Hi. It’s Mr. Andersen and this is AP Physics essentials video 119. It is on specular or
mirror like reflection. And so this lake, since it is perfectly still, is acting like
a mirror. It is reflecting the forest behind it. And so if we look at that in more detail,
if we shine a laser light on a mirror, it is going to come in at an angle. It is going
to bounce off of that surface and it is going to be reflected. We call that specular reflection
because it is all going in the same direction. And there is a rule that the angle of incidence,
in other words if we draw a line right down here that represents zero, we call that the
normal, then the angle between here and here or angle of incidence equals the angle of
reflection. And as we change the angle of incidence we are going to get a changing angle
of reflection. And so why does the forest look like it is underneath the water? Well
let’s use this arrow as an example. And so if we put the normal in like this, when
you see the bottom of the arrow in the water you can see that the angle of incidence, the
angle of the light coming from the arrow bouncing off the water is going to be equal to the
angle at which it bounces back to your eye. And as we move the normal across, as we move
to an area higher and higher on the arrow itself, then we are going to get an angle
of incidence that is greater and so is the angle of reflection. And so what you create
is a virtual image. It is inverted. It is going to appear as though it is underneath
the water, but it is simply specular reflection of the light itself. And so as light moves
from one medium to another one of three things can happen. It can be reflected or bounced.
It can be absorbed or it can be transmitted. And when we are talking about specular reflection
we are just talking about the light that is actually reflected. And so the key point is
that if you ever have a mirror and we have light coming in, we can represent that as
a light ray. And that angle of incidence, in other words the angle between the normal
and the light ray is going to equal the angle of reflection. Now this is specular reflection.
In other words it is perfect mirror like reflection. And you can imagine that is not always going
to happen. If we look at the actual surface it is bouncing off, if it is not perfectly
straight then we are going to get what is called diffused reflection. And so we are
going to have that light, not just bounce off in one ray but it is going to bounce off
in a lot of different directions. And so if you look at how light bounces off of a mirror
that is specular reflection. But if you had some other surface, maybe like a piece of
paper, it does not bounce back exactly the same way it came in. And so you are going
to get this diffuse light on the surface. And so if we model this with a PHET simulation
we now have a laser light bouncing off the surface. You can see as I angle the laser
light, as I move it from a really low angle to a really high angle we are going to have
a corresponding change in the angle of reflection. And if we actually put a protractor on and
start to measure that you can see that the measurement on the left side, between zero
and that angle, is going to match exactly the angle on the right side. In other words
the angle of incidence, this angle right here, is going to equal the angle of the reflection.
If we look at this reflection of this mountain here, so the light on the top is going to
have specular reflection off of the surface and it is going to bounce to our eye. And
so we are going to get a perfect mirror like balance. And that is why the image in the
water is going to be exactly the same as the image that you see just with our naked eye.
But let’s say that we change the surface of the water. Let’s add some irregularities
to that. How would we do that? We could add some waves to it. So we add some waves to
the water itself. Then as the light ray comes down it does not bounce exactly back to our
eye, but we are going to get diffused reflection. And so what would that look like? Just look
into the water for a second. If we add some waves to it, it is going to give us an image
that is not mirror like, or it is not perfectly reflected back to our eye. And that is just
because we have irregularities in the surface itself. What if it was snow? Then we are going
to get total diffused reflection. We do not see any image in the snow itself. And so did
you learn to make predictions about the object and the image position due to reflection?
Remember the angle of incidence always equals the angle of reflection.
And I hope that was helpful.