Electromagnetic radiation and Earth's Atmosphere

Question

We don't get the full spectrum from the Sun because there is for example Hydrogen gas in it's atmosphere which absorbs photon of a specific frequency. How do we know that that dark lines in the spectrum are not because of a gas in our atmosphere?

In our atmosphere we have a lot of gases. Why do we still get white light? Do those gases absorb some frequencies?

Answer 1

Most of the gases in our atmosphere only absorb in the far ultraviolet (and in some cases the infrared). ozone absorbs in the mid-ultraviolet, hence the importance of the ozone layer.

Look up "Fraunhofer lines" in Wikipedia.  We can match the absorption lines in the solar spectrum to the known emission lines that we can get from the elements by, for example, striking an arc.

Generally speaking, these dark lines correspond to species that are only present in vapour form in the Sun's atmosphere because of its very high temperature.

Answer 2

You asked "How do we know that dark lines in the spectrum are not because of gas in our atmosphere ?"

Spectral lines form distinct patterns of frequency depending on the element (examples, hydrogen, helium, oxygen, nitrogen) which is either absorbing or emitting the spectral line. So when examining a spectrum it is usually very clear which element is involved. The frequency of the individual lines, but also the collected pattern of lines together, indicates which element is absorbing or emitting.

So when we see spectral lines for hydrogen, for example, we know for sure it is hydrogen and not some other element.

A second clue is that while spectral lines are a clear 'signature' of an atomic element, those lines do vary a little in strength and frequency, depending on the physical environment of the atom. The sun is very hot compared to the earth's atmosphere. So the environment of the hydrogen atoms around the sun causes the spectral lines to be slightly different than they would be in earth's relatively cool atmosphere. Some conditions that affect spectral lines are kinetic motion (which shift lines lower or higher), ionization, magnetic fields and chemical bonding.

So for example, although the sun is relatively constant in its distance from the earth, there are other stars which are either moving away from the earth or towards the earth. When we observe the spectrum from these stars (or even galaxies) we notice a definite shift in the overall frequency of the spectrum, including all the spectral lines. This is often referred to as the 'red-shift', because many galaxies are moving away from us and their spectra are shifted towards lower frequencies, which is towards 'red'.

From the red shift, we know we are looking at distant stars and not at spectral lines from our own atmosphere. We also assume that the sun is like many stars in composition and emitted light, so we know that our observations of spectral lines are from gases in the sun.

Another clue is that the sun's atmosphere is mostly hydrogen and helium. Helium is almost non-existent in the earth's atmosphere. An amazing fact in history, is that helium was discovered on the sun first, before it was discovered on earth. Most of the helium we haveon earth, such as in helium balloons, comes from underground caverns or pockets in the earth's crust, where helium accumulated from nuclear decay of radioactive rocks.  So when we see spectral lines of helium from the sun, it can't be from our own atmosphere because our atmosphere has almost no helium at all - not enough to explain the spectral lines.

Another good clue is that the sun's spectral lines are in light from the sun. That seems obvious. We can observe spectral lines using our own atmosphere, by doing experiments in dark rooms. We can observe elements such as hydrogen, oxygen, neon, mercury when they floresce in strong electric fields or high temperatures.

The sun is so hot that many atom's lose one or more electrons. The atoms become ions, and they form a different state of matter, called plasma. The spectral lines of plasma are a bit different than atoms that are in stable chemical bonds. In our atmosphere the atoms are bonded into gas or vapour molecules. Hydrogen in our atmosphere is in water molecules (H20), or other molecules such as ammonia. "Pure" hydrogen gas in our atmosphere would be in the form of the H2 molecule, not as plasma. We can tell the difference in the spectral lines between hydrogen plasma and hydrogen gas.

You also asked "Why do we get white light?"

The light from the sun looks to our eyes very bright, but is it really "white" ? Our eyes are really only senstive to three specific frequencies of light.  Light does not have to be full spectrum to look "white" to us. For example, we cannot see the ultraviolet light that gives us sunburn. If the ultraviolet is missing, you can't see a difference, because your eyes don't detect ultraviolet.

So normally you don't notice the 'missing' frequencies of spectral lines, because your eyes do not detect most frequencies. You detect three 'bands' from frequency around red, yellow and blue. Even in the yellow, for example, if a spectral line was there, you would not detect that you were looking at yellow with a spectral line, compared to yellow without a spectral line. You either see the yellow as brighter or dimmer, but you can't tell the exact frequency of it. The only way we see spectral lines is by spreading the light frequencies apart in position, when we bend the light through a prism

Another reason we get white light is that light scatters. When you look at the moon, or an egg, or the sidewalk, or a tree, the spectrum you see has much more to do with the color of the object reflecting the light, than it does with the light source. Many objects change the color of light. For example, ultraviolet light falling on an eggshell causes the eggshell to absorb some ultraviolet and then re-emit a lower frequency.  You can notice this effect if you have an ultraviolet lamp. This is a lamp that emits ultraviolet (invisible) light only. It is sometimes called a "black" light. If you put ordinary objects, like eggs, under a "black" light, you see them glow in the dark !!

So in ordinary conditions, when you are outdoors or indoors, so many objects are reflecting and scattering the light, that spectral lines get lost. The light frequencies you see are all mixed and shifted by the objects reflecting the light to your eyes.

Actually, almost everything gives off light, most of which is invisible to your eyes. The sun is very bright because it is so hot, and your eyes are adapted to the kind of yellow light that the sun emits. But even when you are in a closed room which seems to you completely dark, every object in the room is giving off infra-red light. Your eyes cannot see it, but some types of cameras can see parts of the infra-red. These cameras show warm things (like your body) as bright, and cold things (like ice cubes) as dark. Special goggles or cameras that use infra-red are sometimes called "night vision" or "thermal vision".