7.4 The Eye

Critical Questions:

  • What does an eye do, and how does it work?
  • How do we achieve depth perception?

The complexity of the eye is frequently pointed to by creationists as proof that evolution didn’t happen, because it cannot possibly have resulted in such improbably effective organs. The argument goes something like this: “Just look at this thing. I mean, come on! What the heck?”

A hawk's eye, not to be confused with either Hawkeye the Marvel Avenger or the character from M.A.S.H. (Photo credit: Steve Jurvetson)
A hawk’s eye, not to be confused with the character ‘Hawkeye’ from M.A.S.H.
(Photo credit: Steve Jurvetson)

In fact, there is a pretty decent record of the evolution of the eye, from simple light-sensitive cells to the bafflingly awesome balls of goo we’ve all got stuck into our skulls. But despite this evidence, I suppose anyone might be forgiven for being impressed.


7.3 Lenses, Mirrors, Cameras, and Telescopes

Critical Questions:

  • What does a lens do?
  • Why do things look bigger or smaller or otherwise distorted when seen through a lens?

Lenses have been around for a very long time. This isn’t too suprising – as soon as humans starting working with glass, it’s not hard to imagine someone looking through a curved piece and noticing that it made things look strange.

The most likely uses for lenses through most of history were to see small things more clearly and to start fires. The latter use is fairly easy to understand: lenses are very good at focussing beams of light. Since we know that light tends to increase temperature, it makes sense that a highly focussed beam of light could be strong enough to start a fire.

But why does focussing rays of light make objects appear magnified?


7.2 Light

Critical Questions:

  • What is light made of?
  • Why is the sky blue?

The main reason that light has always been so hard to figure out is that it follows its own rules.

For example, we’re used to things having mass. People, ferrets, churches, very small rocks – all are made up of matter and thus do things like resist acceleration (according to Newton’s Second Law) and create gravitational fields.

Light, on the other hand, has no mass. It’s made up of… nothing, it would seem. And as a result, it neither exerts a gravitational pull on massive objects nor ever slows down.1

We’re also used to dealing with waves. While solid objects run into each other and generally follow Newton’s Laws of motion, waves interfere with each other and generally don’t.

But when scientists developed the technology to look more closely at light, they found that sometimes it behaved like a wave – interfering with other light, for example – and sometimes it behaved like a particle, in that it seemed to come in countable numbers of separate objects.

Photons make their own rules, much like Steve McQueen's character in Bullitt.
Photons make their own rules, much like Steve McQueen’s character in Bullitt.

Once we started to make sense of all this, we had to assign light to a whole new category – neither wave nor particle but wave-particle, something which exhibits characteristics of both. And we eventually named the light wave-particle the photon.


  1. Don’t get clever and tell me that light slows down when travelling through something other than a vaccuum, by the way – it’s not really slowing down, as I’ll explain eventually.

7.1 Introduction to Light and Optics

If you want to really understand just how weird light is, you have to imagine the perspective of someone from a long, long time ago.

Why not choose, for example, a farmer in Southeast Asia a few thousand years ago. Put yourself into the farmer’s shoes – unless this is a barefoot farmer, in which case I’m speaking only metaphorically. You’re relaxing after a long day of work. Feel the rough, scratching fabric of the shirt on your back. Feel the soft evening breeze blowing through your hair as you watch the sun set over the countryside of what is now Vietnam.

As the daylight slowly fades, what do you see? Well, if you’re a particularly perceptive farmer, the answer is: quite a lot.