Electromagnetic Radiation (energy) from the Sun

Chapter 9. Waves, Light, and Climate Change

The chapter’s objectives and major topics:

• waves

• interference of waves

• the electromagnetic wave theory of light

• the reality of fields

• the electromagnetic spectrum

• global ozone depletion

• global warming

Introductory remarks

In Chapter 9, we consider waves in general and electromagnetic waves in particular, emphasizing the field concept. We will then look at the electromagnetic spectrum, and we’ll end this chapter by considering two important social applications of these ideas: global ozone depletion and global warming.

Remember, there are 4 story lines in the text:

1. the scientific process 2. modern physics and its significance 3. energy 4. the social context of physics

All four of the story lines appear in Chapter 9!

The scientific process appears in connection with the concept of fields and the development of the electromagnetic wave theory of light.

Philosophically, fields represent a significant deviation from the Newtonian worldview and toward modern physics.

Energy is the basis from most of the physical analysis, especially in the case of global warming.

Ozone depletion and global warming are prime examples of the relation between physics and society.

Importance of waves “. . . the wave flees the place of its creation, while the water does not;

like the waves made in a field of grain by the wind, where we see the waves running across the field while the grain remains in its place.”

— Leonardo da Vinci

I found this quote in as an undergraduate over a decade ago, and it resonated to the extent that I wrote it in a blue book and have kept it with me since. In it, da Vinci makes an interesting observation: waves run through a field without carrying the grain supporting them.

Waves don’t always leave the medium they travel through behind, however. For example, water molecules move with water waves, but they usually move much more slowly. Interestingly, the molecules of water tend to swirl beneath the surface of the wave in an elliptical pattern, which becomes smaller with the depth beneath the water surface. Similarly described are the displacements of atoms in Rayleigh waves, which apply to sound waves traveling along the surface of a solid.

Wave behavior is all around us. Wind creates waves running across fields of grain and the surface of the ocean. Energy released through the Earth’s crust can travel along as a seismic wave and even as a tsunami. Sound waves are used for detection, not only in medicine, weather, and fishing, but also by organisms that pick up vibrations in their surroundings using tools such our own inner ear bones. All forms of radiation, including the visible light our eyes were built to see and the heat our sense of touch can perceive, travel as waves composed of mutually perpendicular, conjoined electric and magnetic fields. Quantum mechanics is built on the Schrodinger equation, which has many practical uses and philosophical interpretations but describes a wave, at the end of the day.

Perhaps the most important type of wave, for us, is the electromagnetic wave. Because of its ability to travel through space, it delivers the energy from the sun on which life on Earth thrives. Well over 99% of the energy on the Earth is from our sun, including fossil fuels, which are also known as "buried sunshine”.

The atmosphere has evolved to act as both a sunscreen and a blanket to the Earth, both extremely important to the life around us. In recent years, the sunscreen has thinned and the blanket thickened.

Though the result of the latter is increasingly frightening, we’ve already taken the first step in working toward a solution — we’ve recognized that these changes present us with an urgent problem.

Einstein's words echo in my ears: "In the middle of difficulty lies opportunity."

Waves

To better understand what a wave really is, let’s look at a few examples.

Let’s start with a snapshot of a water wave far from shore.

There are a couple of important things to notice: - the surface of the water has crests (high points) and troughs (low points)

- the distance between neighboring crests (the wavelength) is approximately the same

- with time, the crests and troughs move along the surface of the water, but the water molecules themselves don’t move along with the wave

Question: The wave moves without taking the water molecules along with it. So, what is being transferred by the wave??

Waves

Now let’s look at a sound wave.

Important things to notice: - unlike the water wave, where water displaces upward and downward to make crests and troughs, in a sound

wave the molecules move left and right, along the direction the sound wave travels!

- the displacements of molecules in a sound wave create regions of high and low pressure.

- the distance between the midpoints of the regions of high pressure is called the wavelength.

- the molecules carrying a sound wave do not themselves move with the wave.

Question: The wave moves without taking the molecules along with it. So, what is being transferred by the wave??

Now an electromagnetic wave.

Important things to notice: - like the water wave there are crests and troughs, but here in electric and magnetic fields.

- the wavelength (distance between crests) of the electric field is the same as the wavelength of the magnetic field.

- the electric and magnetic fields are perpendicular to each other and to the direction of the motion of the wave.

- electromagnetic waves travel even in the absence of molecules (space!).

Question: The wave moves without taking any molecules along with it. So, what is being transferred by the wave??

Waves

Waves

Question: The wave moves without taking any molecules along with it.

So, what is being transferred by the wave??

Energy!

A wave is a traveling disturbance in space that carries energy, not particles such as molecules.

A common way to categorize waves distinguishes between waves that need a material medium to travel.

