Electromagnetic Energy
Core concepts
Candidates must understand the electromagnetic spectrum from radio waves through microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays, and that all travel at the speed of light. Know the relationships c = fλ and E = hf, so shorter wavelength means higher frequency and higher photon energy. Understand wave behaviours: reflection, refraction, diffraction, and interference. Know visible light as a small band, and how colour relates to wavelength. Understand absorption and emission spectra as evidence for energy levels in atoms. Link radiation to technology and biology: UV causes DNA damage and skin cancer; ionising radiation (X-rays, gamma) can damage cells; infrared is felt as heat. Understand solar radiation and the albedo effect, and how the atmosphere absorbs different wavelengths.
Traps to watch
- Believing frequency changes when light enters a new medium; frequency stays constant, wavelength and speed change.
- Confusing ionising (high-energy, ejects electrons) with non-ionising radiation.
- Mixing up the order of the spectrum, especially placing infrared beyond ultraviolet.
- Forgetting that E = hf means energy depends on frequency, not amplitude.
- Assuming all UV is harmful; UV-A, UV-B, and UV-C differ in energy and penetration.
Worked example
A photon has frequency 5.0 × 10¹⁴ Hz. Using E = hf with h = 6.63 × 10⁻³⁴ J·s: E = (6.63 × 10⁻³⁴)(5.0 × 10¹⁴) ≈ 3.3 × 10⁻¹⁹ J. Rule of thumb: double the frequency, double the photon energy; halve the wavelength, double the energy.