Waves
What's covered
Key facts
Black Body Radiation
All bodies, whatever their temperature, emit and absorb infrared (thermal) radiation continuously.
Electromagnetic Waves
From longest to shortest wavelength: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
Lenses
A convex lens is a converging lens: it bends parallel rays of light inwards so they meet at the principal focus.
Reflection and Refraction
Angles of incidence, reflection, and refraction are all measured from the normal to the ray.
Seismic Waves
P-waves (primary waves) are longitudinal seismic waves: particles vibrate parallel to the direction of wave travel.
Sound Waves
In the ear, the eardrum (tympanic membrane) converts incoming sound waves into mechanical vibrations that are then passed to the inner ear.
Total Internal Reflection
Total internal reflection happens when the angle of incidence exceeds the critical angle.
Transverse and Longitudinal Waves
Light is an example of a transverse wave: electric and magnetic fields oscillate perpendicular to the direction of propagation.
Ultrasound
Ultrasound has a frequency above 20 000 Hz (20 kHz), which is above the upper limit of human hearing.
Uses of Electromagnetic Waves
Fibre-optic broadband cables use visible light.
Sample questions
A taste of the 116 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
What happens to the peak wavelength of radiation emitted as an object gets hotter?
- •It becomes longer
- ✓It becomes shorter
- •It disappears entirely
- •It stays the same
Which list orders the EM spectrum from longest to shortest wavelength?
- •Gamma, X-ray, UV, visible, IR, microwave, radio
- •IR, radio, microwave, visible, UV, gamma, X-ray
- ✓Radio, microwave, IR, visible, UV, X-ray, gamma
- •Radio, microwave, visible, IR, UV, X-ray, gamma
What name describes a convex lens because of the way it bends parallel light rays?
- ✓Converging lens
- •Diverging lens
- •Reflecting lens
- •Scattering lens
Why does light bend (refract) when entering glass?
- •Frequency drops on entering glass
- •Glass repels light particles
- ✓It slows down when entering a denser medium
- •It speeds up in denser materials
What is the difference between P-waves and S-waves in earthquakes?
- •Both P- and S-waves are longitudinal but P-waves have higher amplitude in solids
- •P- and S-waves both travel through solids and liquids, only at different speeds
- ✓P-waves are longitudinal; S-waves are transverse and only travel through solids
- •P-waves are transverse and travel only through solids; S-waves are longitudinal
Why is sound classified as a longitudinal wave?
- •Compressions are peaks of high amplitude and rarefactions are troughs of zero
- •It is transverse because air particles vibrate at right angles to the wave
- ✓Particles vibrate parallel to the direction of energy transfer
- •Sound creates longitudinal waves in a vacuum and pushes electrons forward
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