Light and Shadow

§Text Passage

1Summary 重點摘要

Light travels in straight lines; blocked by opaque things it makes shadows, which lengthen and shorten through the day.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: Find light sourcesThings that glow by themselves are light sources, and knowing them is the first step to learning about light and shadow. The sun, lamps, candles and torches glow by themselves — they are light sources; the moon looks bright but only reflects sunlight, so the moon is not a light source. Look up at night and find which things glow and which are only lit up, and our understanding of light grows clearer. A common mistake is calling the moon a light source; the test is only one question — does it glow by itself?
2. Step 2: Prove light travels straightLight travels in straight lines, and this experiment can prove it. Line up three cards in a row, each with one small hole; the torch light passes through all three only when the holes line up. Move just one card away, and the light is blocked. This proves light never turns corners — it only goes straight ahead. Why learn this? Having your view blocked by the person ahead in a queue, and how hand shadows form, both follow from light travelling straight. A common mistake is thinking light bends around obstacles; light goes straight and cannot pass through anything opaque.
3. Step 3: See how shadows formA shadow is the dark shape made when light is blocked by an object. Hold your hand between the torch and the wall; the light cannot pass through your palm, so a dark hand shape appears on the wall, and it moves as your fingers move. Anything opaque blocks light and makes a shadow; we need both light and an opaque object together for a shadow to appear, and neither can be missing. A common mistake is thinking every object makes a shadow; without light, in a pitch-dark room, no shadow appears at all.
4. Step 4: Watch shadows through the dayA shadow's length and direction keep changing with the sun's position. In the morning the sun is low and the flagpole's shadow stretches long; at noon the sun reaches its highest and the shadow is shortest, hiding under the pole; in the afternoon the sun sinks in the west and the shadow grows long again. Observe three times in a day and draw the shadow, and this secret reveals itself; long ago people invented the sundial to tell time by shadows. A common mistake is thinking a shadow stays the same all day; watch for one whole day and the answer is before your eyes.
5. Step 5: Tell transparent from opaqueThings light can pass through let light through; things it cannot pass through are opaque. Cellophane lets light through — almost all of it passes, so there is almost no shadow; card is opaque — it blocks all the light, and the shadow is dark. Think: curtains, cups and books — which let light through and which do not? Learning to tell them apart adds discoveries to daily life. A common mistake is thinking thin things always let light through; whether light passes depends not on thickness but on material — even thin card is opaque.
6. Step 6: Play hand shadowsHand shadow games are fun and teach us about light and shadow. Shine a small lamp at the wall, cross your hands into a bird, wiggle your fingers, and the shadow flies; change the gesture, and you can make a dog or a rabbit. Try moving your hands nearer to or further from the lamp and watch the shadow grow and shrink: the nearer the hands are to the lamp, the bigger the shadow; turn off the lamp and the shadow disappears, because without light there is no shadow. A common mistake is thinking a shadow's size never changes; the shadow changes with the distance between the object and the lamp — try it and see.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• Light travels in straight lines.
• Blocked light forms shadows.
• Shadows are long morning and evening, short at noon.
• Transparent things let light through; opaque things block it.
• Never look straight at the sun.