The Fingerprints of Light

The best part has been saved for last on our series finale! My favorite part of the basics to astrotools – the most important tool in astronomy – the study of light or spectroscopy! Without ever even visiting a source of light, I can identify what it is based on the properties of the light it emits. Without ever even visiting the matter that warped and re-directed that light and its properties, I can identify the type of matter and interaction that took place for the change to occur. Below, I share with you how to search for and identify the fingerprints of light.

Types of spectra: 

There are three basics types of spectra when studying light:

  1. When a source produces a light spectrum with no features (no dark or bright lines nor spikes or dips), it is called a continuous spectrum. . The visible light spectrum generated by an incandescent light bulb, for instance, is a rainbow. It shows a continuous transition from one color of light to the next, so it is a continuous spectrum. 
  2. When a light spectrum shows very bright lines appearing at specific wavelengths, it is a type of emission spectrum. For instance, clouds of hot gas can emit very bright lines in their spectra! The light emitted is determined by the properties of the cloud such as its temperature and the chemical composition.
  3. A light spectrum that is identified with low-intensity or “dark” lines, or dips at specific wavelengths is an absorption spectrum. In this case, a light source must be interacting with some kind of matter such as a gas cloud, and this gas cloud is absorbing only some of light, corresponding to the dips or dark lines in the spectrum. The light absorbed by the matter is determined by the cloud’s composition and temperature. 

Each emission or absorption line in a spectrum is a direct consequence of electrons in a specific atom, ion or molecule, gaining or losing energy in the form of light. Remember that energy levels of bound electrons are discrete, or quantized, which means that an electron requires a specific amount of energy to move within the levels. 

Thermal Radiation: Consider an object that absorbs almost every photon it runs into, trapping each photon within it, making it very difficult to escape. The photons will bounce randomly around inside the object, so that by the time the photons escape the object, they have randomized energies from exchanging energy while bouncing around. If we were to look at all the photons’ escaping from the object by their wavelength, we would see a wide spread from this randomization. If that sounds familiar, it should, because it is a type of continuous spectrum! These spectra only depend on one thing: the original object’s temperature. The photons literally ended up escaping with energy that matched the energy of the particles that make up the object. This temperature dependence is the definition of thermal radiation. 

There are two laws specific to thermal radiation:

Law 1: the Stefan-Boltzmann law. Hotter objects emit more light per square meter of the surface at all wavelengths than cooler objects. 

Law 2: Wien’s Law. Hotter objects emit photons with higher average energy than cooler objects. 

These laws are exactly why the stars in our night sky have varying colors. Their colors indicate their temperatures! Betelgeuse is bright and yellow compared to its nearby night-sky companion Rigel who is very bright but shines white or even blueish some nights. Rigel must be hotter than Betelgeuse! 

Doppler Effect: Light can also tell us the speed of these distant objects through the Doppler effect. If a star or gas cloud is moving towards us, the light emitted from the object will be measured with a higher frequency, shorter wavelength, and thus higher energy than the true light that left the object. Because shorter wavelengths of visible light are bluer, the Dopper shift wave we observe from this approaching light source is “blue-shifted”. If the light source is instead moving away from us, where we observe the lightwaves as longer wavelengths, frequency, and hence less energy, then the fleeing light source is “red-shifted”. Even when the lightwaves are Doppler shifted outside of the visible light spectrum, we adopt the same terminology. So even if a star’s ultraviolet spectrum is actually observed in the visible light spectrum due to the star moving away from us, we still call this spectrum “red-shifted” to indicate that the spectrum is observed at lower energies than the original light emitted.  

We can study each element and their “rest” spectra in our laboratories. This is how we know what the spectra of elements like Hydrogen, Helium, Sodium, and so on, look like with each having a unique “fingerprint” light spectrum. We can compare the spectrum emitted by stationary neutral Hydrogen with the observed wavelength of the object we are studying. In this way, we can measure exactly how blue- or red-shifted the Hydrogen spectrum is from its spectrum at rest.

Want to know more?

Check out Chapter 5: Light and Matter in the textbook The Cosmic Perspective by Bennet, Donahue, Voit,  and Schneider, Seventh Edition, 2014. Pgs 158-172.

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