Investigating Light through Experiments
FoundationLight travels in straight lines, reflects off mirrors with the angle of incidence equal to the angle of reflection, bends by refraction when leaving water — making a chopstick look broken — and splits into seven colours through a prism. Do the pinhole image, mirror games and the coin-reappearing experiments yourself to test the properties of light.
- Light travels in straight lines through a uniform medium — the origin of shadows and of eclipses.
- A pinhole makes an upside-down image, proving straight-line travel.
- Law of reflection: the angle of incidence equals the angle of reflection; rough surfaces scatter light diffusely so we see non-glowing objects.
文體小知識：實驗探究報告記錄實驗目的、器材、步驟、結果和結論，是科學家交流和複核研究的重要文體。
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1Light travels in straight lines through uniform air or water, proved by shadows and pinhole experiments. The most famous is the pinhole image: place a lit candle in front of a card with a tiny hole, stand a translucent screen behind, and an upside-down image of the flame appears. The light from the top of the flame travels straight through the hole and lands at the bottom of the screen, while light from the bottom lands at the top. Shadows prove straight travel too: blocked by an opaque object, light leaves a shadow behind it — and eclipses are simply enormous shadows among the heavenly bodies.
2Light reflects from mirrors and other smooth surfaces following a clear law: the incident ray, the reflected ray and the normal (the line perpendicular to the mirror) lie in one plane, and the angle of incidence equals the angle of reflection. With an optical disc and a plane mirror, shine a laser pointer at the mirror at thirty degrees and the reflected ray leaves at thirty degrees too. A smooth mirror gives regular reflection with a clear image; the bumpy surfaces of paper and walls scatter light in all directions — diffuse reflection — which is why we can see non-glowing objects from any angle.
3When light passes from one medium into another — say from water into air — its speed changes and its direction bends: this is refraction. Stand a chopstick in a glass of water and look from the side: it seems 'broken' at the surface. The chopstick is whole; the bend is an illusion of refraction. Try the coin experiment: put a coin at the bottom of a bowl, back away until it just disappears, then have a friend pour water into the bowl — the coin reappears, because rays bending downwards as they leave the water raise the coin's image. Fishermen aim below the fish they see for the same reason.
4White light is actually a mixture of colours. Let sunlight pass through a prism and it spreads into red, orange, yellow, green, blue, indigo and violet — dispersion; the rainbow after rain is countless floating droplets acting as tiny prisms. The properties of light inspire inventions: a periscope uses two mirrors set at forty-five degrees so soldiers could watch from inside a trench; optical fibres keep light bouncing inside the tube to carry internet data at the speed of light; and magnifiers and spectacles use refraction to sharpen images.
1Summary 重點摘要
Light travels in straight lines, reflects off mirrors with the angle of incidence equal to the angle of reflection, bends by refraction when leaving water — making a chopstick look broken — and splits into seven colours through a prism. Do the pinhole image, mirror games and the coin-reappearing experiments yourself to test the properties of light.
2Vocabulary cards Vocabulary
Light's property of travelling in straight lines through a uniform medium; shadows and pinhole images arise from it.
Light bouncing back off a surface; mirror images depend on it.
The angle between the incoming ray and the normal; it equals the angle of reflection.
The bending of light as it passes between media and changes speed.
White light splitting into seven colours through a prism or droplets.
A transparent triangular block that splits white light into seven colours.
Rough surfaces scattering light in all directions, letting us see objects from any angle.
A tube with two mirrors at forty-five degrees that reflects light so you can see over an obstacle.
A thin fibre carrying light signals that bounce along inside, sending data at high speed.
3Step by step Steps
- Step 1: Prove light travels straightExample: shine a laser pointer across a dark room and mist a little water into the air — a perfectly straight light path appears, showing that light travels in straight lines through the air. Pinhole images and the forming of shadows both come from this property of light. Never point a laser pointer at eyes or at people, because it can damage the retina.
