Chapter 5: Ray Model of Light (Summary Sheet)
Created by Miss Clarissa Ng | www.clartutors.com
Part A · The Ray Model of Light
1 Light and How We See Things
- Light is a form of energy that our eyes can detect — it is what lets us see.
- Luminous objects make their own light (the Sun, a lamp, a candle flame). Non-luminous objects make none, and are seen only because they reflect light from a source into our eyes (the Moon, this page, a pencil).
- We see a non-luminous object because light from a source bounces off it and enters our eyes — not because anything leaves our eyes.
| Object | What it does | Examples |
|---|---|---|
| Luminous | Produces its own light, which enters our eyes directly | The Sun, a torch bulb, a candle flame, a glow-worm |
| Non-luminous | Produces no light; seen by reflecting light from a source | The Moon, a chair, a mirror, a page of print |
2 What Happens When Light Meets a Material
| Material | What happens to the light | Examples |
|---|---|---|
| Transparent | All or most of it is transmitted through; you see clearly through it | Clear glass, clean water, air |
| Translucent | Only some light passes through; the rest is absorbed or reflected, so the view is blurred | Tracing paper, frosted glass, stained glass |
| Opaque | No light passes through — it is absorbed or reflected | Wood, metal, a brick wall |
- At any surface, light may be transmitted, absorbed, reflected or refracted (bent). Light that is scattered into its colours is dispersed.
3 The Ray Model: Light Travels in Straight Lines
- Draw light as a ray: a straight line with a small arrow on it showing the direction the light is travelling. A bundle of rays is a beam.
- The ray model is our working model because it represents the path taken by light — and light does travel in straight lines, until something changes its direction.
- This straight-line travel is called rectilinear propagation (rectilinear = in straight lines, propagation = travel).
- A useful extra rule: reverse the arrow and the ray still works — reflected and refracted rays run along the same path backwards.
- That is why a ray diagram can predict where light goes: draw the rays in straight lines, then follow them.
Part B · Reflection
4 The Five Words You Need
| Term | What it means |
|---|---|
| Incident ray | The ray arriving at the surface |
| Reflected ray | The ray that leaves the surface after bouncing off it |
| Normal | An imaginary dashed line drawn at right angles (90°) to the surface at the point where the ray hits. It is not a real physical thing. |
| Angle of incidence (i) | The angle between the incident ray and the normal |
| Angle of reflection (r) | The angle between the normal and the reflected ray |
- Both angles are measured from the normal, never from the surface itself — measuring from the mirror is the commonest lost mark.
5 The Law of Reflection
- The angle of reflection is equal to the angle of incidence. i = r, both measured from the normal.
- It holds for every smooth reflecting surface and for every angle — flat or curved, sideways or straight on.
- The incident ray, the reflected ray and the normal all lie in the same plane.
6 Smooth Surfaces and Rough Surfaces
- Even a rough surface reflects by the same law — but its surface is not flat at every point, so each part has its own normal, and the rays end up pointing in many directions.
- That is the whole difference between a clear mirror image and a sheet of white paper.
| Type | Surface | What the reflected rays do |
|---|---|---|
| Regular (specular) reflection | Smooth (mirror, still water, polished metal) | Parallel rays stay parallel after bouncing, so the rays from one point reach your eye together and you see a clear image |
| Diffuse reflection | Rough (paper, cloth, a wall, most surfaces) | Parallel rays are scattered in many directions, so no image is formed — but the surface is still lit and visible |
Part C · Mirror Images and Their Uses
7 The Image in a Plane Mirror
- Upright — the right way up, not upside down.
- Same size as the object — a mirror does not magnify or shrink.
- As far behind the mirror as the object is in front.
- Laterally inverted — left and right are swapped, which is why print held up to a mirror reads backwards.
- Virtual — it cannot be caught on a screen. The rays only appear to come from behind the mirror; no light is actually there.
8 Reflecting Surfaces and Their Uses
| Reflecting surface | How it reflects | What it is used for |
|---|---|---|
| Plane mirror | Flat — image upright, same size, laterally inverted, virtual | A periscope uses two of them at 45° to see over or around an obstacle; bathroom mirrors; seeing round a corner |
| Concave mirror (curves inwards) | Brings parallel rays together (converging); a close object gives a magnified image | Shaving or make-up mirror (magnified face), dentist's mirror, torch and car-headlight reflector, solar cooker |
| Convex mirror (curves outwards) | Spreads rays apart (diverging); always gives a smaller, upright, virtual image | Wider field of view — car wing mirrors, security mirrors at blind corners and shop domes |
Part D · Refraction
9 Why Light Bends
- Light travels at different speeds in different media: fastest in air, slower in water, slowest in glass. The speed change at the boundary is what makes the ray bend.
- If the ray hits the boundary at an angle, one side of it enters the new medium before the other, so that side slows first and the ray swings round — exactly like a trolley turning when one wheel slows.
