Hi. It’s Mr. Andersen. And this is AP Physics essentials video 118. It is on light absorption,
reflection and transmission. And in this picture I have a green laser that I am shining at
gummy bears that have different colors. And you can see that most of the light is being
transmitted through the gummy bears. We get some reflection off the surface and off this
background. But we also see a lot of absorption. That red gummy bear is absorbing that green
light. What is interesting is you could have a red laser and a green gummy bear and you
would see the same type of absorption. And so when light moves from one medium to another,
in the example I gave a second ago it was moving from air to the gummy bear itself,
one of three things can happen. First of all we can have reflection. That is when the light
bounces off the surface. We could also have absorption and that is going to be when the
light is actually converted to another form of energy. You know this occurs when that
light simply disappears as it enters another medium. Or it could be transmitted. In other
words it can be granted safe passage through the medium itself. Sometimes it will be refracted.
But we will talk about that in a later video. Now what is interesting is you can have different
wavelengths of light interacting in a different way with the medium itself. Some could be
reflected, absorbed or transmitted. And that is where we get the color of objects and what
colors they actually have. So let’s start with reflection. If we were to look at this
leaf what color of light is being reflected from the surface of the leaf? Well it is green.
Since it occurs as green we are getting a reflection of that green light. So reflection
remember is a bounce off of the surface. Now what is interesting is on that bounce, if
you draw normal, which is going to be a line that is perpendicular to the surface, as light
is reflected the angle it comes in at, we call that the angle of incidence, is always
going to equal the angle of reflection. In other words this angle right here is going
to equal that angle right there. What if it comes straight in, then there is an angle
of zero. And so it is going to bounce straight back. If we are looking at a mountain in the
reflection of a lake, that is going to be reflection that is causing it. And so you
know the mountain is not down in the lake and so how did that image get to your eye?
Well the light from the top of the mountain is reflecting at an angle off of the surface
of the water and it is going to your eye. And so it appears as though it is down in
the water even though it is not. Absorption is different. Absorption is when the light
hits the surface and it is converted into some other form of energy. Generally it is
going to be heat. And so if we were to look at the leaf, for example, what light is being
reflected? It is going to be the green light. And so what light is going to be absorbed?
It is going to be the red light and the blue light that is being absorbed. And we can even
look at an absorption spectrum of the leaf itself. There are two pigments in there, chlorophyll
A and B. And what they do is they absorb a lot of that bluish purplish light. They absorbed
a lot of the reddish yellow light. But they do not absorbed a lot in the green. And that
is because the green is reflected back to us. Now what is neat is a leaf can actually
make use of that energy to do photosynthesis. Now light can also be transmitted. That means
that it goes from one medium to another and it just goes right through that medium. And
so if we were to look on the underside of a leaf we are going to see a lot of green
light coming through. But also some of that blue and red light is going to make it through
as well. And so some plants have evolved a barrier on the bottom. And so you will see
some leaves, that on the bottom are reddish bluish or purple. And what they are doing
is they are having that layer so they are reflecting that back up through the leaf so
they can get more of the energy out of the sunlight itself. And so if we look at this
gummy bear experiment, so I am shining the laser at the gummy bears we can see all three
of these occurring. And so if I were to say where is the transmitted light. That is going
to be on the top. Where would we look for the absorbed light? It is going to be on that
surface. So right as it goes in some of it is absorbed by the gummy bear itself. And
then if we were to look at the reflected light, you can see the reflected light is going to
be right here on the front of that gummy bear. It is bouncing back where it came from. And
so if we continue the video and watch what happens you can see each of those gummy bears
are transmitting, absorbing a different amount. But when we get to the red we see a bunch
of reflection, a bunch of absorption, but not much transmission. And again that is dependent
upon the pigments and the chemicals that we have inside the gummy bears themselves. And
so did you learn to explain the behavior of light as it travels from one medium to another?
Again it can be reflected, it could be absorbed or transmitted. And I hope that was helpful.