Transverse electric and magnetic fields produce each other in light, radio waves and other electromagnetic radiation. Here we demonstrate that electromagnetic radiation is a transverse rather than a longitudinal wave. This page supports the multimedia tutorial The Nature of Light.
The electric field direction and the direction of propagation
Is light and other electromagnetic radiation longitudinal (like sound) or transverse (like waves in a rope)? Here we transmit and receive UHF radio waves to find out.
The dipole atop the mast transmits UHF radio, while I rotate a receiving antenna in the vertical plane.
A horizontal dipole antenna transmits radiation with an electric field parallel to the dipole axis: i.e. horizontal. In the first experiment, I rotate a receiving antenna in the vertical plane. This dipole antenna is connected to the oscilloscope whose screen we see at left. The signal is maximal when the antenna is horizontal and smallest when vertical. So, in this plane, the strongest signal is in the horizontal direction.
In the next experiment, I rotate the receiving antenna in a plane that is nearly horizontal: it is the plane including the line from the camera to the transmitter and a horizontal line at right angles to this.
The dipole atop the mast transmits UHF radio, while I rotate a receiving antenna in a plane including a line from the transmitter to the camera.
The signal is maximal when the antenna is smallest when it lies in the line pointing to and largest when at right angles to that line. So, in this plane, the strongest signal is at right angles to the direction of propagation. So the electric field is in the direction at right angles to that of propagation.
The magnetic field generated by a current in a wire, as in the transmitting antenna, forms circles around it. So the magnetic field is at right angles to both the electric field and the direction of propagation.
Electric and magnetic fields
The animation below shows the electric field E and magnetic field B in one-dimensional propagation.
An animation showing the electric and magnetic fields in one dimensional propagation
In Maxwell's equations, the spatial varation in E is related to the time varation in B and vice versa. This gives rise to a wave solution, with speed c.
In light, the plane including the direction of propagation and the direction of the electric field is called the plane of polarisation.
In totally polarised light, all photons have the same plane of polarisation. In unpolarised light, photons are present with polarisations in all planes including the direction of propagation. We'll discuss this in more detail in the future chapter on polarisation.
Sunglasses with polarising filters
Light reflected from horizontal surfaces often has significant polarisation in the horizontal direction. The 'glare' reflected from horizontal surfaces can therefore be reduced with sunglasses that admit only light with vertical polarisation. Here we show what happens when light is passed through one polarising filter (one pair of such glasses) and then through another, whose relative orientation is changed.
Much more on this when we do the chapter on polarisation.