1. Electromagnetic waves do not need a material medium to travel, meaning they can travel through space.

2. Mechanical waves need a material medium to travel, meaning they cannot travel through space.

Sound and water waves are examples of mechanical waves.

Wave vocabulary

There are a few important vocabulary words when it comes to wave motion.

Figure 9.5 in the text and the figures embedded in these notes may help with visualizing the definitions.

Wave: a traveling disturbance in space that carries energy, not particles such as molecules

Amplitude: distance associated with the maximum disturbance (e.g., height of crest)

Wavelength: the distance from any point along the wave to the next similar point (e.g., from crest to crest)

Wave vocabulary

Wavespeed: the speed at which the wave travels (e.g., speed at which a given crest moves).

Consider the red, thicker portion of the wave shown below. The wave speed is the distance it travels divided by the time it took to travel. Here, the distance it travels between the top and the bottom frame is one wavelength.

Wave vocabulary

Frequency: the number of wavelengths passing a given point per second (e.g., the number of crests of a water wave passing a particular point in the ocean per second.) The unit of frequency is the Hertz.

Consider the two waves below.

If they move at the same wavespeed, the number of crests passing a given point (say, the dashed line labeled A) is larger for the blue wave than for the red one. Hence, the blue wave has a higher frequency than the red one.

A

Waves

As it turns out:

For waves traveling at the same speed

those with a smaller wavelength

have a higher frequency.

A

Interference of waves

One very cool things about waves:

When waves traveling in different directions meet,

they add together,

but they then continue along unaffected.

The above phenomenon is called wave interference.

Constructive interference occurs when adding the waves gives a larger amplitude. See (a) below.

Destructive interference occurs when adding the waves gives a smaller amplitude. See (b) below.

In (a), when the waves meet, their crests overlap.

In (b), when the waves meet, the crest of one overlaps

with the trough of another.

Interference of waves

Waves moving in two- and three-dimensions are much more interesting.

Consider the picture below.

The picture shows,two sources of waves, created, perhaps, from dropping two pebbles in the water.

Notice that each wave propagates outward from the disturbance in a circular pattern.

Notice, too, that where the waves overlap the amplitude is a combination of the two waves that meet.

Interference of waves

Here’s another example.

Notice that you can actually see places of constructive interference and places of destructive interference.

Notice, too, that the places of constructive and destructive interference travel with the waves!

Interference of waves

If the waves from the previous picture were to hit a horizontal wall, there would be a pattern formed by the points of constructive and destructive interference.

The same effect can be achieved by passing water waves through two closely spaced openings (slits). When the waves pass through, they bend (diffract). Many experiments have shown the interference pattern that forms on a nearby wall.

There’s a really cool animated gif that goes with the image above. Please check it out: http://homepage.univie.ac.at/franz.embacher/KinderUni2005/waves.gif

Interference of waves

Important!

Waves interfere and can produce interference patterns.

When an interference pattern is observed, the source was a wave.

The electromagnetic wave theory of light

Waves interfere and can produce interference patterns.

When an interference pattern is observed, the source was a wave.

Light was a mystery to us for many years. It still is, but it was an even greater mystery.

For centuries, arguments that it was a stream of particles clashed against arguments that it was a wave.

One of the first pieces of evidence (if not the first) that light displayed wave behavior came from Thomas Young, in 1801.

He used the principle on the previous slide, passing light through two small, closely spaced slits, to show that light interfered as any wave would!

Below shows the result of the experiment. Light behaves like a wave!*

*Soon, we’ll discuss the particle-like behavior of light.

The electromagnetic wave theory of light

Light is an electromagnetic wave.

Be sure to read Section 9.4 carefully.

To briefly summarize, we have four relationships expressed elegantly by James Clerk Maxwell.

Three of them, we know. The fourth was added by Maxwell on a hunch that light was an electromagnetic wave and that the equations (relationships) ought to be symmetric.

1. Charges exert forces on each other. These forces can be described in terms of an electric field. An electric field surrounds every charged object. (Electric Force Law in terms of Fields.)

2. Charged objects that are moving exert magnetic forces. These forces can be described in terms of a magnetic field. A magnetic field surrounds every charged object in motion.

(Magnetic Force Law in terms of Fields.)

3. A changing magnetic field produces an electric field. (Faraday’s Law.)

4. A changing electric field produces a magnetic field. (Maxwell’s addition for symmetry.)

The electromagnetic wave theory of light

Every vibrating charged object creates a disturbance (wave) in its own electromagnetic field.

The disturbance spreads outward through the field at lightspeed.

Light is such an electromagnetic wave.

The reality of fields

The argument for the reality of electromagnetic fields is simple and direct, just how we like ‘em!