- Step 2: Do the pinhole image experimentExample: place a lit candle about thirty centimetres in front of a card with a tiny hole, stand a translucent screen behind the card, and an upside-down image of the flame appears on the screen. The image is inverted because light from the top of the flame travels straight through the hole and lands on the lower part of the screen, while light from the bottom lands higher. The pinhole image shows an inverted picture and proves again that light travels in straight lines.
- Step 3: Measure the angles of reflectionExample: on an optical disc, shine the laser at a plane mirror with an incident angle of thirty degrees; the reflected ray also leaves at thirty degrees from the normal — the angle of incidence equals the angle of reflection. The normal is the reference line perpendicular to the mirror, and both angles are measured from it. Trying several different angles shows that this law of reflection holds every time, and it is the basis for designing mirrors and periscopes.
- Step 4: Tell regular from diffuse reflectionExample: looking into a mirror you see a sharp image of yourself — regular reflection, because the smooth mirror sends the reflected light off neatly in one direction; seeing the words in your textbook from any angle is diffuse reflection, because the bumpy paper scatters the light everywhere. Both kinds obey the law of reflection; the only difference is how smooth the surface is. Avoiding very glossy paper when writing at night is exactly a way to reduce the glare of regular reflection.
- Step 5: Watch the 'broken' chopstickExample: stand a chopstick slantwise in a glass of water and look from the side — it seems shifted and 'broken' at the water surface; lift it out and it is perfectly whole. The illusion comes from refraction: as light leaves the water for the air its path bends, but the eye assumes it travelled straight, so the underwater part appears displaced. Objects in water also look shallower than they really are, which is why a fisher aims below the fish — the same principle.
- Step 6: Bring back the hidden coinExample: put a coin at the bottom of a bowl, back away until it just disappears, then ask a classmate to pour water in slowly — the coin reappears. The reason is that light from the coin refracts at the water surface as it leaves the water for the air, bends downwards into your eye, and the eye, tracing straight lines backwards, sees the coin's image raised higher. This experiment confirms the broken-chopstick illusion; both are evidence of refraction.
- Step 7: Disperse sunlight with a prismExample: aim a prism at sunlight and slowly turn it over a sheet of white paper — a ribbon of red, orange, yellow, green, blue, indigo and violet appears. This is dispersion: white light is really a mixture of colours, and the prism bends each colour by a different amount so they separate. The rainbow after rain is the same natural dispersion, made by sunlight refracting and reflecting inside countless water drops. The experiment tells us that plain-looking white light holds a wealth of colours.
4Diagrams Diagrams
5Worked example Example
Example: Sin puts a coin in an opaque empty bowl and sits at the table until the coin just disappears from view, then asks a classmate to pour water slowly into the bowl. Explain why the coin reappears and what property of light this experiment verifies.
- At first the coin is invisible because light travels in straight lines, and the bowl's rim blocks the coin's reflected light from reaching the eye.
- After water is poured in, light refracts at the surface as it leaves the water for the air; the angle of refraction exceeds the angle of incidence, and the bent rays reach the eye.
- Looking back along the refracted rays, the eye sees the coin's image raised higher.
- Only the water level changed while the bowl, coin and seat stayed the same, so the result is reliable.
- Conclusion: light bends by refraction when leaving water for air, and the bend makes a hidden object reappear.
- Answer: The coin reappears because refraction at the water surface raises its image once water is poured in; the experiment verifies that light bends when passing between different media.
6Common mistakes Mistakes
7Quick check Quick check
Q1What kind of image does a pinhole make, and what does it prove?
Q2What is the law of reflection?
Q3Why does a chopstick look broken in water?
Q4What does a prism do to white light?
Q5Which property of light does a periscope use?
8Recap Recap
- Light travels in straight lines through a uniform medium — the origin of shadows and of eclipses.
- A pinhole makes an upside-down image, proving straight-line travel.
- Law of reflection: the angle of incidence equals the angle of reflection; rough surfaces scatter light diffusely so we see non-glowing objects.
- Light bends by refraction between media — the 'broken' chopstick and the reappearing coin are both refraction.
- A prism disperses white light into seven colours; periscopes, optical fibres and spectacles apply the properties of light.