Units of Light

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Global Warming I: The Science and Modeling of Climate Change

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The University of Chicago

Global Warming I: The Science and Modeling of Climate Change

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This class describes the science of global warming and the forecast for humans’ impact on Earth’s climate. Intended for an audience without much scientific background but a healthy sense of curiosity, the class brings together insights and perspectives from physics, chemistry, biology, earth and atmospheric sciences, and even some economics—all based on a foundation of simple mathematics (algebra).

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Heat, Light, and Energy

A primer on how to use units to describe numbers when describing temperature, energy, and light. Even if you don't plan on doing calculations yourself, understanding how units work will help to follow the rest of the lectures in the class. If you are interested in practicing your analysis skills, using units to guide calculations, there are some exercises in the Part II of this class.

Units of Light3:16

Blackbody Radiation6:06

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David Archer
Professor
Geophysical Sciences

[MUSIC]

We're also going to talk a lot about light in this class.

And so you need to be familiar with the various ways of describing the different

kinds of light that are found.

So a light ray consists of waves that we'll talk more about in future lectures.

But one way to characterize the different kinds of light waves is by the wave

length, which is the distance between two adjacent peaks.

It's given the Greek letter lambda to describe the wavelength.

And the units of wavelength are the number of cycles per centimeter.

I have this upside down here.

No, centimeters per cycle.

That's centimeters per cycle.

And, a lot of time the units of cycle are left out, sort of implicit.

And, so, sometimes,

very often you will see wavelength described in units of centimeters.

Another way to describe light is in terms of the frequency of light.

So light is moving at some speed, c, the speed of light.

And so if you're just sitting here watching this light go past you,

you can count the peaks as they're coming.

And so the frequency of the light which is given the Greek letter nu

has units of cycles per second.

Or since they tend to drop the cycles,

you can see this written as just 1 over seconds or seconds to the -1.

Little confusing if you don't know what's going on, but it's not that hard.

So if we want to get some more factor label practice at converting units,

this is a good example.

Let's say we have a wavelength of light and we wanna calculate its frequency.

So the frequency has units of cycles per second.

So that's what we want.

And looking at the wavelength, it's, the wavelength

itself is centimeters per cycle, but looks like we want the cycles to be on top.

So I'm going to put the wavelength upside down, 1 over lambda.

And so that gives us units here of cycles per centimeter,

the inverse of Lambda there.

And then the speed of light, c, can be used to

cancel the centimeters and get us the cycles per second that we're looking for.

Another metric of light that turns out to be useful when you start

talking about infrared light is called the wave number.

And the wave number is simply the number of waves within 1 centimeter of distance.

So you count the number of peaks in there.

And the units of that are cycles per centimeter.

Or by neglecting, ignoring the cycles there, it's oftentimes written

as 1 over centimeters or centimeters raised to the power of -1.

And so the wave number is just given the regular letter n.

[MUSIC]

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