Energy Carried by Electromagnetic Waves

Both fields carry energy. An electromagnetic wave is not merely a geometric pattern of oscillating field DIRECTIONS -- the electric field $\vec{E}$ and the magnetic field $\vec{B}$ that make up the wave each store real energy in the space they occupy, exactly as a charged capacitor stores energy in its electric field and a current-carrying inductor stores energy in its magnetic field (both met in earlier chapters). As the wave travels, it carries this stored field energy along with it, from wherever it was generated to wherever it is eventually absorbed -- which is, physically, the whole reason sunlight can warm a surface on Earth roughly 150 million kilometres from the Sun, with nothing but empty space in between.

Intensity. The rate at which an electromagnetic wave delivers energy, per unit area perpendicular to its direction of travel, is called its intensity (or, in the SI system, its irradiance), with unit watt per square metre ($\text{W/m}^2$). WBCHSE's syllabus asks only for the QUALITATIVE picture here: the intensity of an electromagnetic wave is proportional to the SQUARE of the amplitude of its oscillating electric (or, equivalently, magnetic) field -- so doubling a wave's field amplitude quadruples the energy it delivers per second, not merely doubles it. This is exactly why a laser beam, with its fields concentrated into a very narrow, intense spot, can cut or burn material that ordinary diffuse light of the same total power cannot.

Momentum and radiation pressure (qualitative). An electromagnetic wave carries not only energy but also a small amount of momentum, and so exerts a tiny, genuinely measurable PRESSURE -- radiation pressure -- on any surface that absorbs or reflects it. This effect is far too small to notice in everyday life (sunlight's radiation pressure on a person standing outdoors is many orders of magnitude weaker than the pressure of the surrounding air), but it is large enough, over the vast areas and long durations available in space, to be genuinely used to steer a spacecraft fitted with a large, lightweight reflective 'solar sail', and it is also part of what supports a star against its own gravity from the inside out. …