- Define and explain The Electromagnetic Spectrum in your own words
- Use key terms such as wavelength accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on The Electromagnetic Spectrum: ordering the spectrum from radio waves to gamma rays and meeting their uses.
Ordering the spectrum from radio waves to gamma rays and meeting their uses.
Key ideas
The electromagnetic spectrum is one family
All EM waves travel at 3 × 10⁸ m/s in a vacuum and differ only in wavelength and frequency. From longest to shortest: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Long waves suit communications; short waves carry more energy per photon and can ionise atoms, making them hazardous.
Waves carry energy without carrying matter
In a transverse wave (like light or a rope wave) the vibrations are at right angles to the direction of travel; in a longitudinal wave (like sound) they are parallel, forming compressions and rarefactions. The medium oscillates but does not travel with the wave — only energy moves onwards, which is why a floating duck bobs up and down as ripples pass.
Key term — wavelength: The distance between two matching points on neighbouring waves, e.g. crest to crest, in metres.
Name two uses of microwaves and one danger of excessive infrared exposure.
Microwaves: cooking food, mobile phone and satellite communication. Infrared: overexposure can cause skin burns.
Answer: Microwaves: cooking food, mobile phone and satellite communication. Infrared: overexposure can cause skin burns.
- Thinking different EM waves travel at different speeds In a vacuum every EM wave travels at 3 × 10⁸ m/s — speed does not depend on wavelength.
- Saying the medium moves along with the wave Particles oscillate in place; only energy is transferred along the wave.
Practice
50 × 4 = 200 m/s.
Longitudinal — air particles vibrate parallel to the direction the sound travels, in compressions and rarefactions.
Blue light — with speed constant, shorter wavelength means higher frequency.
Refraction — light slows down in glass, and the change in speed at the boundary bends the ray.
Quick check
Which of these best defines "wavelength"?
An EM wave has wavelength 0.5 m. Calculate its frequency (c = 3 × 10⁸ m/s).
- The Electromagnetic Spectrum: ordering the spectrum from radio waves to gamma rays and meeting their uses.
- The electromagnetic spectrum is one family: All EM waves travel at 3 × 10⁸ m/s in a vacuum and differ only in wavelength and frequency.
- frequency: The number of waves passing a point per second, in hertz.
- Watch out for: thinking different EM waves travel at different speeds