JIK

This webpage contains important comments about mistakes or misconceptions that were common in your essays on the second midterm. Almost all of you picked essay question no. 1, about infrared light, so that is the only essay I have commented on here (for the very few of you who picked essay question 2, I have sent you individual comments by e-mail). Not everything that I have written here applies to everyone (obviously), but many of them apply to very many of you. In any case, it is a good idea to at least look over everything on this page, whether you think you got it right or not. Many of the things you were supposed to mention in your essay are central to the course, and you can be sure that at least something that I write on this page is going to appear on the final, either as a multiple-choice question or as a part of one of the essays.

In my e-mailed feedback, I asked many of you to read specific parts of this webpage. If so, make sure that you read those especially carefully (and if you can’t find it, e-mail me and let me know, I may have forgotten to write it). The issues I want to comment on are the following (and do send me an e-mail or talk to me if anything here is confusing):

“Emit heat” issue / What things emit IR

When writing about what things emit IR, a lot of you stated something to the effect of “everything that emits heat, emits IR” or some variation of that. Some of you furthermore stated that “heat is IR” or “IR is heat”. Many others also didn’t give a very good answer for what emits IR, and very, very few of you said why things emit IR (the question didn’t ask explicitly why, but you can’t really give a good answer in this case without something about that). There are at least two issues here (only one may apply to some of those of you whom I told to read this, but both apply to most of you):

1. First of all, the things that emit IR are all things that are not very, very cold (though note that “very, very cold” here does not necessarily mean absolute zero). The key here is that all things with a temperature above 0K emit some form of electromagnetic radiation, although not necessarily IR. Remember: temperature is the kinetic energy in the random shaking or motion of the atoms in a material. These atoms contain charges (protons and electrons), and when you shake an electric charge, it will emit electromagnetic waves. The faster you shake it, the higher frequencies it will emit. And unless something is really, really cold, some of the frequencies a material emits will be in the infrared range (if something is extremely cold, like only a few Kelvin, it will emit only microwaves and not much IR). If it gets hot enough, the frequencies will be high enough to contain visible light, UV or even higher frequencies, but they will still emit IR as well. So the right answer here is: everything that is not very, very cold emits IR (preferably including a concise version of what I wrote above, but most of you didn’t, so I haven’t deducted a whole lot of points for that).
2. Things don’t “emit heat”. They have a certain temperature, and if the temperature is not too low, they will emit IR. A lot of you said that “anything that emits heat, emits IR”, and this sentence does not quite make sense. It makes it sound as though heat and IR are two different things that an object can emit. That is not so. Hot objects in general emit IR, they don’t “emit heat”. The phrase “emit heat” is admittedly a little ambiguous, so I can’t know whether those of you who wrote it really misunderstood what is going on or whether you are just phrasing yourselves poorly. For that reason, I have not subtracted a whole lot of points for this, but make sure you understand what IR really is and what it’s relation to heat and hot objects is (make sure you read the next section too), and make sure you write something that is clearer on the final. You should also note that there is nothing special about IR in relation to heat, in particular IR is not “heat radiation” or the only kind of radiation that will cause things to get hot (see the next section, IR is not heat, and vice versa).

IR is not heat, and vice versa

A common misunderstanding among many of you was having the impression that only IR is felt as heat (whereas e.g. visible light is not), or that IR is “heat radiation”, or that IR and heat are somehow the same thing. All of the above are false. IR is light that has a longer wavelength (and therefore lower frequency) than visible light, but shorter wavelength (higher frequency) than microwaves or radio waves. The only reason it is connected to heat in any way, is that, as mentioned in What things emit IR above, it has a frequency which is emitted by most things that are not very cold (warmer than a few degrees Kelvin), even those that are not hot enough to emit visible light. But if things are hot enough, they will emit visible light and other frequencies as well, and if they are too cold, they will only emit microwaves and not much IR. And, most importantly, IR is not the only kind of radiation that will warm things that it hits. Any kind of electromagnetic radiation will heat whatever it is absorbed by. Visible light will, UV will, microwaves will (microwave ovens heat things with microwaves, not with IR). You can feel this for yourselves: Let the sun shine on you through a closed window. You will feel heat. This is not because IR from the sun is hitting you, most of the IR doesn’t make it through the glass. It is because of the visible light that is hitting you and causing the molecules in your skin to shake around faster. The fact that visible light (and all other kinds of light), not just IR, can heat things, is crucial to understanding how the greenhouse effect works (see below).

