Light passing through nothing
Teaching Guidance for 14-16
Light can travel through a vacuum
Wrong Track: Well, light and sound waves involve something oscillating back and forth. That's what a wave is. Like the coils of the slinky spring. So there must be something, maybe small amounts of air, that is vibrating to make up the light wave.
Right Lines: In fact there is nothing, no stuff, vibrating back and forwards in a light wave. The vibrations consist of changing electric and magnetic fields.
The rise and fall of the Aether (ether)
Thinking about the learning
Following on from the reasonable-sounding fact that sound can travel through air but can't travel through nothing, it comes as a shock for some students to learn that light from the Sun does, in fact, travel through nothing.
Thinking about the teaching
The idea that there
must be something vibrating to make a light wave is not a new one. In the history of physics, it was generally accepted right up to the start of the 20th century that there must be some kind of material medium – the ether – to support the passage of electromagnetic radiation.
The argument in favour of the ether went something like this:
The electric component of electromagnetic radiation consists of changing electric fields, and all such changing fields require separated and opposite electric charges. Electric charge is a fundamental property of matter, so some form of matter is required to provide the changing field that needs to exist at any point along the path of the electromagnetic wave. The passage of waves in a true vacuum would imply the existence of electric fields without associated electric charge, or of electric charge without associated matter, and this is not possible.
The concept of the ether eventually fell from favour and the path of its demise is an interesting story in the development of physics. The basic point in all of this is that if students have problems in accepting that light can travel through nothing (
but how can nothing carry oscillations?), they are in good company.