So, for my first blog post, I thought I’d take a slightly
different approach to introducing one of the core ideas of quantum physics by
considering a fairly simple problem: why does everything go dark when you close
your eyes?
It seems like a dumb statement, at first. I mean, when you
close your eyes, it goes dark because there’s no light, right? And to perceive something
(as opposed to the darkness we experience), you need the energy given to our
eyes by light. It all seems straight forward enough.
That is, until you consider that visible light is just one
part of a larger spectrum of electromagnetic waves by which we’re constantly
surrounded, the closest source being your own body, which radiates heat as
infra-red rays. The visible light section of the spectrum only makes up a small
section, even when your eye is closed it’s still being struck by electromagnetic
waves!
Before exploring what’s going wrong to mean that everything
goes dark, we first need to consider what wave theory predicts should happen
when you leave waves to impact on something. Picture a lamp shining on your desk:
the longer you leave it, the more waves travel from the light of the lamp to
the desk and the warmer the patch on your desk feels – the energy from the waves
compound! So why is there such a noticeable difference between when you have
your eyes open as opposed to closed, if the energy levels involved barely
change?
The difference is down to how our eyes interpret
light into the image our brain receives. Light strikes the eye’s retina, where
its energy causes the isomerisation of the chemical retinol. This transfers the energy into a chemical form, which travels to the brain, where it is interpreted. The secret lies in the specifics of isomerisation,
which is the process by which the energy from the light changes the structure of
the chemical without actually changing the chemical formula. To do this, the
chemical’s electrons need to be given a very specific amount of energy in one
go via the process of photoexcitation. The energy can’t compound and cause a change in the same way that it can
slowly heat up the surface of the desk from the previous example.
The solution to the problem can only be
solved by taking a different approach to how we think about light: rather than
consider it as waves, we have to consider it as made up of tiny, massless particles
by the name of photons. A photon’s energy is defined as Planck’s Constant
(usually represented as “h” and has a value of approximately 6.63x10^-34) multiplied
by its frequency, F. By considering light as photons, it becomes clear how the
specific energy is supplied: the photons colliding with the electrons in the
chemical have to have enough energy individually, as it’s impossible to collide
an electron with two photons at the same time. As infrared waves have a lower
frequency, their photons have less energy than that of visible light, and thus fail to cause an excitation of the electrons in the retinol, meaning there's no signal to interpret by the brain. In other words, everything goes dark!
tldr: As useful as the wave theory of light is, it fails to explain many phenomena, particularly where the excitation of electrons is involved. In such situations, the problem can be explained by considering light as photons with energy "hf".
-So, that's it for my first blog post! I hope you've found the ideas as interesting as I do, and hopefully I'll be able to explore them in more depth in posts to come. If you have any questions, feel free to comment below, and I'll do my best to answer them to the best of my knowledge. The original problem can be found here: http://ajp.dickinson.edu/Readers/Purcell/February1984-Problem1.pdf
Finally, we're still finalising a name for the blog. If you have a suggestion, again, leave a comment.
Finally, we're still finalising a name for the blog. If you have a suggestion, again, leave a comment.
Thanks for reading,
GM ^^
