Light & Energy: an interactive course on light

Light & Energy is an interactive light physics course that explains what light is and how it behaves. Its 23 short sections cover waves and the wave equation, the electromagnetic spectrum, photons and the photoelectric effect, reflection, refraction and Snell's law, total internal reflection, diffraction, interference and energy transfer by thermal radiation, with a live demo or practice question in nearly every one.

It opens with a wave you can drag from radio to gamma, reading off its colour, frequency and photon energy as it changes, which sums up the course: one kind of wave, an enormous range of behaviour.

What the course covers

Light as a wave

Every electromagnetic wave travels through empty space at the same speed, c, about 3.00 × 10⁸ m/s. A source sends out f waves each second, each λ metres long, which gives the wave equation. Radio waves are metres long; visible light is a few hundred nanometres; gamma rays are far shorter still.

Light as particles

Light also arrives in packets called photons, each with energy E = hf, so higher frequency means more energy per photon. The photoelectric effect, light knocking electrons out of a metal, is the evidence: below a certain frequency, no electrons come out however bright the light.

Rays: reflection and refraction

In ray optics, every angle is measured from the normal, drawn perpendicular to the surface. In reflection, the angle of incidence equals the angle of reflection. In refraction, light changes speed and bends, by an amount Snell's law predicts, and going from glass to air beyond the critical angle it reflects completely instead, which is how optical fibres carry signals.

Try a question from the course

Rearrange c = fλ for whatever you need. A radio station broadcasting at 100 MHz, which is 1.00 × 10⁸ Hz, sends waves of wavelength λ = c ÷ f = 3.00 × 10⁸ ÷ 1.00 × 10⁸ = 3.00 m.

Now find a wavelength or a frequency yourself, then press Mark to check your answer in the course.

Who this course is for

• GCSE physics students revising waves, the electromagnetic spectrum, reflection and refraction.
• A-level and other advanced physics students meeting photons, the photoelectric effect, Snell's law, total internal reflection and interference.
• Curious learners who want to know why the sky has colours, how fibres carry the internet and why light is called both a wave and a particle.

What you need to know first

Basic algebra, especially rearranging a formula such as c = fλ, and standard form for very large and very small numbers. The course page suggests Algebra first. Snell's law uses sine, which the Trigonometry course covers.

How long it takes

There are 23 sections and most take a few minutes. Learners without a plan can open two sections of a course a day, in order, so the whole course takes 12 days. A plan removes the daily limit and lets you go straight to any topic, such as Snell's law.

Read next

Physics · Waves and lightRefraction ray diagrams: how to draw and explain themHow to draw a refraction ray diagram, which way light bends at a boundary, the glass block practical, and why refraction happens, using wave fronts.