This mini-lesson walks you through the whole of Eduqas Topic 6 — Light and electromagnetic waves: the EM spectrum, how each region interacts with matter (uses & dangers), lenses and ray diagrams, why objects look coloured, and black body radiation.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
6.1 · The EM spectrum
One family, seven members
EM waves are all transverse waves made of oscillating electric and magnetic fields, so they need no particles to travel. In a vacuum they all travel at the same speed — the speed of light, 3 × 10⁸ m/s.
Order to memorise (low→high frequency): radio → microwave → IR → visible → UV → X-ray → gamma.
Watch out: a common slip is to think gamma rays "travel fastest". They don't — in a vacuum every EM wave travels at the same speed. Gamma rays have the highest frequency and energy, not the highest speed.
Quick check
Order of the spectrum
?Which list puts the EM regions in order of increasing frequency (lowest first)?
6.2 · Interactions & applications
Uses and dangers
Because speed is fixed, as frequency rises the energy rises too — so each region behaves differently and is useful for different jobs:
Radio — broadcasting & communications. Made by oscillating currents in a circuit; they can also induce currents in a receiving aerial.
Microwave — satellite communication and heating food (absorbed by water molecules, making them vibrate).
Infrared (IR) — heating and TV remote controls.
Visible light — illumination and seeing.
Ultraviolet (UV) — sterilisation and fluorescent lamps.
X-rays — medical imaging of bones.
Gamma — sterilising equipment and cancer radiotherapy.
Dangers:UV, X-rays and gamma have very short wavelength and high energy, so they are ionising — they can damage cells and cause mutations (cancer). UV ages skin and raises skin-cancer risk; this is why we use lead aprons and sun protection.
Match it
Region → use
Tap a region on the left, then tap its main use on the right.
EM region
Use
6.2 · Reflection
Specular vs diffuse reflection
At a plane (flat) surface the law of reflection holds: the angle of incidence = angle of reflection, both measured from the normal.
A mirror gives a clear reflection (specular); a rough wall scatters light in all directions (diffuse) so no image forms.
The surface still obeys i = r for each tiny ray in diffuse reflection — it's the roughness that scatters parallel rays in different directions.
6.3 · Lenses
Two shapes of lens
A lens refracts light to form an image. A ray through the centre of a lens carries straight on.
Convex lenses bulge out and focus rays to a focal point F. Concave lenses cave in and spread rays so they appear to come from F.
Watch out: a converging (convex) lens can make a real OR a virtual image — it depends on where the object sits. Closer than one focal length it acts as a magnifying glass (virtual image); further away it forms a real image.
6.3 · Converging lens
Real image from a convex lens
When the object is beyond the focal point, two rays locate the image:
a ray parallel to the axis bends to pass through F on the far side;
a ray straight through the centre of the lens carries on unchanged.
Object beyond 2F → the image is real, inverted and diminished (this is how a camera or eye works). Between F and 2F it would be real, inverted and magnified — a projector.
A real image is where light rays actually cross, so it can be caught on a screen.
Quick check
Reading the lens
?An object is placed further than 2F from a converging lens. What is the image like?
6.3 · Diverging lens
Virtual image from a concave lens
A concave (diverging) lens always makes the same kind of image, wherever the object is: virtual, upright and diminished. The rays only appear to come from a point in front of the lens.
The blue and red rays diverge; tracing them back (dashed) meets on the same side as the object → a virtual, upright, smaller image. Used to correct short-sightedness.
Virtual means no light actually meets there, so it cannot be projected onto a screen — your eye just sees it.
6.3 · Magnification
How big is the image?
Magnification compares the size of the image with the size of the object:
m = image height ÷ object heightmagnification (no units) = height of image ÷ height of object
It has no units because it is a ratio of two lengths. If m > 1 the image is magnified; if m < 1 it is diminished.
Worked example
A 4 mm tall object forms a 12 mm tall image.
m = 12 ÷ 4 = 3 (the image is 3× the object's height).
Calculate
Your turn — magnification
1A magnifying glass forms an image 15 mm tall from an object 5 mm tall. Calculate the magnification.
×
Hint: m = image height ÷ object height = 15 ÷ 5.
Calculate
Your turn — find a height
2A converging lens produces a magnification of 0.5 from an object 8 cm tall. Calculate the image height in cm.
cm
Hint: image height = m × object height = 0.5 × 8.
6.4 · Colour and frequency
Why objects look coloured
White light contains every frequency (colour) of the visible spectrum. When it hits an opaque object some frequencies are absorbed and the rest are reflected — and the reflected ones reach your eye.
A red object reflects red light and absorbs the other colours. A white object reflects all colours; a black object absorbs them all.
Materials are described as transparent (all light transmitted, e.g. clear glass), translucent (only some transmitted, the rest scattered/absorbed) or opaque (none transmitted). A colour filter transmits its own colour and absorbs the rest.
Watch out: a red object does not "turn light red". It reflects the red already present in white light and absorbs the rest. Under pure blue light a red object looks black — there's no red for it to reflect.
Quick check
Through a filter
?White light is shone through a green colour filter. What happens?
6.5 · Black body radiation
Emitting and absorbing (qualitative)
Every object both emits and absorbs EM radiation (mainly infrared). The hotter the surface, the more intense the radiation and the shorter its peak wavelength.
Dark, matt surfaces are the best absorbers and emitters; shiny white surfaces reflect radiation and are poor at both.
A perfect black body is an idealised object that absorbs all radiation falling on it (reflecting and transmitting none). Because a good absorber is a good emitter, it is also a perfect emitter. Stars are the closest real things to black bodies.
Watch out: "good absorber = good emitter." A matt black object isn't only good at soaking up radiation — it is equally good at radiating it away. That's one object, two skills.
Quick check
Best radiator
?Four identical cans of hot water have different surfaces. Which one cools down fastest (best emitter)?
6.5 · Earth's temperature
Earth's energy balance
Earth's temperature depends on the balance between the radiation it absorbs (mostly from the Sun) and the radiation (mostly infrared) it emits back into space:
absorption = emission → temperature stays constant;
absorption > emission → temperature rises;
absorption < emission → temperature falls.
Greenhouse gases absorb some outgoing IR and re-radiate it, keeping Earth warm. More greenhouse gas → more IR trapped → temperature rises.Sort it
Rising or falling?
Tap a statement, then tap whether Earth's temperature would rise or fall.
🔺 Temperature rises
🔻 Temperature falls
Recap
The key facts to know
EM order (↑freq): radio · micro · IR · visible · UV · X-ray · gamma