Light and temperature

Understanding some things about light and temperature are useful for understanding the greenhouse effect and the role of greenhouse gases (GHGs), such as carbon-dioxide, $CO_2$.

In this topic, we'll be relying on a couple of concepts that we'll look at in more detail later on. But you've probably heard of some of these:

• Energy comes in many forms--such as thermal energy, light energy, electric energy, gravitational energy, chemical energy...
• Energy conservation: Energy can be changed from one form into another. But energy is never lost: The total amount of energy always stays the same.
• Matter consists of atoms.

The picture shows a brick, just taken out of a kiln, a high temperature oven for firing ceramics. The brick is so hot that it's glowing white-hot.

If we leave the brick out of the oven, how will it change over the next minutes and hours?

People (and animals) take shelter in the shade from the light of the sun when it's hot outside, because...why?

We say that...

• Hot objects give off light (energy) and cool down (lose thermal energy),
• Cold objects can absorb light (energy) and heat up (gain thermal energy).

If you're out in the sun, you get hot the fastest when you're wearing what color?

This process is called radiative cooling (and radiative heating), because light is a kind of radiation--electromagnetic radiation.

It does not depend on two objects being in physical contact:

The sun (light from the sun) heats up Earth. But space is pretty empty. By the time you get 10-20 km above Earth, there is no more air. Space is a much better "vacuum" than anything we can make on Earth.

Temperature and color

Based on your experience with hot objects: Using colors, rank the temperatures in this picture from highest to lowest temperature.

Here is a diagram of the Electromagnetic spectrum by frequency and by wavelength:

Where is "white" on this spectrum?

Light coming out of the sun (or any hot object) is more complicated because it's not just one wavelength, but a mixture of waves of different wavelengths. Still, can you tell...

As temperature increases is the frequency or wavelength of the light increasing?

Wait!...Did you say wavelength?

Yes! Light consists of electro-magnetic waves. Waves have things like wavelength, frequency and they move with a certain speed.

Talking about waves

A hand shaking a piece of string, (or a slinky) can send a bunch of waves traveling down the string / slinky.

Wavelength, $\lambda$ = distance between crests in [m] (or [m / (wave)])

Wavespeed, $c$ - units [m/s]

Amplitude = maximum distance away from 'undisturbed' (yellow line) - units [m]

Frequency, $f$ = the number of waves passing a point per second: [(waves)/sec] or [(cycles)/sec] = [Herz] =[Hz]

Or, how 'fast' your hand shakes as it sends out waves on the slinky.

When you shake faster the wave peaks get closer

Shaking *really fast* results in waves with (a) a low frequency, or (b) a high frequency?

• Online wave simulator (PHET - University of Colorado)

Shaking *really fast* results in waves with (a) a small wavelength, or (b) a long wavelength?

It turns out that speed, $c$, wavelength, $\lambda$, and frequency $f$, are related by: $$\lambda = \frac{c}{f}$$ More commonly written as $c=\lambda f$.

Some common wave speeds:
- Sound ~ 760 mph= 340 m / s - Light ~ $3*10^8$ m/s = 300,000,000 m / s

It turns out that most of the day, electromagnetic waves with a frequency of 91.1 MegaHertz = 91,100,000 cycles / second are hitting your body! What is the wavelength of these waves? And where are they coming from?

Our sun has a surface temperature of ~5400 C = ~5700 K = ~9800 F, and gives off a lot of light in the "visible" part of the spectrum.

We mammals are warm too. Typically warmer than "room temperature". Do we glow? If so, what part of the electromagnetic spectrum would it be, given our temperature of about 98.6 F = 37 C?

"Black body" radiation

Hot objects give off not a single frequency, but E-M waves of many different frequencies. (See the white light at the right...)

It turns out the distribution of frequencies from a hot object has a universal shape. This is called "blackbody radiation". Some common features of this radiation:

• As the temperature increases the peak frequency increases (frequency with the most waves emitted).
• As the temperature increases, the overall amount of energy radiated increases (the "brightness").

An Infra-Red (IR) camera

Special sensors can sense waves in the IR (infra-red) region of the spectrum, and then transpose it up into the visible range for us to see. This is the basis of an IR camera.

By figuring out the peak freqency (most common frequency) of an image, assuming the black body distribution, the camera can also estimate the temperature of an object. (The camera displays the temperature of whatever is in the circle on the screen.)

Interesting: The incandescent bulb is much hotter than the fluorescent light in my ceiling... But much cooler than 1000 F? What happens when we turn off the light bulb?

When light strikes "stuff"

Three kinds of things can happen when light strikes something, depending on frequency and the material:

• Transmission: the light can pass right through (windows).
• Reflection (scattering): the light "bounces" off (white surface), or
• Absorption (friction): the light comes to a stop, and its energy *heats* the object up.

Some materials differ in their transmission (transparency) and absorption (opaqueness) and reflection (color) depending on the frequency of light.

It appears that air is pretty transparent for both visible light and IR light. But...

How do the garbage bag and the glass coffee carafe differ?

Fingers are in*visible* behind a garbage bag. But in the IR image they can be plainly seen and counted.

Fingers are visible inside the glass carafe. But in the IR image they cannot be seen through the glass.

the Greenhouse Effect

We can "see" the sun very easily. Gases in the atmosphere like Nitrogen, oxygen, Carbon dioxide, water vapor (though not condensed water vapor = clouds), methane, hydrogen, etc are transparent to visible light.

But some gases in the atmosphere are less transparent to infrared light (like the glass coffee carafe). These are the "greenhouse gases":

• Carbon dioxide,
• water vapor,
• methane,
• $NO_x$: This means $NO$ and $NO_2$.
• CFC's (ChloroFluoroCarbons) - These are refrigerants from refrigerators and air-conditioners in cars and buildings.

The Greenhouse Effect

1. Visible light from the sun is transmitted through the atmosphere (including the GHGs):
2. The visible light strikes Earth's surface where it is absorbed by land/roads/trees: These objects heat up.
3. Hot objects give off infrared (IR) radiation.
4. But GHGs absorb IR radiation; They are not as transparent to IR light.

So some energy from the sun remains trapped in the atmosphere, which heats up the atmosphere.

Substitute "car windows" for "GHGs" in the description above, and you have an explanation for why the inside of your car on a sunny day is hotter than the surroundings.

Thanks to the natural greenhouse effect the surface of earth has an average temperature of about 14 C (~60 F).
Without this, (Oxygen and nitrogen but no GHGs) it is estimated the average temperature would drop to -18 C (~ 0 F).

Test your understanding

What would happen to the temperature inside a sealed glass jar with nothing inside it if you put it out in the sun?

1. It would heat up.
2. It would cool down.
3. There would be little or no change.

IR applications

IR cameras are being used to quickly "take people's temperatures" at airports, to see if they have a fever (maybe from COVID-19...).

Oviyandi Emnur, via Science Magazine

• The best way to sense a fever is 0.5 meters away, looking at an ear.
• But someone might be infected with coronavirus for 2-14 days before showing symptoms including fever.

You can also use this technology to look for heat leaks: Here are two sets of windows on a 6th St house on the same cold night:

Older window	Replacement, highly insulated window

Some snakes have "pit organs" that can sense IR radiation. Why would that be useful to a snake?

Finally, IR imaging can be used to detect leaking methane: In the visible range of light, methane is an invisible gas. But in the IR it stands out (perhaps because it's a different temperature from the surrounding air?)

Texas Methane Super-emitters (NYTimes)

Image credits

Holding hands coffee mug