Dr. Bozidar Mitrovic: Here is the naked eye
observer, and this is the light coming from
a
distant star.
We collect with our naked eye only small amount
of
the total energy emitted by the star because
all of star's energy
is emitted.
It's broadcast uniformly throughout the space.
At a large distance from it, you will see
only a tiny amount of
that energy.
The question is, "How can we perhaps gather
more star
light into our eye?"
There are basically two ways to do that.
We can use refraction of
light, bending as it passes from one medium
to another, or we can
use the reflection of light by mirrors.
Here we have on one side air, and on the other
side we have glass,
denser medium, and there is a ray of light
falling from less dense
medium onto this boundary, between less dense
and more dense
medium.
It turns out that what this ray of light would
do in
passing from less dense to a more dense medium,
it will bend
towards the vertical.
This angle here is smaller than the incident
angle here.
Why does it do that?
It does that because the speed of propagation
through a medium is
less than the speed of propagation through
air, to a less dense
medium.
You can understand this through an analogy.
Suppose that you work
as a lifeguard and here is the shore, and
here is the lifeguard 
and
there is a person in water, who is in trouble,
who is screaming,
"help."
You want to reach the person who is drowning
or is in
trouble in the shortest possible time, that's
your goal.
That should be your strategy to get to this
person in the shortest
possible time.
You also know, most people run faster than
they can
swim.
Then, the wrong strategy would not be to go
in a straight
line.
You can actually minimize the time by running
a little bit
longer on the shore, because you can run faster,
so that you reduce
the distance that you have to swim.
In order to minimize the time, you run a little
bit longer on the
shore, and then you swim the rest of the way
to the person.
Why?
You can run faster on the ground than you
can swim.
That's exactly
what the light does.
You can actually formulate theory of light
propagation where you say that the light propagates,
so that it
goes from a point A to point B in the shortest
possible time.
It's so called Fermat principle that governs
the way the light
behaves.
It's the same basic strategy as the one that
would be used
by the lifeguard who is trying to help a person.
He would not go in
a straight line because that line does not
minimize time.
That's why the light refracts in going from
less dense to a more
dense medium.
You can use refraction to actually channel
the light
collected, to more light into your eye.
Let's look into the detail,
how we can use refraction to channel more
light.
Here are the light rays coming from the star,
it is at such great
distance from us that we can assume that near
the object of any
earthly size, they're essentially parallel
to each other.
Then, you
shape a more dense medium such that all these
rays would, through
refraction, be all collected in the same point,
so called focal
point.
What the lens does?
It's using the refraction, the bending of
light, it can actually channel all the light
rays into the same
point that is called focal point.
That way, by using the lens, I
could collect all this light into a single
point.
If I didn't have lens there, then all the
light that would reach
this point if my eye is here would be basically
just this ray here.
You can see how using lenses and refraction,
you can gather more
light into your eye that you place in the
focal point.
The light rays arriving at the lens are all
parallel to each other.
Then each one of them is bent just by the
right amount, and all the
rays are collected in this single point that
is called focus or
focal point.
The distance between the lens and the focal
point,
this distance here, is called focal length.
The focal length of a
lens is the distance between the lens and
its focal point.
