How light travels
Light is a form of energy that travels as an electromagnetic wave. In a vacuum, it moves at about 300 000 km/s, the fastest speed in the universe. It travels in straight lines, which we draw as rays.
When light hits an object, three things can happen:
- Reflection: it bounces off.
- Transmission: it passes through (and may refract).
- Absorption: it’s taken in and turned into heat.
Reflection
The law of reflection
The law of reflection: the angle of incidence equals the angle of reflection.
- Both angles are measured from the normal, an imaginary line perpendicular to the surface.
- The incident ray, reflected ray and normal all lie in the same plane.
Types of reflection
| Type | Surface | Result |
|---|---|---|
| Specular (regular) | Smooth (mirror, still water) | Clear image |
| Diffuse | Rough (paper, cloth) | Scattered light, no image |
Mirrors
| Mirror | Shape | Image | Uses |
|---|---|---|---|
| Plane (flat) | Flat | Upright, same size, laterally inverted (left and right swapped) | Bathroom mirror |
| Concave | Curves inward | Can be magnified or inverted | Makeup mirrors, flashlights, telescopes |
| Convex | Curves outward | Smaller, upright, wide view | Car side mirrors, store security mirrors |
That’s why car side mirrors say "Objects in mirror are closer than they appear."
Refraction
Refraction is the bending of light when it moves from one medium to another at an angle, because its speed changes.
- Into a slower medium (air → water or glass): light bends toward the normal.
- Into a faster medium (water → air): light bends away from the normal.
| Medium | Speed of light (approx.) |
|---|---|
| Vacuum / air | 300 000 km/s |
| Water | 225 000 km/s |
| Glass | 200 000 km/s |
| Diamond | 124 000 km/s |
Effects of refraction:
- A straw looks bent in water.
- A pool looks shallower than it really is.
- Mirages appear on hot roads.
- Stars twinkle as their light passes through moving air.
Lenses
A lens uses refraction to focus light.
| Lens | Shape | Effect | Uses |
|---|---|---|---|
| Convex (converging) | Thicker in the middle | Brings rays together at a focal point | Magnifying glasses, cameras, the eye, glasses for farsightedness |
| Concave (diverging) | Thinner in the middle | Spreads rays apart | Glasses for nearsightedness, peepholes |
Colour
The visible spectrum
White light is a mix of colours. A prism refracts each colour by a slightly different amount, separating them into the visible spectrum:
Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROY G BIV)
- Red has the longest wavelength (about 700 nm) and bends least.
- Violet has the shortest (about 400 nm) and bends most.
Rainbows form when sunlight refracts, reflects and disperses inside raindrops. You see a rainbow only with the Sun behind you.
Why objects have colour
An object’s colour depends on which wavelengths it reflects and which it absorbs.
| Object | Reflects | Absorbs |
|---|---|---|
| Red apple | Red | All other colours |
| White paper | All colours | Very little |
| Black shirt | Very little | All colours (gets warm in sunlight) |
| Green leaf | Green | Red and blue (used for photosynthesis) |
Mixing light vs. mixing paint
| Light (additive) | Pigment/paint (subtractive) | |
|---|---|---|
| Primary colours | Red, green, blue | Cyan, magenta, yellow |
| All three combined | White | Black (or dark brown) |
| Used in | Screens, stage lights | Printers, paints |
The primary colours of light combine on screens: red + green = yellow; red + blue = magenta; green + blue = cyan.
Why is the sky blue?
Air molecules scatter short wavelengths (blue) much more than long ones (red). This scattering sends blue light across the sky. At sunset, light travels through more air, so most blue is scattered away and reds and oranges remain.
Common mistakes
- Measuring angles from the surface instead of the normal.
- Thinking mirrors reverse up and down. They swap front and back (we perceive left and right swapping).
- Mixing up additive and subtractive colour.
Try it yourself
- Shine a flashlight at a mirror at different angles and measure the angles of incidence and reflection. Are they equal?
- Place a coin in an empty cup and move back until you can’t see it. Have someone slowly fill the cup with water. Explain what happens.
- View a white screen through a magnifying glass. What three colours of tiny lights make it up?
Quick recap
- Light travels in straight lines at about 300 000 km/s.
- Law of reflection: angle of incidence = angle of reflection.
- Refraction bends light when it changes speed; lenses use refraction.
- White light splits into the visible spectrum.
- Objects show the colours they reflect; light mixes additively, paint subtractively.