P1C: An Introduction To Waves

P1C: An Introduction To Waves

1. TRANSVERSE WAVES

   • Waves transfer energy from one place to another
   • In transverse waves, the oscillations (vibrations) are at right angles to the direction of travel and energy transfer. Water waves are transverse waves.
   • Transverse waves move like a Mexican wave in a football crowd. The wave moves around the stadium, while each spectator stays in their seat - only moving up, and then down, when it is their turn.
   • Light and other types of electromagnetic radiation are transverse waves. All types of electromagnetic waves travel at the same speed through a vacuum, such as through space

2. AMPLITUDE

   • As waves travel, they set up patterns of disturbance.
     • The amplitude of a wave is its maximum disturbance from its undisturbed position.
       • The amplitude is not the distance between the top and bottom of a wave.

3. WAVELENGTH

   • The wavelength of a wave is the distance between a point on one wave and the same point on the next wave.
     • It is often easiest to measure this from the crest of one wave to the crest of the next wave, but it doesn't matter where as long as it is the same point in each wave.

4. FREQUENCY

   • The frequency of a wave is the number of waves produced by a source each second.
     • It is also the number of waves that pass a certain point each second. The unit of frequency is the hertz (Hz), which is one wave per second.

5. WAVE SPEED

   • The speed of a wave - its wave speed - is related to its frequency and wavelength, according to this equation:
     • wave speed (m/s) = frequency (hertz, Hz) × wavelength (metre, m)
       • All waves obey this wave equation. For example, a wave with a frequency of 100 Hz and a wavelength of 2m travels at 100 × 2 = 200 m/s.

6. REFLECTION

   • Light waves reflect from surfaces. When waves reflect, they obey the law of reflection: the angle of incidence equals the angle of reflection.
   • The normal is a line drawn at right angles to the reflector.
     • The angle of incidence is between the incident (incoming) ray and the normal.
       • The angle of reflection is between the reflected ray and the normal.
   • Smooth surfaces produce strong echoes when sound waves hit them, and they can act as mirrors when light waves hit them.
   • Constructing a ray diagram
     • In a ray diagram, the mirror is drawn a straight line with thick hatchings to show which side has the reflective coating. The light rays are drawn as solid straight lines, each with an arrowhead to show the direction of travel.

7. REFRACTION

   • Light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction.
   • Refraction doesn't happen if they cross the boundary at an angle of 90° (called the normal) - in that case they carry straight on.
   • The refraction follows a regular pattern.

8. DIFFRACTION

   • When waves meet a gap in a barrier, they carry on through the gap. However, the waves spread out to some extent into the area beyond the gap. This is diffraction.
   • The extent of the spreading depends on how the width of the gap compares to the wavelength of the waves. Significant diffraction only happens when the wavelength is of the same order of magnitude as the gap. For example:
     • a gap much larger than the wavelength causes little spreading and a sharp shadow eg light through a doorway
     • a gap similar to the wavelength causes a lot of spreading with no sharp shadow eg sound through a doorway
   • Diffraction reduces the quality of images seen in microscopes and telescopes. It can cause rings or spikes around the image of the object being viewed.

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