Why do you feel warm when you walk under the sun How do you think the heat from the sun reaches the earth?

Heat is a form of energy, and it travels through radiation. Radiation is a form of energy that does not need a medium to travel, which is why heat can travel through a vacuum.

Table of Contents Show

• What is heat?
• Transfer of heat
• Heat transfer through radiation
• Suggested Reading

‘The outer space is a near-perfect vacuum; so, how does heat travel through space?’

A lot of people get befuddled by this question. In simple words, does heat need a medium to travel? If it does indeed, then how do sun’s ‘heat rays’ travel through the vacuum of space before reaching Earth?

The answer is quite simple: heat is a form of energy released from the sun and travels through radiation, which is why the sun feels hot.

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What is heat?

This probably seems like a very dumb question to ask, but the concept of ‘heat’ is much more than ‘something that a thermometer measures’ if you really dig deeper. In everyday terminology, we say that something emits ‘heat’ when it feels hot to the touch, or we can say that air is being ‘heated up’ by the effects of global warming and so on. However, what is ‘heat’ at its most basic definition?

One of the many daily-life examples of applying heat to something (Photo Credit : Shutterstock)

Heat is a form of energy. It is the energy that an object possesses by virtue of the movement of its constituent particles. These particles are continuously moving, hitting and bouncing off each other (solids allow minimal movement, while gases allow maximum movement of constituent particles). The faster these particles move and hit each other, the hotter the object in question becomes.

When you ‘heat’ something up using a burner (or any heat source), what you do is essentially raise the average kinetic energy of the substance’s particles, which in turn raises its overall temperature.

Transfer of heat

Heat can be transferred in three different ways: conduction, convection and radiation.

In basic terms, conduction occurs when two bodies are in contact with each other. This is the most significant and common method of heat transfer and it occurs when rapidly moving or vibrating particles interact with particles of a neighboring object and transfer some of their energy to the latter.

On the other hand, convection occurs when a heated fluid (e.g., air, water etc.) moves away from the source of heat and comes in contact with other substances, transferring some of their energy in the process.

There are numerous examples of heat transfer through both conduction and convection, so it’s easy to mistakenly assume that these are the only two methods by which heat is transferred.

Heat transfer through radiation

The third method of transferring heat – the one responsible for heating the planet and everyone on it – is radiation. In space, there are hardly any particles (making it a near-perfect vacuum), but there is radiation, which gets converted into heat when it collides with an object. Radiation is responsible for heating not only Earth-bound objects, but also objects that are not (physically) adhered to our planet, such as the ISS, the moon and other celestial bodies.

You see, the reason that the sun ‘burns’ all the time is that it plays host to nuclear fusion reactions of epic proportions. These reactions, quite predictably, release massive amounts of energy all the time, which is then released all around the sun into space via electromagnetic waves. The sun emits radiation at many wavelengths across the EM spectrum, including infrared, UV and X-rays (Source). It also emits EM waves in the visible range of the spectrum, which is the reason we can see the Sun in the first place!

Now, if you remember reading about electromagnetic waves/radiation in your high school physics class, then you might recall one singular truth about them…

Precisely! EM waves don’t need a medium to propagate, meaning that they CAN travel through a vacuum. This is why see the sun and feel the ‘sunlight’ on our planet. The sun’s radiation consists of small, massless packets of energy called photons. They travel seamlessly through space; whenever they strike any object, the object absorbs photons and its energy is increased, which then heats it up.

So, these photons travel through a vacuum without any problem, but as soon as they collide with an object, like the Earth or other celestial bodies, they get absorbed and impart heat energy to the host object in the process.

In addition to that, our atmosphere does a very good job keeping the planet warm by trapping 50% of the sun’s heat energy that reaches the planet and preventing it from escaping back into space.

The atmosphere keeps the planet warm by preventing the heat energy from escaping into space (Credit: Vadim Sadovski/Shutterstock)

Next time someone asks you how heat can possibly travel through the vacuum of space, just remember that it’s not the ‘heat’ traveling through the vacuum, but rather the electromagnetic radiation, and which doesn’t need a medium to propagate!

Suggested Reading

References

1. Caltech
2. Wikipedia - Heat Transfer
3. NASA
4. NASA - Science

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There's two main things to consider - energy and absorption charasteristics of different photon wavelengths.

