Now that we have a terminology for discussing waves and periodic phenomena
and have discussed sound and how sound is a wave
and the properties of a sound. We're ready to move on to light
and we are going to start with some of the historically interesting things that happened
with discussing whether light is a wave or a particle.
So that’s where we'll start. As an overview with light,
we'll talk about the wave phenomena first
and then move on to the properties of radiation,
electromagnetic radiation,
and then talk about some of the light spectrum properties.
First, with some wave phenomena,
we'll start with interference and the idea that light waves
can interfere with each other in the same way
that sound waves interfere with themselves.
We go back to the historical question first,
which is this light, this thing that we're seeing all the time
and which we know and have known for many centuries
is very important to our perception.
What is it? Is it a wave? Is it a particle? How does it behave?
What can we do with it? Can we manipulate it or capture it?
And this is a picture here of Thomas Young, who performed
one of the very famous experiment to try to solve this hotly debated question
as to whether light behaves like a wave or behaves like a particle.
Here’s what he did, he took two screens like you see here.
One on the left, one on the right and in one of them he put two holes.
He cut two holes that were very small.
So we often call this the double-slit experiment
because he cut two very small holes in this first screen.
And then he asked what would happen if I sent light through these two holes
and look at what happen when it impacted the other screen,
the screen that’s on the far right here.
He wanted to deduce whether light was a wave or a particle
by sending the light through these two slits.
Here’s one that would help,
suppose first of all that light were a particle, what would happen?
If we sent a bunch of particles through these two slits
what we would expect
is that we'll have two streams of particles as you see here,
impacting the back screen
and we should see two bright spots on the back screen
exactly where those two holes were that we created in the first screen.
What happens if light were a wave?
We know from wave phenomena
that if a wave impact on an opening in some barrier,
maybe ocean waves hitting some opening, they would do it called diffraction.
We'll talk more about diffraction and where it comes from in a little bit
but for now all we need to know is that
when waves impact on an opening like this,
they bend outwards in sort of a spherical shape.
What would then happen is if both of these waves are impacting this boundary
and bending outwards in this spherical shapes,
the parts of the waves that are going to be constructively interfering
both that are peaks, will constructively interfere on the back screen
creating a bright spot as the light constructively interferes.
On the other hand, if instead of... it constructively interfering
with peaks of the light wave meeting peaks of the light wave.
We can also have the peaks meeting with the troughs
and as we saw one we talked about wave addition.
When these peaks and troughs meet with each other,
they destructively interfere because you add a positive with a negative.
In those cases, the light when it's destructively interfered
would create these dark spots on the screen behind it.
And again, this is in contrast with the constructive interference
which comes when peaks meet peaks or troughs meet troughs.
And so we have these two different phenomena.
So Thomas Young did exactly this, he sent light through these two slits,
these two openings, to see what happens
and found that light in fact behaved like a wave.
Instead of getting two bright bands,
he instead observed these many, many bands
which as we've just discussed comes from constructive
and destructive interference of the light waves.
We can see on the screen on the back,
both the bright bands and the dark bands.
And it is very important to conceptionally understand
that the bright ones correspond with constructive interference
and the dark ones with destructive interference.
So as we know, constructive causes the bright bands,
destructive causes the dark bands.
So now that we know
that light certainly has at least one wave phenomenon to it.
We can ask and discover some things
about some other properties that we've seen many times in nature.
So for example,
if light is going down and interferes with or impact on some medium,
maybe it goes from air to some soapy layer that’s on top of a layer of water.
What will happen is some of that light,
as we'll see when we get to optics very soon,
we'll go into the medium at a slightly bent angle
and then come back out of the medium.
Whereas some of the other light will instead just reflect immediately
rather than going in and then back out.
What happens in this case is that we say these two rays of light.
One that went into the medium and one that just bounced off the medium
will have a different phase. We say that the phase is altered.
Remember our definition for the phase,
the phase is how much the light is shifted
or where it is in its path as the wave goes up and down.
So for example now, the wave that bounced off the surface might be at a peak
while the wave that entered into the medium and came back out
might have evolved in such a way that it is now at a trough
or it could have been the exact opposite where the bouncing wave was at peak
and the one that went in and back out was also at a peak.
And so we could have either of these two different types of phenomena.
When this happens, these waves interfere with each other.
Sometimes creating peaks and sometimes troughs
depending on where the different phases were that we just discussed.
And we see this sort of diffraction or bending phenomena
as light goes into and out of these thin surfaces.
And so we call this the thin film effect
because as we just saw in our example
it happens when you have a thin film of something.
Maybe a thin film of oil over water or a thin film of soap over water.
And we see this sort of shiny bands that you can see here
coming from bubbles or soap or like we said oil.