Nature of Electromagnetic Waves
- In 1808, Eteinne-Louis Malus discovered light could be partially polarised by reflection.
- A polarised wave is a wave in which all the vibrations are in the same plane, and polarisation is a property of transverse waves only.
- Huygens' wave theory predicted that light spread like sound, as a longitudinal wave, which caused scientists to struggle to explain polarisation.
- In 1817, Young suggested light was a transverse wave consisting of vibrating electric and magnetic fields at right angles to each other and the direction of travel, which explains why light can be polarised.
James Clerk Maxwell's Formula for the Speed of Ele
Maxwell predicted the existence of EM waves before they were even discovered. In the 1860's, he showed theoretically that all EM waves should travel at the same speed in a vacuum, c.
- c = the speed of the wave in ms^-1 = 3.00 x 10^8 ms^-1
- Uo = the permeability of free space = 4Pie x 10^-7 Hm^-1
- Eo = the permittivity of free space = 8.85 x 10^-12 Fm^-1
- Uo relates to magnetic flux density due to a current-carrying wire in free space.
- Eo relates to electric field strength due to a charged object in free space.
By the time Maxwell did this calculation, the velocity of light could be measured quite accurately, and it was found to be very close to Maxwell's value of c.
This suggested that light was an electromagnetic wave.
Heinrich Hertz Discovery of Radio Waves
- In the late 1880's, Hertz produced and detected radio waves using electric sparks.
- He used an induction coil and a capacitor to produce a high voltage, and showed that radio waves were produced when high voltage sparks jumped across a gap of air.
- An induction coil is like a transformer - whenever the current is interrupted a high voltage is induced in the secondary coil ( wire loop).
- He detected these radio waves using a loop of wire with a gap in it in which sparks were induced by the radio waves.
- In later experiments Hertz used a flat metal sheet to create stationary radio waves, showing that they could both be reflected and show interference.
- He also went on to show that they could be refracted, defracted and polarised.
- Hertz produced stationary waves at a fixed resonant frequency.
- He moved the radio wave detector between the transmitter and the reflecting sheet and measured the distance between nodes.
- Since the distance between nodes is half a wavelength, he could work out the wavelenth of the waves and use c = f x Landa to calculate the wavespeed.
He measured the speed of radio waves to be the same in a vacuum as the rest of the electromagnetic spectrum.
This helped confirm that radio waves, like light, are electromagnetic waves.
This is the end of the topic