Electromagnetic fields are real:

they contain radiant energy,

and radiation (such as light) travels through them.

The electromagnetic spectrum

Section 9.6 is fun.

We’ve all heard of gamma rays, X-rays, ultraviolet (or UV) rays, infrared (or IR) rays, microwaves, and radio waves.

Want to know one way of making them?

Shake a charge!

By moving a charge up and down, you’re creating a change in the electric field associated with the charge. That change creates a changing magnetic field. And the changing magnetic field creates a changing electric field . . .

. . . and you’ve created an electromagnetic wave!

The number of times per second you shake the charge determines the frequency and thus type of radiation you create! Remember, all forms of radiation travels at the same speed, so the higher the frequency, the smaller the wavelength.

Radiation of higher frequency carries greater amounts of energy and can interact with smaller objects due to the smaller wavelength. (UV, for example, is small enough to interact with the DNA inside the nuclei of our cells.)

All forms of radiation can be found on a continuum of frequencies (wavelengths).

This continuum is called the electromagnetic spectrum. (Next slide.)

The electromagnetic spectrum

This is one of the best descriptions of the spectrum. Spend some time with it.

Notice that infrared is just before red and ultraviolet is just after violet on the visible portion of color

The electromagnetic spectrum

Here are the major concepts to know for each type of radiation (radiant energy carrier) on the spectrum.

Solar radiation spectrum

Let’s consider, for a moment, the forms of radiation that are delivered to us from the sun.

Both Figure 9.30 from the text (below) shows that the great majority of the radiation from the sun comes as:

- ultraviolet

- visible

- infrared

Solar radiation spectrum

The great majority of the radiation from the sun comes as:

- ultraviolet

- visible

- infrared

Most life on Earth has evolved to take advantage of these three forms of radiation.

Radiation and our atmosphere

Notice that not all types of radiation make it to the Earth’s surface!

Ozone depletion and global warming

Technology is creating truly global issues, not merely in the sense that they occur all over the globe, but in the sense that action in one part of the world has direct consequences in another.

As one might expect, the first problems of this sort are atmospheric in nature. For example, we have ozone depletion, global warming, acid rain, and fallouts from, e.g., Cherynobyl.

Politically, these problems are international.

To understand ozone depletion and global warming, let’s use our knowledge of electromagnetic radiation:

- ultraviolet in the case of ozone

- infrared in the case of global warming

Ozone depletion

Ozone depletion is a useful case study: Its history is finished in the sense that the final outcome is now up to nature and out of human control.

We are just at the uncertain beginning of global warming, and we might find the lessons of ozone depletion useful in considering possible global warming futures.

The text follows a historical approach, as the history of ozone depletion is now fairly clear and because this approach puts the physics within its social context.

Don’t worry too much about the chemical reactions that are discussed in the text. The main point is that chlorofluorocarbons (CFCs) and other manmade chemicals that are similar (e.g., HCFCs) and often used as refrigerants and in aerosol spray cans can climb high into the atmosphere—the region called the stratosphere—and break down the naturally formed ozone molecules.

(Ozone (O3) is similar to the oxygen molecules around us, O2, but instead of being formed by two oxygen atoms bonded together,

it’s formed by three.)

Ozone is in the stratosphere is very important to us because it filters out a lot of the UV radiation, which can cause skin cancer, cataracts, and premature aging in humans and reduce the health and population of some crops and marine life.


 The process of stratospheric ozone depletion is shown schematically on the next slide.

Ozone depletion

Don’t worry about memorizing this! I just want you to see the mechanics of the process.

Interestingly, ozone is depleted more rapidly when the poles are in winter. Then, there are more clouds in the stratosphere, and these clouds act as a catalyst, speeding up the ozone depletion process.

Note that there’s no “hole” in the ozone; its just thinned out.

Ozone depletion

After realizing that the ozone content of the stratosphere was declining and finally understanding the CFCs and the like led to the decline, several nations decided to take action through the Montreal Protocol in the late 1980s/early 1990s, which the text refers to as the Ozone Treaty.

This treaty banned further production and use of ozone-depleting chemicals. Yay, humanity!

It now seems that the stratospheric ozone layer has begun rebuilding itself. It’s projected that the ozone layer will be fully recovered by 2050.

1979

2012

Global warming

Global warming is an important, fascinating, and broadly interdisciplinary topic.

I just want to present the most important points, for us.

First are the greenhouse gases. They are called greenhouse gases, because, like the walls of a greenhouse, they absorb infrared radiation. This is called the greenhouse effect.

Here’s what happens: - greenhouse gases let UV and visible radiation from the sun pass through the atmosphere (though some is reflected

by clouds, ice, and other reflective surfaces.)

