The law of reflection

When light hits a surface it reflects (bounces back). We measure every angle from the normal — an imaginary dashed line drawn at 90° to the surface:

• Specular reflection — a smooth surface (a mirror): parallel rays stay parallel, giving a clear image.
• Diffuse reflection — a rough surface: the bumps face different ways, so rays scatter in many directions (no clear image).

Watch out: in diffuse reflection the law of reflection still holds at every point — it's the surface that varies, not the law.

Find the angle

Bending at a boundary

Refraction is the change of speed — and so, usually, direction — of a wave as it crosses into a new medium. The frequency stays the same; the wavelength changes with the speed.

Analogy: picture a toy car (or supermarket trolley) rolling onto mud at an angle. The wheel that hits the mud first slows down, so the car swings round towards the normal — exactly how light bends entering glass.

Watch out: refraction is caused by a change of speed, not by "light wanting to bend". If a ray meets the boundary along the normal (90° to the surface) it slows but does not change direction.

Which way does it bend?

Why does a straight straw look snapped?

Investigating refraction

Edexcel Core Practical 3 asks you to investigate refraction using a rectangular glass block and a ray box:

• Place the block on paper and draw round it. Shine a single ray into one face.
• Mark the incoming ray, the point it enters, and the ray that leaves. Remove the block and join the dots.
• Draw the normal at the entry point and measure the angle of incidence and the angle of refraction with a protractor.
• Repeat for several incidence angles — the angle of refraction inside the glass is always smaller than the angle of incidence (except at 0°).

Tip: a sharp pencil and a thin ray line cut measurement error. Dim the room lights so you can see the ray clearly, and view from directly above to avoid parallax.

Total internal reflection

When light travels from glass towards air, at large angles it can be trapped inside.

How can one lens make two kinds of image?

Converging & diverging lenses

A lens forms an image by refracting light. Edexcel needs both types:

• Convex (converging) — fatter in the middle; parallel rays converge to the principal focus.
• Concave (diverging) — thinner in the middle; parallel rays spread out, appearing to come from the focus.

The focal length is the distance from the lens centre to the principal focus. A fatter lens has a shorter focal length and more power.

Real vs virtual: a real image is where rays actually meet (it can be projected). A virtual image is where rays only appear to come from (e.g. a magnifying glass close up) — it can't be projected.

How can one lens make two kinds of image?

Magnification

Magnification compares the image size with the object size. It has no units (it's a ratio):

Watch out: both heights must be in the same units. Magnification is just a number — never write "cm" after it.

Your turn — magnification

Why objects look coloured

White light is a mix of all the colours of the visible spectrum — each colour is a different wavelength. Materials are sorted by how they handle light:

• Transparent — light passes straight through (clear glass).
• Translucent — some light passes through but is scattered (frosted glass).
• Opaque — no light passes through; it is reflected or absorbed.

A colour filter works the same way: it transmits its own colour and absorbs the rest. A red filter only lets red light through.

Watch out: a red object does not "make" red light. It simply reflects the red already in white light and absorbs the rest — under pure green light it would look almost black.

Light through a filter

Why are some light-speed waves dangerous?

One family of waves

The EM spectrum is a continuous family of transverse waves. They all transfer energy from a source to an absorber and all travel at the same speed in a vacuum (3 × 10⁸ m/s). They are grouped by wavelength and frequency:

Watch out: in a vacuum every EM wave travels at the same speed. Gamma rays are not "faster" than radio — they have a higher frequency and shorter wavelength, which makes them more energetic, not quicker.

Order the spectrum

Tap each wave, then tap whether it sits at the long-wavelength or short-wavelength end.

Put to work — and the risks

Each group's energy decides what it's used for and how it can harm us. Higher-frequency waves carry more energy, so they are more dangerous:

• Radio — TV & radio broadcasting (made by, and can induce, oscillations in circuits).
• Microwave — satellite communications and cooking · danger: internal heating of cells.
• Infrared — cooking, thermal imaging, short-range remotes, optical fibres · danger: skin burns.
• Visible — vision, photography, fibre-optic communication.
• Ultraviolet — fluorescent lamps, security marking, disinfecting water · danger: skin cancer, eye damage.
• X-ray — medical imaging, airport security scanners · danger: mutation/cell damage (ionising).
• Gamma — sterilising equipment/food, treating cancer · danger: mutation/cell damage (ionising).

Link to atoms: EM radiation is generated by changes in atoms and their nuclei (and absorbed radiation can cause such changes), spanning a wide frequency range — gamma rays come from the nucleus.

Wave to use

Tap a wave on the left, then its correct use on the right.

Spot the risk

Why are some light-speed waves dangerous?

Which car cools down faster at night?

Emitting & absorbing infrared

All objects emit and absorb infrared. The hotter an object, the more infrared it emits each second. Surfaces also differ:

• Matt black surfaces are the best emitters and absorbers of infrared.
• Shiny, silvery surfaces are the worst emitters and absorbers (they reflect IR).

CP10 method: fill a Leslie cube with hot water and hold an infrared detector the same distance from each face. Black/matt faces give the largest reading; shiny faces the smallest.

Best at radiating

Keeping the Earth's temperature steady

An object at a constant temperature emits radiation at the same average rate as it absorbs it. If it absorbs more than it emits its temperature rises; if it emits more than it absorbs it falls.

Earth's temperature depends on the balance between energy received from the Sun and energy radiated back to space. Trapping more outgoing IR shifts that balance and warms the planet.

The energy balance

Which car cools down faster at night?

The Topic 5 essentials

You've covered the key ideas in Edexcel Topic 5 — Light and the electromagnetic spectrum. Press Finish to see your score.