P1c: Introduction to waves

P1c: Introduction to waves

1. Electromagnetic waves travel in straight lines

2. If wavelength increases then frequency decreases.

3. Refraction occurs because waves go at different speeds in different materials.

4. Diffraction occurs the most when the gap is a similar size to the wavelength.

5. The speed of a wave can be calculated using its frequency and wavelength

6. Lasers produce intense narrow beams of light.

7. Transverse waves

   • Light consists of transverse waves in which the electromagnetic vibrations are at 90º to the direction of travel
   • Waves are vibrations that transfer energy from place to place without matter - solid, liquid or gas - being transferred.
   • In transverse waves, the oscillations (vibrations) are at right angles to the direction of travel and energy transfer. Water waves are transverse waves.
   • 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

8. As waves travel, they set up patterns of disturbance. The amplitude of a wave is its maximum disturbance from its undisturbed position.

9. 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

10. 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

    • It is common for kilohertz (kHz), megahertz (MHz) and gigahertz (GHz) to be used when waves have very high frequencies
    • The unit of frequency is the hertz (Hz), which is one wave per second.
    • most people cannot hear a high-pitched sound above 20 kHz, radio stations broadcast radio waves with frequencies of about 100 MHz, while most wireless computer networks operate at 2.4GHz.

11. wave speed (m/s) = frequency (hertz, Hz) × wavelength (metre, m)

12. 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.

13. Smooth surfaces produce strong echoes when sound waves hit them, and they can act as mirrors when light waves hit them. The waves are reflected uniformly and light can form images

14. 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.

    • There is one special case you need to know. Refraction doesn't happen if they cross the boundary at an angle of 90° (called the normal) - in that case they carry straight on.

15. 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

    • 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.

16. 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

    • 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