I'm going to be explaining what diffraction gratings are and how they work and I'm going
to be performing an at-home lab experiment where I melt chocolate over a diffraction
grating and get the chocolate to reflect white light as a rainbow. First, let's talk about the
difference between constructive and destructive interference of waves. So here I have two waves,
and let's imagine them traveling together in the same direction and at the same speed.
The wave that we would actually be able to observe would be the sum of these two waves,
so I've given these y axes where the peaks are at an amplitude of positive one,
crossing the line is at an amplitude of zero, and the valley is at negative one.
Let's add these two waves together. So, at the very beginning we have zero plus zero equals zero,
and at this peak we have one plus one equals two. Here, we're crossing the line again so zero plus
zero equals zero. Now the valley: negative one plus negative one equals negative two.
And at the end crossing the line again, zero plus zero equals zero. The resultant wave that
we would get would be same shape but with a larger amplitude. Now, one thing that travels in waves
would be electromagnetic radiation and today we are specifically going to be talking about the
visible region of the electromagnetic spectrum. That is what most people call a rainbow. But what
you just observed here was called constructive interference. We have the peaks of the waves
aligning the valleys of the waves aligning in our resultant sum wave was just a larger amplitude.
Our eyes perceive this larger amplitude to be brighter light. Now let's talk about the
destructive interference of waves. So at the top I have two original waves and just like last
time they are traveling together same speed same direction. The wave that I'm actually going to be
able to observe is the sum of these two waves, but this time these two waves are out of phase. What I
mean by that is when one has a peak the other has a valley. Let's find the sum of these two waves
at main points along these lines. So at the very beginning, we have zero plus zero,
so our resultant wave would have an amplitude of zero. This peak has an amplitude of positive one.
It's aligned with a valley with an amplitude of negative one. So one plus negative one: our
resultant wave has an amplitude of zero. Crossing the line once again zero plus zero equals zero.
At this valley of negative one we are aligned with a peak amplitude positive one. Negative
one plus positive one once again an amplitude of zero, and at the end zero plus zero equals zero.
What you are seeing here are two waves that are completely destroying each other and canceling
each other out so in our resultant wave at the bottom, you are literally seeing an amplitude of
zero. If this was a visible light wave we would perceive an amplitude of zero as the absence of
light. So this is destructive interference because these are out of phase and destroying each other.
If you look at any of these waves you can see that they have a repeating pattern to them,
but if you find the smallest unit that repeats and measure the length along the axis line that
length is called the wavelength and wavelength is an important way to characterize these waves.
So if we are talking about the visible region of the electromagnetic spectrum, we perceive
different wavelengths to be different colors. So at a wavelength of about 750 nanometers,
we would perceive that as red. We can see all the way down to a wavelength of about 400 nanometers,
which is violet, and everything in between would be various colors of the rainbow.
So a diffraction grating is a plate made out of a reflective material and on one side of this plate
there is an imprint of lots and lots of little grooves in a grid pattern. Because of the fact
that it's made out of a reflective material, that means that each and every one of those little
grooves acts as its own mirror. So if you were to turn this on its edge and you zoom in really,
really far to this narrow edge, you would see something like this. Each of these surfaces
can act as a mirror. Now think about what would happen if you had two light waves coming in,
currently constructively interfering, but they hit neighboring mirrors. When they reflect off
the neighboring mirrors they end up traveling a slightly different distance than each other.
If that difference in distance is a whole number of wavelengths, these are still
going to be constructively interfering and this color can continue on and be reflected.
If, however, we change the wavelength, you could end up in a situation where,
after it reflects off and they traveled their slightly different distances, they could now be
destructively interfering and you would not be able to see that color anymore. This isn't just
dependent on the wavelength: It's actually also going to be dependent on the angle of incidence,
meaning the angle at which the original light is hitting the diffraction grating. So at some
other angle, you are going to have a different wavelength of light constructively interfering.
So what happens when I shine white light on this diffraction grating is I see a very complex
rainbow pattern. I am only able to see the colors that are currently constructively interfering
at that particular location, so it depends on the wavelength, but it also depends on the angle of
incidence of the original light hitting this diffraction grating. So what I'm going to do
in my at-home lab experiment is: I'm going to melt chocolate and pour it on this diffraction grating,
using the diffraction grating like a stamp. So when the chocolate freezes, I'm hoping that that
chocolate is going to be reflective enough that it will also disperse white light into a rainbow
just like the diffraction grating. Now for the lab experiment. I'm starting by boiling water.
I'm simplifying this procedure compared to a lot of other videos. So, you don't want to caramelize
the sugar in the chocolate and you don't want to burn the chocolate, but with my method you
don't need a candy thermometer and I've still decreased the risk of burning the chocolate: So
the boiling water is not going to get hotter than the boiling point of water. Any water molecules
that change into the gas phase or escaping as water vapor. So I'm breaking up chocolate and
putting it into a metal measuring cup. Make sure you put it into something that can transfer the
heat and not be damaged when it's down in the boiling water. This is just what I had on hand.
I don't want to get the chocolate wet so I'm just putting the cup down in the water
to let the heat from the water transfer to the chocolate. A chopstick is just a makeshift clamp
to hold it in place. I turn the stove off here because once my water boils, that's hot enough.
And once the chocolate melts, it's ready. The chocolate does not need to get as hot
as the boiling water. Just take it out as soon as it melts. Only one side of my diffraction
grating has the grooves, so I have to look pretty closely to make sure I'm using the correct side.
But I'm pouring the melted chocolate onto the diffraction grating and waiting for it to freeze.
This is the moment of truth. Look at this! The chocolate is now a diffraction grating!
Sunlight is dispersing into rainbows all over the surface of the chocolate, so success!
Thanks for watching Chemistry in a Nutshell. Here's another video you'll like.
Tastes like chocolate.