The Sun emits a lot of energy, obviously. Even at Earth's distance from the Sun, the energy concentration is still far from negligible - when this energy impacts your body and is absorbed, it mostly causes heating (a bit complicated by wavelength, but we'll get to that). How much energy is that? Well, at rest, the adult human body radiates around 300-600 W, or 300-600 J/s. This is actually in roughly the same magnitude as the insolation energy - the average varies a lot by lattitude, mostly, and it's different at different times of the year, of course. For example, southern Spain has an average of about 200 W per square metre (more in the summer, less in the winter), while the equator has around 1000. The amount of energy you absorb depends on how much surface area you're exposing, and at what angle. But we're only dealing with ballparks here, we already have a rough magnitude.

How does the X-ray compare? Well, the X-ray machines are actually operating at an extremely low power output. According to wikipedia, 300 J is the lethal dose of X-rays - so even a single second of X-ray exposure energetically equivallent to normal sunlight would kill you! This makes it obvious that your routine X-ray scan is way, way below the kind of energy you receive from the Sun, and far too low to be detected by human senses - don't forget that everything in the room is also radiating, and at room temperature, this is again the same order of magnitude as the solar light, and your own heat radiation (this is why you don't really "feel" losing those 300 W of heat - roughly the same amount is absorbed from the environment). The X-rays are utterly insignificant in comparison.

Now, wavelength. The most important thing here is that different wavelengths of light have different behaviour when interacting with different kinds of matter. Wavelength depends on the energy of the individual photons, which in turn affects how those photons interact with matter.

Visible light is more or less the range where the individual photons have enough energy to induce chemical changes, but not enough to break the stronger bonds - this is what makes it perfect for seeing, it easily influences the light-sensitive molecules in our eyes, but doesn't destroy them (and us) utterly. It's easy to understand the principles like "opacity" with visible light - if something absorbs light, it is opaque; if it reflects light, it is "shiny"; and finally, if it doesn't interact with light, it is transparent. For example, clear glass is usually transparent and shiny, so it reflects part of the incident visible light, and absorbs very little, letting most through. Clouds tend to reflect visible light a lot, which makes them distinctly white from the top, and blocking some of the sunlight from coming to the surface (especially when the cloud gets really deep - basically, the darker a cloud appears from the bottom, the more light it reflected back up, and the deeper it is).

At lower energies lies the infrared (literally "below red", red being the lowest energy visible light). This is what we commonly call "heat radiation", because it far dominates at the low temperatures we commonly encounter - almost all of your own radiation is infrared, for example. It actually dominates even at much higher temperatures, but since those also cause the emission of visible light, we tend to take that as the more important, even though most of the energy is still emitted as infrared. The visible radiation begins around 480 °C, a dull red colour; the surface of the sun is somewhere around 6000 °C. The sunlight is energetically mostly composed of infrared (~50%) and visible light (~40%). Glass is usually opaque to infrared light - this is one of the things that makes greenhouses work; the material in the greenhouse absorbs some of the incident visible light, but when that energy is radiated back outside, it will be radiated as infrared, unable to pass through the glass. Clouds absorb near-infrared quite effectively, which is why even very shallow clouds in the summer can make you feel "cold" - the sunlight that reaches you can lose a big portion of that 50% slice mentioned earlier. Deep clouds can block almost all of the energy coming from the sun, both infrared and visible.

At higher energies we have ultraviolet ("above violet"). UV light (only a tiny fraction being "X-rays" - technically, they are ultraviolet, but are usually grouped separately), is around 8% of sunlight by energy at the top of the Earth's atmosphere, and around 4% when it finally comes through the atmosphere and the ozone layer. So in terms of the total energy flux, they are mostly negligible. Their main danger comes from their per-photon energy - they have enough energy to strip atoms of their electrons, and break even strong molecular bonds; the most important for us, they are powerful enough to damage our DNA. The lower energy ultraviolet light tends to be absorbed by clear glass quite readily, so you're not going to get tanned when sunbathing behind a window (tanning salons use different kind of glass, transparent to the "desired" band of UV light). As you get to the really high energies, you find that less and less photons are absorbed by matter - and whenever they are, they cause big changes.

X-rays are energetic enough to pass almost unhindered through the human body - basically, at those energy levels, the most important thing is density of the matter. This is what makes X-rays so useful in medicine - many X-rays pass or scatter right through us and we can use that information to build a fairly accurate image of our insides, clearly showing e.g. bones and inner organs. The drawback is that when the X-ray hits a chain of DNA, it causes damage - if there's enough X-rays absorbed (or even scattered) by the DNA, it can easily overwhelm our ability to fix the errors, causing radiation poisoning (cells not able to fix their DNA self-destruct, basically) and cancer (some of the previously mentioned cells don't self-destruct). The lethal dose is pathetically small compared to the energies we encounter every day, so much so that if you feel the heat from an X-ray source, you're pretty much guaranteed to die.