Greenhouse effect, and (not!) ozone layer

Your answers to how IR affects the Earth’s climate were many and
varied. Most of you had at least an inkling about it, but very many of you did not get it entirely right. There were a lot of different misunderstandings out there, so not everything I write here will apply to all of you, but you should still read all of it, and definitely ask me if there is anything you can’t make sense of.

Most of what you need to know has already been outlined in what I posted on the blog before the midterm, so read that again before the final. Here I will summarize the main points that many of you missed or misunderstood:

• The most important way in which IR (really the interplay between visible light and IR) influences our climate is through the greenhouse effect (most of you got this right, but some of you forgot), which makes our planet on average about 30C warmer than it would have been without the greenhouse effect. This is the natural greenhouse effect (more about the possibly not-so-natural one at the end)
• The greenhouse effect is caused by greenhouse gases. There are many such gases, notably CO2, water vapor, methane and, to some extent, ozone. These gases make up a tiny portion of our atmosphere (much less than 1%), but have a big effect on climate.
• Greenhouse gases let visible light from the sun through and lets that light hit and be absorbed by the ground, thereby heating the surface of the Earth. The Earth in turn cools down by emitting infrared light. Remember, unlike the Sun the Earth is not hot enough to emit visible light, so it emits only infrared. But although the greenhouse gases are transparent to visible light and let the light from the sun through, they are not transparent to IR. They therefore absorb the IR being emitted from the Earth, so the Earth does not cool down as effectively as it would have without the greenhouse gases. This causes the Earth to get warmer than it otherwise would have been, and that is the greenhouse effect. So, in a nutshell: greenhouse gases let visible light from the sun through, so the Earth can receive heat, but does not let as much IR from the Earth back out, so the Earth can’t cool down very effectively.
• The greenhouse gases also of course absorb IR coming from the sun, and some of you tried to say that this (or something like it) is what causes the greenhouse effect. But that is not so. The fact that greenhouse gases absorb IR coming from the sun means that less energy from the sun hits the ground, so this actually has a cooling effect on the climate. If the sun and the Earth both emitted only IR, then greenhouse gases would not warm the Earth at all; the incoming IR energy from the sun blocked out would balance the outgoing energy from the Earth’s surface that is held back. The reason greenhouse gases have a net heating effect is that the Sun emits much of its energy as visible light (which gets through the greenhouse gases and reaches the surface), whereas the Earth’s surface emits almost only infrared light.
• An extra note for some of you: The Earth absorbs light from the sun, and then emits this energy again as IR. The light or the IR involved in the greenhouse effect is not being “reflected” or “bounced off” the Earth, it is absorbed and then emitted.
• The greenhouse effect is not mainly caused by the ozone layer. A concerningly large number of you stated much of what I said above about the greenhouse effect, but you stated that it is the ozone layer that is absorbing the IR, rather than greenhouse gases in general. It is true that ozone is a (weak) greenhouse gas, but it is plain wrong to say that it is the ozone layer that causes all or most of the greenhouse effect. Many other greenhouse gases, notably CO2, are far more important.
• What I’ve said so far is about the greenhouse effect in general. It is mostly a natural phenomenon, caused by greenhouse gases that are naturally present in the Earth’s atmosphere, and without it Earth would be a freezing place. But since we have been pumping extra greenhouse gases into the atmosphere throughout the industrial age, adding a lot more CO2 and methane to the atmosphere, there is a worry (which is starting to be backed up by very solid research) that we are causing excessive global warming.

Applications of IR

Some of you had a somewhat skimpy list of practical applications of infrared light. Here is a (not necessarily complete) list of applications you could include:

1. Heat lamps
2. Weather satellites, detecting IR from the Earth to measure temperature, see clouds in the dark and spot water vapor (water vapor emits IR at a very characteristic frequency)
3. Stinger missiles, and other heat-seeking missiles
4. Infrared goggles which detect long-wavelength infrared light, which can be used to see people or other warm things in the dark
5. Infrared googles that detect short-wavelength infrared light, at a wavelength where people and other not-too-hot things don’t emit much light, but which can detect light given off by an infrared flashlight (which you can then use as a flashlight which cannot be seen by anyone who does not have the appropriate IR goggles
6. Thermal imaging for medical use (can be used to spot e.g. infected and inflamed areas in the body just under the skin)
7. Remote controls (for TVs, video players, stereos etc.), Bluetooth and other devices that use IR to communicate or send signals (note that cell phones do not use IR, they use microwaves)