Light and Shadow
FoundationLight travels in straight lines; blocked by an opaque object, a shadow forms on the far side. Move the light, and the shadow changes in length and direction.
- Light travels in straight lines; block it and a shadow forms.
- The sun and lamps are light sources; the moon is not.
- Shadows always lie away from the light.
文體小知識：說明文用手影遊戲引入，解釋影子形成的條件，再教你用實驗找出影子變化的規律。
§Text Passage
1A lit lamp, the sun and a torch all give out light — they are light sources. Light travels in straight lines; when it hits an opaque object and is blocked, a dark shadow forms in the space behind where the light cannot reach.
2Different materials let light through differently: clear glass lets nearly all light pass, so the shadow is faint; translucent waxed paper lets some light pass, so the shadow is lighter; an opaque book blocks all the light, so the shadow is darkest.
3Shadows follow the light. In the morning the sun is in the east, and shadows stretch long to the west; at noon the sun is highest and the shadow shrinks to your feet, its shortest; in the afternoon the shadow lengthens again and swings to the east. Using exactly this, people in ancient times invented the sundial to tell the time.
4We can play shadow games: raise a hand before a lamp and a bird or a puppy appears on the wall. Move the hand nearer the lamp and the shadow grows; hold it against the wall and it shrinks. Try it, and the secrets of light appear.
1Summary 重點摘要
Light travels in straight lines; blocked by an opaque object, a shadow forms on the far side. Move the light, and the shadow changes in length and direction.
2Vocabulary cards Vocabulary
Something that gives out its own light, such as the sun and lamps.
A path with no bends; light travels in straight lines.
Letting no light through, such as a book.
Letting some light through, such as waxed paper.
Letting nearly all light through, such as glass.
The dark area formed behind an object that blocks the light.
An ancient tool that tells time by the changing position of a shadow.
A small handheld light.
Where something is; move the light and the shadow follows.
3Step by step Steps
- Step 1: Find the light sourcesOnly things that make their own light are light sources, such as the sun, a burning candle, a lamp and a torch. The moon looks bright but merely reflects the sun's light; a mirror only bounces light back — neither is a light source. Some people think everything shiny is a source. Search your room at night, tell which things give light and which only reflect it, and only when they are told apart do you truly know light sources — the first step in studying light.
- Step 2: Learn what makes a shadowHow does a shadow form? Try a small experiment: switch off the room light, shine a torch at a white wall, and put your hand between the torch and the wall — the hand's shadow appears at once. A shadow needs three conditions together: a light source, an object light cannot pass through, and a screen to catch the shadow, such as a wall or the ground. Some people think light alone makes a shadow, but all three are indispensable; lacking any one, no shadow can form.
- Step 3: Test different materialsLight meets different materials with different results, so shadows differ too. Shine the same torch at glass, waxed paper and a book: with glass the wall shows almost no shadow, for the light passes through; with waxed paper a pale shadow appears, for it lets only some light through; with the book the shadow is completely dark, for no light gets through. Some people think all shadows are alike, but how much light a material lets through decides how dark the shadow is.
- Step 4: Watch shadows change with timeAs the sun moves across the sky, the shadow's length follows. At nine in the morning, twelve at noon and three in the afternoon, measure the flagpole's shadow on the playground with a tape and write the numbers down. Comparing the three readings shows the shadow longer in the morning and afternoon and shortest at noon, because the sun is highest then and its light falls almost straight down. Some people think shadow length never changes, but measure once and you will find it changes every day.
- Step 5: Verify with shadow gamesHand-shadow games verify the link between distance and shadow. Make a bird or a dog between the lamp and the wall: bring your hand near the bulb and the shadow on the wall grows at once; pull your hand back near your chest and the shadow shrinks. It is because the nearer an object is to the lamp, the wider the light it blocks. Some people think a shadow keeps one size, but watching while you play fixes the principle in mind — the most delightful way to verify science.
- Step 6: Record and find the ruleDraw the three measured shadow lengths as a simple bar chart: the noon bar shortest, the morning and afternoon bars longer — the rule is plain to see. Now look at direction: a shadow always lies on the side away from the light; the sun in the east means the shadow in the west. Some people think observation ends when the measuring stops, but records and charts turn watching into conclusions — the very method of scientific thinking, worth cultivating from childhood.
4Diagrams Diagrams
5Worked example Example
Example: on the playground, Tin finds his shadow to the west in the morning but to the east in the afternoon. Why does the shadow change direction?
- A shadow always lies on the side opposite the light — the side away from it.
- In the morning the sun is in the east, so the shadow falls to the west.
- In the afternoon the sun moves to the west, so the shadow turns to the east.
- Conclusion: as the sun's position changes the shadow follows, and at noon it is shortest.
- Answer: The shadow always lies away from the light: in the morning the sun is east and the shadow west; in the afternoon the sun is west and the shadow east. As the sun moves, the shadow changes direction with it.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is a light source?
Q2On which side of the object does the shadow lie?
Q3Which material makes the darkest shadow?
Q4When is the shadow shortest in a day?
Q5What happens to the shadow when the hand nears the bulb?
8Recap Recap
- Light travels in straight lines; block it and a shadow forms.
- The sun and lamps are light sources; the moon is not.
- Shadows always lie away from the light.
- Shadows are long in the morning and short at noon.
- Never stare at the sun; protect your eyes while experimenting.