Know that all electromagnetic waves travel at the same high speed in a vacuum
3.3 Electromagnetic Spectrum
Key Concept: Speed of All EM Waves
In a vacuum, every electromagnetic wave—whether it’s a radio wave, light, or X‑ray—travels at the same speed, called the speed of light, denoted by $c$: $$c \approx 3.0 \times 10^8 \,\text{m/s}$$ ⚡️ Think of it like a super‑fast roller coaster that never slows down, no matter how steep the track. The track (frequency or wavelength) changes, but the coaster’s speed stays constant.
Because the speed is constant, the relationship between frequency $f$ and wavelength $\lambda$ is: $$c = f \lambda$$ 📡 If you double the frequency, the wavelength halves, but the product (speed) remains the same.
Exam Tip: When asked to calculate wavelength or frequency, remember to use the equation $c = f \lambda$ and that $c = 3.0 \times 10^8 \,\text{m/s}$ in a vacuum. Always check units—meters for wavelength, hertz for frequency.
The Electromagnetic Spectrum (Colourful Overview)
| Type | Wavelength (m) | Frequency (Hz) | Typical Example |
|---|---|---|---|
| Radio Waves | > 1 mm | < 3 × 10^11 | 📻 AM/FM radio |
| Microwaves | 1 mm – 1 m | 3 × 10^11 – 3 × 10^10 | 🍲 Microwave oven |
| Infrared | 700 nm – 1 mm | 4 × 10^14 – 3 × 10^11 | 🌞 Sunlight warmth |
| Visible Light | 400 nm – 700 nm | 4 × 10^14 – 7 × 10^14 | 👀 Human vision |
| Ultraviolet | 10 nm – 400 nm | 3 × 10^16 – 4 × 10^14 | ☀️ Sunburn risk |
| X‑Rays | 0.01 nm – 10 nm | 3 × 10^19 – 3 × 10^16 | 🩻 Medical imaging |
| Gamma Rays | < 0.01 nm | > 3 × 10^19 | ☢️ Nuclear reactions |
Exam Tip: When you see a question about wavelength or frequency, remember:
- Use $c = f \lambda$ with $c = 3.0 \times 10^8$ m/s.
- Check the units: meters for wavelength, hertz for frequency.
- Convert between units (e.g., nm to m) before plugging into the formula.
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