Light: Straight Lines, Reflection and Refraction
FoundationLight travels in straight lines and reflects off mirrors with the angle of incidence equal to the angle of reflection; entering water or glass it refracts and changes direction. Shadows, car mirrors and periscopes all use these rules.
- Light travels in straight lines in a uniform medium; shadows and pinhole images prove it.
- Light reflects off mirrors; the angle of incidence equals the angle of reflection, both from the normal.
- Light refracts entering another transparent substance; the bent-looking object in water is an illusion.
文體小知識：物理科學說明文用實驗解釋光的規律，直進、反射和折射都能夠親手驗證。
§Text Passage
1Light travels in straight lines. The beam of a torch at night is a straight line, and sunlight through a small hole makes a bright dot on the floor. Shadows best prove it: light blocked by the body leaves a shadow behind, and the lower the sun, the longer the shadow. The pinhole image shows the same rule: light from a candle through a pinhole forms an upside-down picture on the screen of a dark box.
2Light reflects when it meets a surface. A plane mirror is the commonest example: the angle between the incoming ray and the normal is the angle of incidence; the angle between the reflected ray and the normal is the angle of reflection. Experiments show the two are equal — the law of reflection. Example: point a laser pen at thirty degrees to a mirror, and the reflected ray also makes thirty degrees with the normal.
3When light passes from one transparent substance into another, it changes direction — refraction. Stand a chopstick in a glass of water and look from the side: the chopstick seems bent at the water surface, because light bends as it leaves the water for the air. Pool water looks shallower than it really is for the same reason, so never judge water depth by eyesight alone.
4The rules of light serve daily life. Car mirrors use reflection to show drivers what is behind; a periscope uses two plane mirrors at forty-five degrees so that someone behind an obstacle can see ahead; optical fibres bounce light along to carry internet signals around the world. Grasp the rules of light and many wonders around you make sense.
1Summary 重點摘要
Light travels in straight lines and reflects off mirrors with the angle of incidence equal to the angle of reflection; entering water or glass it refracts and changes direction. Shadows, car mirrors and periscopes all use these rules.
2Vocabulary cards Vocabulary
Something that makes its own light, such as the sun and lamps.
The property of light moving in straight lines through a medium such as air.
The dark area behind an object that blocks light.
Light bouncing back off a surface into the medium it came from.
The angle between the incoming ray and the normal.
The angle between the reflected ray and the normal, equal to the angle of incidence.
The line at right angles to the mirror at the point the ray strikes, for measuring angles.
Light changing direction as it passes between transparent substances.
A tube with two mirrors for looking out from behind an obstacle.
3Step by step Steps
- Step 1: Prove light travels in straight linesLight travels in straight lines in air — the most basic rule of optics, which we can prove by experiment. For example, stand three cards, each with a pinhole, in a row; only when the three holes line up can the eye see the candlelight at the far end, showing that light moves in straight lines. If one card shifts a little, the card blocks the ray and the candlelight disappears. The straight beam of a torch at night follows the same rule.
- Step 2: Use shadows as proofShadows also prove that light travels in straight lines, because an opaque object blocks the light and leaves a dark area behind it. For example, the sun hangs high at noon and shadows are short; at dusk it shines aslant and shadows are long. Stand a pencil in clay like a sundial's needle and its shadow moves with the sun, pointing the opposite way. The ancients used exactly this principle to make sundials, telling the time of day by the shadow.
- Step 3: Meet the pinhole imageThe pinhole image is another proof of straight-line travel. For example, make a pinhole in the front of a dark box; the candle's light crosses at the hole and an upside-down flame appears on the back screen, because light from the top of the flame travels straight to the bottom of the screen and light from the bottom to the top — the rays cross at the hole. The hole must be small for the image to be sharp; too large a hole lets the rays jumble and the image blurs.
- Step 4: Do a reflection experimentLight bounces back when it hits a mirror; this is called reflection, and we can measure it in an experiment. For example, lay a plane mirror on a protractor; with an adult shining a laser pen at thirty degrees, the measured angle between the reflected ray and the normal is also thirty degrees — the law of reflection. Never shine a laser pen into anyone's eyes, and always do this experiment with an adult present; safety comes first at all times.
- Step 5: See the law of reflection at workReflection can be seen everywhere in daily life, and the commonest example is the mirror. For example, in a mirror the image is reversed left to right; raise your right hand and the you in the mirror raises the other hand. A plane mirror forms a clear, orderly image, so it is used in dressing mirrors, car mirrors and periscopes, helping us see beyond our line of sight — safe and convenient at once.
- Step 6: Watch refractionWhen light passes from one transparent material into another, its direction changes; this is called refraction. For example, put a coin at the bottom of a cup, step back until it just disappears, then let water be poured in slowly — the coin reappears, because light bends at the water surface towards your eyes. A spoon standing aslant in water looks bent at the surface, and pool water looks shallower than it really is — all the same rule at work.
- Step 7: Explain light in daily lifeOnce we know the three rules of light, we can explain many things around us. For example, pool water looking shallow is refraction, the image in a car mirror is reflection, and shadows following the sun are straight-line travel; explain each with its rule and the sights are no longer mysterious. Understanding the world with scientific principles keeps us from believing strange superstitions when we meet odd sights, and builds the good habits of observing and thinking.
4Diagrams Diagrams
5Worked example Example
Example: with an adult's help, Kit shines a laser pen at a plane mirror so that the incoming ray makes thirty degrees with the mirror surface. What angle does the reflected ray make with the surface, and with the normal?
- Draw the normal first: a line at right angles to the mirror at the point where the ray strikes, the base for measuring angles.
- The incoming ray makes thirty degrees with the surface and the normal makes ninety, so the angle of incidence is ninety minus thirty, that is, sixty degrees.
- By the law of reflection, the angle of reflection equals the angle of incidence, so it is also sixty degrees.
- The reflected ray makes ninety minus sixty, that is thirty degrees, with the surface — perfectly symmetrical with the incoming ray.
- Answer: The angle of reflection is sixty degrees from the normal and thirty degrees from the mirror surface, perfectly symmetrical with the incoming ray — exactly what the law of reflection predicts.
6Common mistakes Mistakes
7Quick check Quick check
Q1What property of light does a shadow prove?
Q2What is the law of reflection?
Q3What is refraction?
Q4Which rule of light does a periscope use?
Q5Why does pool water look shallower than it is?
8Recap Recap
- Light travels in straight lines in a uniform medium; shadows and pinhole images prove it.
- Light reflects off mirrors; the angle of incidence equals the angle of reflection, both from the normal.
- Light refracts entering another transparent substance; the bent-looking object in water is an illusion.
- Periscopes and car mirrors use reflection; optical fibres carry signals with light.
- Laser experiments belong to adults — never look into a beam or aim at eyes.