Chapter map — Ray Model of Light

The model

Ray model of light Light travels along straight line paths called rays. A beam is a bundle of rays. In a ray diagram the straight line shows the path and the arrow shows the direction of travel. A model is a scientific representation used to explain and predict behaviour.

Evidence that light travels in straight lines: the three-card experiment. Aligned holes → the flame is visible. Move one card → the light is blocked and the flame cannot be seen.

In a vacuum light travels 300 million metres per second — 7.5 times round the Earth in one second. Brain signals travel at about 120 m/s.

How we see

Luminous objects emit light on their own, in all directions. Natural: the sun, the northern lights, the Milky Way, fireflies. Man-made: lamp, candle, mobile phone.
Non-luminous objects emit no light. You see them only when light from a source is reflected off them into your eyes — so two straight lines: source → object → eye.
The moon is non-luminous; it reflects sunlight. One orbit takes about 27 days.
In total darkness you see nothing at all — not even a white object. No source, nothing to reflect.
The sun's core reaches 15 million °C, producing heat and light by nuclear fusion (hydrogen → helium).

Reflection

ReflectionThe bouncing of light off a surface. Light travels in straight lines even when it bounces.

Term	Definition
Incident ray	The ray hitting the reflecting surface
Point of incidence	Where the incident ray meets the surface
Normal	The line perpendicular to the surface at the point of incidence (drawn dashed)
Angle of incidence, i	Between the incident ray and the normal
Angle of reflection, r	Between the reflected ray and the normal
Reflected ray	The ray travelling away from the surface after bouncing off

The lawThe angle of reflection is equal to the angle of incidence. i = r — measured from the normal, never from the surface. If a question gives an angle to the surface, subtract from 90° first.

Images in a plane mirror — all five

Same size as the object
Upright
Virtual — formed inside the mirror, cannot be formed on a screen (a screen = any physical object such as a wall or paper)
Laterally inverted — left and right reversed
Same distance from the mirror as the object is

Uses of plane mirrors: checking appearance · lining walls to make an interior look wider · periscopes (submarines) · reflecting an eye chart so it appears further away.

Ambulance lettering is reversed so a driver ahead reads it correctly in their rear-view mirror, which laterally inverts it back.

Curved mirrors

	Convex	Concave
Surface	curves outwards	curves inwards
Also called	diverging mirror	converging mirror
Effect	spreads rays apart → wider range of vision	brings rays together → magnifies / concentrates
Uses	road junctions (blind corners) · shop corners · rear-view and side mirrors	dentist's mirror · shaving and make-up mirrors · car headlights · microscope mirror

A spoon shows both: the top is concave, the bottom is convex. Curved mirrors distort the images they form — as in a funhouse mirror.

Smooth vs rough surfaces

Smooth surface	Rough surface
Each ray reflected in the same direction	Each ray reflected in a different direction
Clear images form	Light is scattered; no image
plane mirror · polished metal · calm water	paper · walls · rippled water

The examinable subtlety For both smooth and rough surfaces, i = r for every individual ray. What differs on a rough surface is the direction of the normal at each point, because the surface tilts differently from place to place. Paper looks smooth but is uneven under a microscope, which is why it scatters light and shows no reflection.

Refraction

Refraction The bending of light at the boundary between two different transparent materials (optical mediums), due to a change in the speed of light as it passes from one into the other.

Optical density affects the speed of light in a medium: the more optically dense, the slower light travels. It is not the same as the density from Chapter 2.

Going from	Speed	Bends
air → glass (less dense → denser)	slows down	towards the normal
glass → air (denser → less dense)	speeds up	away from the normal
Entering along the normal (i = 0°)	still changes	no bending — straight through

The cart model: wheel A hits the grass first, slows first, travels a shorter distance than B, so the cart swings. Perpendicular entry → both wheels slow together → straight on at lower speed.

What you observe: a spoon or pencil in water looks bent or broken · letters under a glass block appear nearer the surface · a pool's perceived depth is less than its actual depth, so it looks shallower than it is.

Dispersion

White light is a spectrum of seven colours: ROYGBIV — red, orange, yellow, green, blue, indigo, violet.

Each colour slows to a different extent on entering a new medium, so each bends by a different angle. Violet bends the most, red bends the least. The splitting of white light into its component colours is dispersion.

Rainbows form as sunlight crosses the air–water boundary into raindrops.
A secondary rainbow comes from light reflected twice inside each droplet — fainter, harder to spot, colours in reverse order.
Newton's colour disc, spun quickly, appears white — recombining the seven colours.
In a real prism experiment the spectrum is only visible on a white screen placed far from the prism.

Radiation and its impact

Visible light is only one type of electromagnetic (EM) radiation. Others we cannot see include infrared and ultraviolet. Snakes see infrared; bees see UV.

	Applications	Harmful effects
Infrared	Thermal imaging cameras screen for fever (used during COVID-19) · robotic vacuum cleaners avoid stairs · vehicles detect a drowsy driver	Overexposure in welding, cutting, brazing, furnaces and molten metal — workers wear goggles · trapped in the atmosphere it drives climate change
Ultraviolet	Increases vitamin D production · treats skin diseases such as psoriasis · UV sterilisation of equipment, food and water with no harmful chemicals or by-products	Overexposure harms the eyes and causes skin cancer · sunscreen filters out UV
Visible light	Photosynthesis · daily activities, reading, driving safely at night · Edison's incandescent bulb (1879) → fluorescent lamps → LEDs	Chemical changes fade documents, photographs and paintings (no flash photography in museums) · light pollution

Light pollution Too much artificial light in the environment. It harms living things that depend on the day–night cycle for migration, reproduction, searching for food, sleeping and escaping predators.

Migratory birds gauge the seasons by sunlight; artificial light makes them migrate too early or too late, which can be fatal. Turtle hatchlings are guided to the sea by natural light in the sky; artificial lamps draw them away and they die (NParks hatchery at Sisters' Islands Marine Park, 2018). Street lighting also uses a lot of electricity — LEDs and CFLs reduce both problems.