Waves
Wave properties, electromagnetic spectrum, light, and sound.
In this topic
Wave Properties
Waves transfer energy without transferring matter.
Transverse waves: oscillations are perpendicular to direction of travel (e.g., light, water waves, electromagnetic waves). Longitudinal waves: oscillations are parallel to direction of travel (e.g., sound, ultrasound).
Key terms: - Wavelength (λ): distance between two consecutive identical points (peak to peak) - Frequency (f): number of waves per second, measured in hertz (Hz) - Amplitude: maximum displacement from the rest position - Period (T): time for one complete wave (T = 1/f)
Wave speed equation: v = fλ (speed = frequency × wavelength)
Key Points
- All electromagnetic waves travel at 3 × 10⁸ m/s in a vacuum
- Sound waves need a medium to travel through
- Higher frequency = higher pitch (sound) or more energy (EM)
- Larger amplitude = louder (sound) or brighter (light)
Example Questions
2A wave has a frequency of 500 Hz and a wavelength of 0.66 m. Calculate the wave speed.
v = fλ = 500 × 0.66 = 330 m/s
[2 marks]
3A sound wave has a speed of 340 m/s and a wavelength of 1.7 m. Calculate the frequency and the period.
f = v/λ = 340/1.7 = 200 Hz. Period T = 1/f = 1/200 = 0.005 s
[3 marks]
Electromagnetic Spectrum
The electromagnetic (EM) spectrum is a continuous range of waves, all travelling at the speed of light (3 × 10⁸ m/s) in a vacuum.
From longest wavelength to shortest: Radio → Microwave → Infrared → Visible light → Ultraviolet → X-ray → Gamma ray
Uses: - Radio: TV, radio communication - Microwave: cooking, satellite communication, mobile phones - Infrared: thermal imaging, remote controls, heating - Visible light: seeing, photography, optical fibres - Ultraviolet: sterilising, fluorescence, detecting forged notes - X-rays: medical imaging, airport security - Gamma rays: cancer treatment, sterilising medical equipment
Dangers increase with frequency: UV causes skin cancer, X-rays and gamma rays can cause mutations.
Key Points
- All EM waves are transverse
- They all travel at the same speed in a vacuum
- Frequency increases from radio to gamma
- Wavelength decreases from radio to gamma
- Higher frequency = more energy = more dangerous
Example Questions
1Put these EM waves in order of increasing wavelength: gamma rays, visible light, radio waves, X-rays.
Gamma rays, X-rays, visible light, radio waves
[1 mark]
2Explain why UV radiation is more dangerous than visible light.
UV radiation has a higher frequency and therefore more energy per photon than visible light. This higher energy can damage or kill cells and cause mutations in DNA, leading to skin cancer. Visible light does not have enough energy to cause this damage.
[2 marks]