Reflection of Light
FoundationObjects that glow by themselves are light sources, like the sun; the moon is seen only by reflecting sunlight. Light bounces off smooth surfaces, forming mirror images and water reflections, while shadows lengthen and shorten as the sun moves.
- Sources glow by themselves: the sun, lit lamps and torches.
- The moon does not glow; moonlight is reflected sunlight.
- Smooth surfaces reflect light into mirror images and water reflections.
文體小知識：說明文解釋光的小秘密。這一篇教你分辨光源和反射光，看看鏡子和水面怎樣把光送到我們眼中。
§Text Passage
1Without light we could see nothing. Some objects glow by themselves — the sun, a burning candle, a switched-on torch or lamp. They are light sources. Desks, books and the moon do not glow; we see them because light from elsewhere shines on them and bounces into our eyes.
2Lifting our eyes to the moon at night, its light looks soft and bright, yet the moon itself does not glow and is not a light source. It is like a great mirror hung in the sky, bouncing the sun's light towards the Earth; what we see is reflected light. In the same way, words on a wall and clouds in the sky do not glow — they only reflect the light that falls on them.
3When light strikes a smooth surface it bounces back in a set direction; this is reflection. A mirror, flat and slick, reflects best of all and forms a clear image. Still water is a natural mirror: trees on the bank and clouds above cast their reflections on it, and smooth metal shows our shape too. Rough surfaces scatter light in all directions, so they form no image.
4Light travels in straight lines. Blocked by an object that lets no light through, it leaves a dark shadow on the side away from the light. Through one day the sun's position never stops changing: early morning, with the sun low and slanting, shadows are long and slanted; at noon the sun is highest and shadows shortest; at dusk, as the sun sinks west, shadows stretch out again, pointing east.
5The ancients invented the sundial from these changing shadows, reading time from where the pin's shadow fell. We can run a small experiment too: choose a lamp post, and note its shadow's length and direction in the morning, at noon and in the evening. The shadow changes regularly every day — the sun's journey was written on the ground all along.
1Summary 重點摘要
Objects that glow by themselves are light sources, like the sun; the moon is seen only by reflecting sunlight. Light bounces off smooth surfaces, forming mirror images and water reflections, while shadows lengthen and shorten as the sun moves.
2Vocabulary cards Vocabulary
An object that glows by itself, such as the sun and a switched-on torch.
Light striking a surface and bouncing back.
Light bounced off an object; moonlight is reflected light.
An object with a smooth surface that reflects light into a clear image.
The picture formed by reflection in a mirror or on water.
The dark area behind an object that blocks the light.
The body circling the Earth; it does not glow but is seen by reflecting sunlight.
An ancient tool that tells time from where the pin's shadow falls in the sun.
Light moves in straight lines; blocked, it leaves a shadow.
A flat, slick surface that reflects light especially well.
3Step by step Steps
- Step 1: Know light sourcesWe see things only because light enters our eyes. Where does light come from? Objects that glow by themselves are light sources. The greatest and brightest source in the sky is the sun; a burning candle, a switched-on torch, a lit lamp and a glowing phone screen are all sources. The test is simple: does it still glow after the power is cut or the flame put out? A lamp lit is a source; switched off, it is not. The flickering tail of a firefly is a rare animal light source.
- Step 2: Being seen is not glowingSome think everything we can see must glow — a common mistake. We see books, desks, buildings and the moon, but none of them glows. In fact, light from a source strikes their surfaces and bounces into our eyes, and so we see them. Try this: in a completely darkened room, lay a book on the desk; even before your eyes, it cannot be seen without light. Switch on a torch and the book appears at once. Seeing means light has been reflected into your eyes, not that the object glows.
- Step 3: The moon is not a light sourceThe moon is the brightest body in the night sky, yet it is not a light source. The moon cannot glow; the moonlight we see is sunlight reflected from the moon's surface. As the moon circles the Earth, the sun lights only the half facing it; from the Earth the lit part looks larger or smaller, so the moon waxes and wanes — a new moon is barely visible, a full moon a bright round disc. Knowing the moon shines by reflection, we will not mistake its light for its own.
- Step 4: The phenomenon of reflectionLight striking a surface bounces back — reflection. The smoother the surface, the more orderly the bounce: mirror reflection forms clear images, which is why a dressing mirror shows our face. Still water is a mirror laid on the land: mountains and rosy clouds cast reflections in the lake, and the glass walls of towers mirror blue sky and white clouds. Rough surfaces such as white walls and paper scatter light in every direction; they form no image, yet that scattering is exactly why we can see them from many angles.
- Step 5: How shadows formLight travels in straight lines. Blocked by an object that lets no light through, it leaves a patch of darkness behind the object — a shadow. The shadow's shape follows the object's outline: raise your hands and a bird-shaped shadow spreads its wings on the wall. Transparent objects let most light pass and cast faint shadows; translucent ones cast pale ones. Shadow and light always face away from each other: light from the left, shadow on the right. Hand-shadow games use exactly this, a few fingers bringing a bird to life.
- Step 6: Shadows follow the sunThe sun's position changes all day, and shadows follow. Early morning, the sun low in the east, shadows stretch long and slanted, pointing west; through the morning they shorten; at noon, when the sun is highest, shadows are shortest; in the afternoon they lengthen again, and by dusk they point long towards the east. Record a lamp post's shadow length and direction, morning, noon and evening, and draw a chart: the pattern stands out plainly — and it repeats every single day.
- Step 7: Using reflection and shadowsReflection and shadows serve us well. The ancients invented the sundial: the pin's shadow moves with the sun, and where the tip falls on the scale, there is the time — the oldest clock. A periscope uses two parallel mirrors to bounce light twice, so we can see over a wall while staying behind it. Car mirrors and the big convex mirrors at blind bends widen our view by reflection. Architects weigh the sun's angle and shadows when designing buildings so rooms get enough light. Little secrets of light have always served human life.
5Worked example Example
Example: Little sister asks her brother, 「It is so dark at night — why can we still see the moon?」 The brother cannot answer. Explain for him: why can we see the moon at night? Is the moon a light source? And why does the moon's shape seem to change?
- The moon cannot glow by itself; it is not a light source. The moonlight we see is sunlight reflected from the moon's surface.
- Sunlight strikes the moon, and the moon, like a mirror, bounces the light towards the Earth — that is why we can see it at night.
- As the moon circles the Earth, the angle of the sunlit half keeps changing, so the bright face we see waxes and wanes.
- Supporting example: shine a torch on a ball in a dark room — only the lit side glows, just as the sun lights only half the moon.
- Answer: The moon is not a light source; it does not glow itself. We see it at night because the moon reflects sunlight into our eyes. As the moon orbits the Earth, the sunlit part seems larger or smaller, so the moon waxes and wanes — at full moon we see its whole lit face.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is a light source?
Q2Is the moon a light source? Why?
Q3What phenomenon makes reflections on water?
Q4When are shadows shortest in a day?
Q5What does a sundial rely on to show time?
8Recap Recap
- Sources glow by themselves: the sun, lit lamps and torches.
- The moon does not glow; moonlight is reflected sunlight.
- Smooth surfaces reflect light into mirror images and water reflections.
- Shadows form where light is blocked, and change length as the sun moves.
- Long at dawn, short at noon — the ancients read time by sundials.