So, how do we introduce color into our
Schlieren images? By using color filters!
We have a strip of red filter and a strip of green filter
butted together, sitting in front of the camera lens. Our
white light source, point light source, which bathes the mirror
with white light, that light is focused right at the junction of the red and green filter. If
there is a refraction that's happening down there where the mirror is that tends to
refract the light
upwards, then it passes through the red filter. If it refracts it down, then it passes through the green filter, and that's the color
that the camera sees when that refraction occurs.
So, let's see what information the color gives us.
Remember, it is the change in density in the air that
refracts the light.
So if there is, for example, a hot soldering iron
the density of the air around the soldering iron is
less than the surrounding air, because this is hot air.
So if I hold this in front of the mirror
you see that
when the density of the air changes from a high density in this cooler air down here to the hot air, then the
light is refracted and bent through the green filter, and
it appears green.
Whereas at the top, where we're going from a low density to a high density,
the opposite happens. Then the light is refracted in the other direction through the red filter.
So this is true also for cold objects. My
assistant Allen is going to bring an ice cube tray out of the refrigerator. So that is cold, and
the cold air around it is going to be more dense than the surrounding air.
So you see, when we go from a less dense to a more dense
region, then the
light is refracted through the red filter, whereas when we go from a more dense to a less dense region
its refracted into the green filter.
So it's exactly the opposite of this cold object and the red object, er, hot object.
So now that we have an understanding of what the colors mean we can
take another closer look at the standing wave that we've seen before in white light.
To do that we're going to zoom in a little closer and
see what colors appear, if any.
So right now I'm turning up the intensity of the light, and
lo and behold! We see
alternating bands of red and
green.
So let's remember, the red bands show a change in density of the air
from less dense to more dense, and the green in the opposite. So where those colors appear,
that's where the largest changes in density--or pressure, if you like--appear. Those are the pressure
antinodes.
For our final demonstration, let's use our standing wave to levitate some small objects, in this case
Styrofoam balls. What I'm interested in is to see where those balls want to settle down. So
I'll turn up the power...
I'm going to take a little Styrofoam ball and place it in the space
and you notice the ball settles down in a red band. If I take another one
that one too is in a red band.
Now I've got two of them in one area.
One of them is in a green band!
So you see the balls settle down
where there are bands of color.
Those bands of color are
rapid changes of pressure--or density--and those are the pressure
antinodes.
That is where the balls settle down.