an autodidact meets a dilettante…

‘Rise above yourself and grasp the world’ Archimedes – attribution

towards James Clerk Maxwell 5: a bit about light

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Canto: Our last piece in this Maxwell series dealt with the apparently irrelevant matter of Newton’s laws of motion…

Jacinta: But not irrelevant in that Newton was so seminal to the foundation of, and mathematisation of, modern physics, and he set the course…

Canto: Yes and we’ll have to go back to his work on gravity to really get a feel for the maths side of things I think.

Jacinta: No doubt, but in keeping with our disorganised approach to out topic I’m going to fast forward to give a partial account of Maxwell himself, before he did his major work. Maxwell was clearly indefatigably curious about the physical world even in childhood. He was conducting various chemical, electrical and magnetic experiments at home and later at the University of Edinburgh, from his early teens, and writing papers – the first at the age of fourteen – which were accepted by the Royal Society of Edinburgh, though he was considered too young to present them himself.

Canto: But we’re going to focus here on his focus on light, since we’ve been going on mostly about electricity and magnetism thus far. Light, and its wave properties, is something we’re going to have to get our heads around as we approach Maxwell’s work from various angles, and it’s horribly mathematical.

Jacinta: Yes, the properties of polarised light were among Maxwell’s earliest and most abiding areas of interest and research, so we need to understand what that’s all about.

Canto: Okay, here’s a simple definition of the term ‘polarisation’. It’s ‘a property applying to transverse waves that specifies the geometrical orientation of the oscillations’. That’s from Wikipedia.

Jacinta: That’s not simple. Do you understand that?

Canto: No, not yet. So waves are generally of two types, transverse and longitudinal. A moving wave oscillates. That’s the up-and-down movement you might see on a graph. In a transverse wave, the oscillations are at right angles to the movement of the wave. Light waves are transverse waves apparently, as opposed to sound waves, which are longitudinal – in which the wave oscillates, or vibrates, in the direction of propagation. That doesn’t make immediate sense to me, but for now we’ll focus on transverse light waves and polarisation. A light wave, we now know, is an electromagnetic wave, but don’t worry about that for now. Let me try to explain unpolarised light. The light from the sun is unpolarised, as is your bedroom light or that from a struck match. The light waves from these sources are vibrating in a multitude of directions – every direction, you might say. Polarised light is light that we can get to vibrate on a single plane, or in some other specific way..- .

Jacinta: So how do we polarise light is presumably the question. And why do we call it polarised?

Canto: I don’t know why it’s called polarised, but it’s light that’s controlled in a specific way, for example by filtration. The filter might be a horizontal grid or a vertical grid. Let me quote two sentences from one of many explaining sites, and we’ll drill down into them:

Natural sunlight and almost every other form of artificial illumination transmits light waves whose electric field vectors vibrate in all perpendicular planes with respect to the direction of propagation. When the electric field vectors are restricted to a single plane by filtration, then the light is said to be polarized with respect to the direction of propagation and all waves vibrate in the same plane.

So electric field vectors (and we know that vectors have something to do with directionality, I think) are directions of a field, maybe. And a ‘field’ here is an area of electric charge – the area in which that charge has an influence, say on other charges. It was Michael Faraday who apparently came up with this idea of an electric field, which weakens in proportion to distance, in the same manner as gravity. A field is not actually a force, but more a region of potential force.

Jacinta: It seems we might have to start at the beginning with light, which is a huge fundamental force or energy, which has been speculated on and researched for millennia. I’ve just been exploring the tip of that particular iceberg, and it makes me think about how particular forces or phenomena, which are kind of universal with regard to humans on our modern earth, are taken for granted until they aren’t. Think for example of gravity, which wasn’t even a thing before Newton came along, it was just ‘natural’ that things fell down to earth. And think of air, which many people still think of as ’empty’. Light is another of those phenomena, but it’s been explored for longer than the others because it’s much more variable and multi-faceted, at least at first glance haha.. Darkness, half-light, firelight, shadow effects, the behaviour of light in water, rainbows and other tricks of light would’ve challenged the curious from the beginning, so it’s not surprising that theories of light and optics go back such a long way.

Canto: Yes and the horror of it – for some – is that mathematics is key to understanding so much of it – especially trigonometry. But returning to those electric field vectors – and maybe we’ll go back to the beginning with light in the future, – in a light wave, the oscillations are the electric and magnetic fields, pointing in all directions perpendicular to the wave’s propagation.

Jacinta: Yes, I get that, and polarised light limits all those perpendicular directions, or perpendicular planes, to one, by filtration, or maybe some other means.

Canto: Right, but notice I spoke of electric and magnetic fields, which is why light is described as an electromagnetic wave. It should also be pointed out that we tend to call them light waves only in the part of the spectrum visible to humans, but physics deals with all electromagnetic waves. Our eyes, and it’s different for many other species, detect light from a very small part of the entire wave, or electromagnetic, spectrum. Wavelengths of less than about 380 nanometres (even less when we’re young) at the ‘ultraviolet’ end, and of more than about 750 nm at the long ‘infrared’ end, form the visible spectrum for humans. Beyond UV light we have x-rays and then gamma rays, and beyond the infrared we have microwaves and then radio waves.

Jacinta: I wonder if Maxwell knew about all this in his day.

Canto: We’ll no doubt find out…

References

https://en.wikipedia.org/wiki/Transverse_wave

https://www.physicsclassroom.com/class/light/Lesson-1/Polarization

https://en.wikipedia.org/wiki/History_of_optics

https://www.britannica.com/biography/James-Clerk-Maxwell

Written by stewart henderson

June 29, 2019 at 10:55 am

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