- energy carried by the UV and visible radiation is partially used by plants in the photosynthesis process

- the energy that’s not used is emitted by the plants and soils as infrared radiation

Remember, infrared radiation carries less energy than UV and visible radiation.

- the infrared radiation is absorbed by the greenhouse gases instead of traveling immediately out to space

- the greenhouse gases then re-radiate the infrared radiation in all directions, including back down toward Earth

- excess infrared radiation (warmth) is thus kept by the Earth under the greenhouse gas blanket

The greenhouse effect is a good thing! If it weren’t for greenhouse gases, much of the Earth would be frozen and the average surface temperature would be very, very cold. You, I, and all the little dogs, too, would probably not be here.

The problem is that over time we’ve increased the greenhouse gas content in the atmosphere, trapping more heat than is probably good for us.

Greenhouse effect

The figure below sums up the process quite nicely.

Greenhouse gases include water vapor, which is the most abundant, carbon dioxide, methane, and nitrogen oxides.

Greenhouse gases

The figure below shows how the content of important greenhouse gases has increased since we began using fossil fuels extensively with the arrival of the Industrial Revolution, where much physical labor was replaced with machinery. (We can look back in time using proxies, such as ice cores, ocean sediment cores, tree rings, etc. )

Though water is the most important contributor, it is difficult to track.

Year

Greenhouse gases

One very cool thing to note is that though the overall trend is increasing, there is a yearly cycle in the carbon dioxide content of the atmosphere. This is because when the Northern Hemisphere is in winter, the a larger number of the Earth’s deciduous trees are dormant.

(Most of the Earth’s land is in the Northern Hemisphere, and a very important, lush forest for us is the boreal forest.)

The NOAA is like the NASA of our atmosphere and oceans.

Global warming

What we know from looking at proxies for historical data and having taken real-time data for over 200 years, is that increased greenhouse gases and increased global mean surface and sea temperatures go hand in hand. This is not the Earth’s first ride at the fair, but it may be ours!

(This graph shows, specifically, the relationship between carbon dioxide and temperature.)

Global warming What happens when the global mean surface and sea temperatures of the Earth increase?

We’ve observed what intuition would suggest:

- Earth ice melts This has at least one implication that feeds the rate of global warming: with less ice, less of the incoming solar radiation is reflected.

- sea levels rise You know how when you heat up a marshmallow or a piece of metal, it expands? Well, so do the oceans! As they warm up, they take up more volume. This is the primary cause of rising sea levels, which could submerge island nations and parts of, e.g., Florida and Manhattan.

- weather patterns shift In addition, there is more evaporation of water, which collects as clouds and rains down hard somewhere else. But, more importantly, this also has an implication that feeds the rate of global warming: water vapor is a strong greenhouse gas, so with more water molecules in the atmosphere and less as a liquid, temperatures rise more quickly.

- extreme weather events become more frequent This is related to the above dash-bullet. Extreme weather events include droughts, floods, tornados, hurricanes, typhoons, etc. Wildfires are particularly harmful, taking away one of the atmosphere’s carbon dioxide scrubbers (trees and other plants) and dumping greenhouse-gas laden smoke into the atmosphere.

- plant and animal (including insect) populations shift geographically In fact, malaria is moving to higher elevations in Africa because mosquitoes can live in regions that were previously too cold. This affects not only insects, but nearly every living species.

There are additional effects, such as increases in ocean acidity, which is having negative effects on life at the bottom of our food chain.

Global warming

Don’t worry about remembering everything on the previous slide or this one.

I simply want to be sure you’re informed about what nearly every scientist who has studied the issue agrees with.

So, WTH do we do? There is encouraging news. Positive action that can be taken to avert some or even much of the predicted damage.

Remember, we faced a similar situation during the 1980s, involving destruction of atmospheric ozone, and solved it.

We currently have an international working group that was created by the UN and WMO in 1988.

It’s called the Intergovernmental Panel on Climate Change (IPCC), and at the moment representatives and scientists from over 120 countries are involved. (http://www.ipcc.ch/)

Every few years, the IPCC publishes a report that highlights, conservatively:

- recent scientific work and observed data regarding greenhouse gas concentrations, temperature, global warming, and the current state of the global climate and local climates

- impacts, adaptations, and mitigation of climate change

- economic and social dimensions of climate change

The most recent report came out in 2014, and I will use it as a case study in a course I’ll teach in the Spring.

The results of their work and the actions we take based on it will have impacts that affect humanity for, likely, centuries to come. I believe that reducing the rate of climate change is the most urgent issue we face. The second and third, in my mind, are developing “clean” and easily accessible sources of energy and reducing the rate of population growth.

So, everybody: reduce, reuse, recycle.

Words of encouragement, from the one and only Dr. Seuss