and-shadows-shadow) turned with me.” Sam says the [shadow](#def-g1-light-and-shadows-shadow) never moved at all. Think of [Example 3.9](#ex-g1-light-and-shadows-opposite): who is right, and why did it *look* like the [shadow](#def-g1-light-and-shadows-shadow) turned? **Solution of Exercise 3.9.** Sam is right. The [shadow](#def-g1-light-and-shadows-shadow) stayed exactly opposite the Sun the whole time. Mia turned, so the [shadow](#def-g1-light-and-shadows-shadow) changed its place *compared to her own front and back* — but on the ground it never moved.