Light and Colour: How We See the World

§Text Passage

1Summary 重點摘要

Light travels in straight lines, reflects off mirrors — and the rainbow hides inside white light.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: Light travels in straight linesLight travels in straight lines in a uniform medium, with evidence all around: a torch beam is straight, shadow edges form straight lines, and a pinhole image is upside down. A person in front of you in a queue blocks your view precisely because light does not bend and cannot reach your eye round the obstacle. This property is the basis of drawing ray diagrams — reflection and imaging later all start from 'light moves in straight lines'.
2. Step 2: How we see objectsWe see things because light enters our eyes: luminous objects like the Sun and lamps emit their own light, while the Moon and books give out none and are seen only by reflecting light into the eye. Everything you see is in fact light that has entered your eye. That is why nothing is visible in a pitch-dark room — not because objects vanish, but because no light is available to reflect — the real meaning of lighting.
3. Step 3: The law of reflectionLight bounces off surfaces by a precise rule: the angle between the incident ray and the normal is the angle of incidence, that between the reflected ray and the normal the angle of reflection, and the law of reflection states that the two are equal. Remember when measuring: angles are taken from the normal, not from the mirror surface — the most common mistake in experiments and exams. The normal is an auxiliary line perpendicular to the surface; draw it first, then measure.
4. Step 4: Specular vs diffuse reflectionReflection comes in two kinds: specular reflection on smooth surfaces sends rays back in one orderly direction and forms clear images, as in mirrors and calm water; diffuse reflection on rough surfaces scatters rays in all directions, forming no image but making objects visible from every angle. Both in fact obey the law of reflection — the only difference is smoothness. That a sheet of paper is visible from any angle is diffuse reflection at work.
5. Step 5: Plane mirror imagesA plane mirror forms images with three features: the image is the same size as the object; it is laterally inverted — raise your right hand and the image raises its left; and the image distance equals the object distance. The image behind the mirror can be seen by eye but never cast onto a screen, so it is a virtual image. Dentists use small plane mirrors to view the inner sides of teeth, exploiting the same-size, side-swapped image. Mark these three features when drawing ray diagrams.
6. Step 6: The secret of colourWhite light is actually a mixture of seven colours, shown by Newton dispersing it with a prism into a rainbow. Opaque objects reflect the light of their own colour and absorb the rest, so red paper reflects only red light; transparent objects transmit their own colour, so a red filter passes only red. Shine red light on green paper and, with no green light to reflect, it looks black. Light-coloured clothes feel cooler in summer — the same science of colour and light.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• Light travels in straight lines — shadows prove it.
• Law of reflection: incidence equals reflection (from the normal).
• Mirrors form images; diffuse reflection shows objects from all angles.
• White light holds seven colours — Newton proved it.
• An object's colour comes from the light it reflects — black absorbs all.