- If the ray arrives straight along the normal, both sides slow together, so the ray carries straight on — it changes speed but does not bend.
- Three things are needed for bending: two media of different optical density, both able to pass light, and a ray that is not along the normal.
10 Which Way the Ray Bends
| Direction of travel | How it bends | Speed |
|---|---|---|
| Air → water or glass (less dense → more dense) | Towards the normal | Decreases |
| Water or glass → air (more dense → less dense) | Away from the normal | Increases |
| Straight along the normal (either way) | No bending | Changes |
- Memory hook: going into the denser medium the ray bends towards the normal, because it is slowed down.
11 The Effects You Can See
- A straw in a glass of water looks broken at the water surface, and a pencil in a beaker looks shifted sideways.
- A swimming pool looks shallower than it really is — and a coin at the bottom of a basin of water looks raised.
- A fish seen from above appears closer to the surface, and your legs look shorter in water.
- All of these happen for the same reason: we see each part of the object where the light seems to come from, along the straight line our eyes assume.
12 Apparent Depth and Real Depth
- Real depth = the actual distance of the object below the surface.
- Apparent depth = where the object appears to be. Because the rays bend away from the normal as they leave the water, the object appears raised.
- So apparent depth is always less than real depth — the pool is never as shallow as it looks.
Part E · Dispersion of White Light
13 Splitting White Light
- Dispersion is white light being split into its separate colours by a prism (or any transparent medium that refracts it).
- White light is a mixture of all the colours of the spectrum travelling together.
- Each colour refracts by a different amount because each travels at a slightly different speed in the glass, so they spread out into a band.
- A rainbow is the same effect: sunlight is dispersed by tiny water droplets in the air.
14 The Spectrum and Its Order
- The order is always Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV).
- Red bends the least (it travels fastest in glass), so it appears at the top of the band.
- Violet bends the most (slowest in glass), so it appears at the bottom.
Part F · EM Radiation and Light Technology
15 EM Radiation: Helpful and Harmful
- Light is one kind of EM radiation. Infrared and ultraviolet are two more, and all three have both beneficial and harmful effects.
| EM radiation | Beneficial | Harmful |
|---|---|---|
| Infrared | Remote controls, thermal imaging, keeping food warm, heaters | Too much causes burns and overheating; hot surfaces radiate it |
| Ultraviolet | Helps the skin make vitamin D; sterilises water and equipment; used in some lamps | Sunburn, skin ageing, skin cancer and eye damage (cataracts) after too much exposure |
| Light (visible) | Lets us see; powers photosynthesis; photography and vision aids | Glare and very bright light strain and damage the eyes; wasted light adds to light pollution |
16 Light We Make, and What It Costs
- Technology has given us light whenever we want it, and that has changed society for the better: streets, homes and workplaces stay usable after dark.
- City lights improve night visibility and safety — but they also cause light pollution, hiding the stars from city dwellers.
- Bright artificial light disorients birds and other animals that navigate or feed by night.
- All that light uses a great deal of electrical energy, which costs money and adds to the environment's burden.
- So the fair answer to "is artificial light good?" is: it depends how much we use, where we point it, and when we switch it off.
Part G · Exam Tips
17 Exam Tips: What Each Syllabus Pointer Asks For
| The pointer | What earns the mark |
|---|---|
| Show an understanding that the ray model represents the path taken by light | Draw rays as straight lines with arrows and say the model is used because light travels in straight lines — so the rays show the path light follows |
| Describe the effects and uses of reflecting surfaces (e.g., plane and curved) | Name the surface, then what it does to the rays and one real use: periscope (plane), magnified shaving mirror (concave), wide-view wing mirror (convex) |
| Explain how reflection is affected by a smooth and rough surface using the ray model of light | Say the law still holds, but on a rough surface the normal points in different directions at each point, so rays scatter (diffuse) instead of staying parallel (regular) |
| Show an understanding that the change in the speed of light in different media can cause refraction | Say light travels at different speeds in air, water and glass, and that hitting the boundary at an angle makes it bend — no angles are calculated |
| Describe some effects of refraction | Give a named everyday effect: the broken-looking straw, a pool that looks shallower, a coin that appears raised |
| Describe the dispersion of white light by a prism using the ray model of light | Say white light is a mixture of colours that refract by different amounts, so a prism spreads them into ROYGBIV — red least, violet most |
| Show an awareness that EM radiation (infrared, ultraviolet and light) has both beneficial and harmful effects | Give one of each for infrared (heaters / burns), ultraviolet (vitamin D / sunburn and skin cancer) and light (vision / glare and eye strain) |
| Show an awareness about the impact of light produced by technology on society and environment | Balance both sides: better night visibility against light pollution, disoriented birds and the electrical energy used |
Always mark the normal before you mark any angle, and put an arrow on every ray. A ray diagram with no normal, no arrows, or an angle measured from the mirror surface loses the marks even when the physics is right.