Electromagnetic Spectrum
High-Yield Summary
- EMR = oscillating electric and magnetic fields, perpendicular to each other and to travel direction; needs no medium, so it travels through vacuum.
- EMR also behaves as particles — photons — whose interaction with matter depends on wavelength/frequency.
- Wavelength and frequency are inversely proportional, linked by c = λν, c ≈ 3.00×10⁸ m/s in vacuum (same speed for all EMR in vacuum).
- Spectrum order, longest to shortest λ: radio → microwave → infrared → visible → UV → X-ray → gamma ray.
- Visible light spans ~700 nm (red) to ~400 nm (violet), sitting between infrared and UV.
Speed of Light Relation
c = λν
- c = Speed of light in vacuum ≈ 3.00×10⁸ m/s
- λ = Wavelength
- ν = Frequency
- Worked example: λ = 500 nm = 5.00×10⁻⁷ m → ν = c/λ = 6.00×10¹⁴ Hz.
- All EMR travels at the same c in vacuum — only λ and ν vary across the spectrum.
EM Spectrum, Longest to Shortest Wavelength
| Region (wavelength range) | Key application |
|---|---|
| Radio waves (~10⁶ m to 1 m) | Radio and TV broadcast |
| Microwaves (1 m to 1 mm) | Radar, microwave heating |
| Infrared (1 mm to 700 nm) | Heat, thermal imaging |
| Visible light (700 nm to 400 nm) | Human vision |
| Ultraviolet (400 nm to ~50 nm) | Sunburn, ionization |
| X-rays (50 nm to 10⁻² nm) | Medical imaging |
| Gamma rays (< 10⁻² nm) | Nuclear reactions, cosmic events |
Common MCAT Trap
- Wavelength and frequency move in opposite directions — a shorter-wavelength photon always has higher frequency and higher energy, never assume they scale together.
- Don't assume EMR needs a medium — unlike sound, it propagates through vacuum, since the oscillating E and B fields regenerate each other.
Quick Recall
A photon has wavelength 400 nm. Is it visible, UV, or infrared?
Which travels faster in vacuum: a radio wave or a gamma ray?