Electromagnetic spectrum and emittance

EM Spectrum

The Sun emits electromagnetic radiation in many different wave lengths. The actual EM radiated by the Sun, is known as the solar spectrum, and is made up of some ultraviolet (A, B, and C), but mostly ‘visible light’, and infrared (A, B, and C) - as heat. It extends from around 290 nm to more than 3200 nm. Some stronger rays are emitted by the Sun (such as X-Rays), as mentioned in previous sections, but these do not make it through the Earth’s lower atmosphere. Small amounts of radio waves are also given off by the Sun and other stars. The energy spectrum given off by a star depends upon its size and temperature.

The ‘visible spectrum’ - that part of the spectrum that we can detect with our eyes is only a small part of the entire spectrum of electromagnetic waves. To the left of visible light are wavelengths that increase (higher) in frequency - short waves, and to the right are waves that decrease (lower) in frequency - and therefore long waves. As stated above, all forms of electromagnetic energy travel at the speed of light, but the amount of energy of a photon differs across the spectrum - from left to right, from more powerful gamma rays (i.e., 106 electron volts), X rays and Ultraviolet (and more harmful to life) on the left, to less powerful infra-red, and radio waves (less harmful for life - i.e., 10-8 election volts) to the right.

As is shown in following web pages, the Earth’s atmosphere filters out nearly all the most dangerous forms of electromagnetic waves.

Emittance

All objects have charged particles moving around within them and interactions occurring - which changes their velocities, and elements vibrating and rotating, therefore emits (and absorbs) EM radiation (unless the object’s temperature is OºKelvin - absolute zero). Therefore all objects, such as ourselves, the air, walls, and an apple pie, emit (and absorb) electromagnetic energy, and the Sun can be said to produce EM radiation, as it emits, but absorbs virtually no EM radiation.

The Importance of Temperature (Thermal Radiation)

In understanding, and being able to calculate objects emittance of EM radiation, scientists developed experimental test rigs using empty boxes, internally covered in black soot, for instance called a blackbody, creating an idealised model, to compare against experimental results.

What scientists (such as Planck and others) eventually found with these experiments, was that the amount (emission rate or intensity), and type (spectral distribution) of EM radiation emitted and absorbed, depends on temperature. They found that:

Hot objects emit energy at a higher rate (greater intensity), cold objects far less.

And that most objects emit EM radiation at many different wavelengths - although there is usually one particular wavelength that is emitted the most (known as lambda (λ) max) - and:

Hotter objects emit a greater range of wavelengths, and as temperature goes up λmax shifts to the left.

To understand what this means, and the terminology described above, the first picture to the right shows a light bulb (which is connected to an electrical circuit for power…).

Incandescent lamps glow bright as electric current flows through a filament. The filament is a conductor, which allows electricity to flow through it. However, filaments are made short and thin, which increases the resistance, and so the flowing current basically bumps into the atoms in the filament a lot, as they ‘try’ to travel through it, increasing the temperature. As already stated above, hot objects emit (more) EM radiation, and so the objective of the incandescent bulb is actually to get the filament really hot - although this means a lot of energy is wasted as heat (IR radiation).

The first image ‘Off’ (to the left) shows that there is a measure of EM radiation emitting from the bulb even though the light is switched off. As the atoms are being accelerated and/or decelerated in the bulb, like in any object, they are emitting some EM radiation to the room, however, as the light bulb is off and cold, it is only giving off very, very small amounts of long-wave infrared into the room, that is too cold to feel as heat.

The next image ‘Slightly On’ shows that when the light bulb is slightly on, the emitted radiation shifts to the left - or as scientists say - shifts to the blue as it starts to go towards the blue spectrum of visible light. Here the bulb does emit small amounts of visible light in the red spectrum (as well as more IR), and so part of the EM radiation is now visible as low intensity red light, and perhaps a small amount of heat can be felt.

The last image ‘Fully On’ shows, again, how the curve shifts further to the left, now that the light is fully on, and the mix of the light spectrum now being emitted gives the lamp a “warm” white glow. The amount of emitted IR radiation has also increased again, and so the bulb is hot to touch.

This basic light bulb example confirms that hot objects emit energy at a higher rate (greater intensity), cold objects far less. It also shows that most objects emit EM radiation at many different wavelengths - although there is usually one particular wavelength that is emitted the most (known as lambda (λ) max). And it shows that hotter objects emit a greater range of wavelengths, and as temperature goes up λmax shifts to the left.

The experiment also shows that the intensity spectrum is a bell curve - there is a max intensity, and after which, short-wave length emission drops to zero. This was not obvious, at least in theory to scientists for a long time - although experiments showed bell curves as observed results. Their calculations showed that the curve should keep increasing from the right: as temperature increased, intensity would increase to infinity as wave length shortened (known a the UV catastrophe). It took Plank, to suggest that light was not a continuous wave, and instead ‘packets’ (as discussed in part 2) to make the maths work, and later Einstein to confirm that light was actually packets (also particles) in the photo-electric effect experiment that made the calculations (which are relatively complex) show what was observed in experiments.

Black Bodies

Black body radiation is an ideal model (black bodies do not really exist) for the EM radiation given off by an ‘ideal’ (or ‘perfect’) radiator. In general, an object can absorb the same EM radiation as it can emit, however, as a perfect absorber, it will reflect no light (as all is absorbed), and therefore will look black (hence ‘black body’). Black bodies also emit EM radiation at all wavelengths.

Black body radiation calculations purely consider temperature (which must be constant - in thermal equilibrium), and it ignores size, and material composition (discussed below). As a model it can be used to compare against what is observed, and although it is a theoretical model, surprisingly, the Sun is very close to a black body concerning absorption, as any light that hits it’s gas/plasma surface is scattered and totally absorbed. However, it is not a perfect black body, concerning emittance: The sun does emits light across the entire EM radiation spectrum, but it doesn’t follow exactly the ideal model, due to different dynamics happening inside the plasma, but it can be considered a black body for most calculations. The first of the images to the right shows the ideal black body radiation, with all the properties discussed with the light-bulb example above. And the second image to the right shows that actual emittance of the Sun (orange line filled with yellow. As can be seen from this graph, the ideal black body radiation for an object of this temperature (5778 Kelvin) follows very closely to the measured emittance.

The Importance of Material Type

As mentioned above, most objects are not black bodies, in that others factors, particularly what the object is made of is also very important. For instance, when iron is heated to the same temperature as glass, it emits and absorbs a different amount and type of EM radiation - it has a different emission spectrum.

Understanding how EM radiation (energy) interacts with materials (matter), at the atomics level and at the molecular level will be discussed further in some of the further web pages; as the EM radiation begins to penetrate the Earth’s atmosphere and interact with the Earths surface.