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?

[source]
[source]
The first thing to know is that light always travels in straight lines, unless it comes into contact with solid objects. From the previous sections, we know that the reason we can see things is because they emit light towards our eyes.1 But this emission of light goes out from every point on an object, and in all directions. There’s nothing special about the spot where our eyes are located – they just pick up some of this electromagnetic signal that’s spreading out from the source.

So, for example, consider this dog:

cute little guy.
cute little guy.

If we choose just one point on this dog, and a few light rays coming from that point, we have a picture that looks a bit like this:

aw, his ear's glowing.
aw, his ear’s glowing.

Now let’s watch what happens when some of that light reaches someone’s eye:

aw, the light rays from his ear are being detected by an eyeball.
aw, the light rays from his ear are being detected by an eyeball.

(Remember, I’m only drawing some of the light coming from one point on the dog. If I were to try to draw this same diagram for every ray coming from every point, it would be entirely too messy to understand.)

As you can see, it’s not just one ray that reaches the eye from this point, but a collection of rays. And they have a very distinct shape.

It turns out that our eyes are able to detect how far away an object is by estimating where the light came from – that is, where those diverging rays of light all originated. (There’ll be more information on this in the very next section.) We do this for every point on every object we look at, and when we put them together, we get a three-dimensional view of the world around us.

I’ve already said that a lens can be used to focus light. So what would that do to the light coming from this dog?

aw, the light reflecting off his ear is being focussed through a converging lens!
aw, the light reflecting off his ear is being focussed through a converging lens!

The light comes from the dog’s ear, being emitted in all directions. Some of it passes through the lens and gets focussed at another spot, then keeps going.

Now here’s the important question: if those light rays kept going to the right and eventually entered an eye, how would this look any different than if the dog’s ear were located at that second point, where the light beams cross?

The answer is that there would be no difference. All this lens does is to take some of the light leaving the dog and cause it to meet up again somewhere else. That new point acts just like the original light source, giving off light rays that spread out in all directions.

So if you looked through that lens, and your eye was at the correct distance, the dog would appear much closer to you than before.

Using this kind of logic, you can set up different lenses of different shapes and sizes to do all kinds of things to light. While the lens above caused light to focus, other shapes of lenses can make light beams spread farther apart. You can make things appear closer or farther away, bigger or smaller, even upside-down. Telescopes use combinations of lenses that either focus light or spread it farther apart in order to make distant objects seem much larger and closer.

Cameras use lenses to focus light at one particular spot: on the film or digital detector. Depending on the distance of the object being photographed, the lenses inside have to be closer or farther apart from each other in order to focus the light at the correct location. When you adjust a camera’s focus, you’re changing the distances between the lenses inside.

Eyeglasses do something a little bit more complicated: they make the light from either distant or very close objects focus at a spot within the visible range of the wearer – in other words, they basically move the light source from a place where the person can’t focus to a place where they can.

In the next section, we’ll learn how we animals created lenses out of living tissue in order to see.

Big Ideas:

  • Lenses cause light beams to focus at one spot or spread farther apart.
  • Since our eyes can detect where groups of light beams originated, lenses “move” light sources by making them seem to have originated from a different location.
  • Optical devices such as cameras and telescopes use combinations of lenses to achieve various results.

Next: 7.4 – The Eye

Previous: 7.2 – Light

  1. To be clear, this “emission” can be a reflection, a scattering, or a glow, as in a light bulb.