Astronomical Concepts – Week 4

This week’s main topic was light.

James Maxwell

James Maxwell was a Scottish scientist who lived from 1831 to 1879. He is well known for the research he did on electromagnetism and light, building on the work of Michael Faraday. He produced a set of equations that explain the properties of magnetic and electric fields which helped to show that light was an electromagnetic wave. He was able to bring together well established laws of electricity and magnetism and with Faraday’s law they could imply that any disturbance in the electric and magnetic fields will travel out together in space at the speed of light.

He also described Saturn’s rings as numerous small particles, and this theory was proved later on in the 20th century by a space probe.

Einstein’s Photoelectric experiment

Einstein’s experiment showed that packets of light, called photons, contained a fixed amount of energy that depends on the light’s frequency. When a metal plate is exposed to light electrons are expelled. This is the photoelectric effect. The effect was discovered by German scientist Heinrich Hertz in 1887. His observations showed there was an interaction between light and matter. But Albert Einstein was needed to explain the theory further in 1905, his theory of light. Einstein said that light is a particle, called a photon. Einstein was awarded the Nobel Prize in physics in 1921 for his experiment.


Spectroscopy is a technique used to measure the light that is emitted or absorbed or scattered by materials. It breaks light into its component parts and this information can be used to identify and quantify those materials.


When light is absorbed or reflected by materials not all light behaves the same way. Only certain wavelengths of light are absorbed other get reflected. When you seperate the light that is passing through a sample you end up with an emission spectrum or absorption line.

An emission spectrum in the visible light range may look like this.


A spectrum like this would be created when material is given extra energy and that extra energy is later emitted as light energy.

An absorption spectrum would look like this.


A spectrum like this is created when light is passed through a gas or liquid or strikes a solid. Certain wavelengths of light will be absorbed by the material and later emitted in random directions. Most wavelengths will pass through the material without being absorbed. sun_spectrum

The image above shows a spectrum of our sun. From this spectra astronomers can tell what elements the sun is made from, for example hydrogen and helium. It is like a rainbow with holes, the holes are coming from the absorption of energy at a particular wavelength, at a particular colour, by the atoms in the cloud. This goes back to the energy levels of the atom, of only taking energy at very particular energies, as electrons move from one excited state to another excited state. So what you’re seeing is the absorption of photons by atoms. When energy is absorbed you are seeing the energy raising the energy level of an electron. So, we see the rainbow because the inside of the sun is hot and it emits a continuous thermal spectrum. The atoms in the outer layer of the sun absorb some of the energy and use it to promote electrons from a low energy level to a high energy level.


Astronomers also take pictures of light, usually through a filter. Astronomers want to see what the light looks like in red light or green light.

Astronomers also do timing with light, which means to measure the brightness or phase changes as things happen in time.

The combination of spectroscopy, imaging and timing can tell us all kinds of information from the thing we are looking at. We can tell hoe fast something is rotating, if it is moving towards or away from us, temperature, density and composition. Light is our spaceship, we can’t travel to stars and planets light years away but light does travel to us, it is the only way to get the information we need.

Doppler effect

Light waves from a moving source experience either a red shift or a blue shift in the lights frequency. A light source moving away from a stationary observer causes a shift towards the red end of the light spectrum, called a red shift. When the light source moves towards an observer the frequency shifts towards the blue end of the spectrum.


Why is the sky blue?

The sky is blue because atoms in our atmosphere scatter blue light more than they scatter red light. When we look towards the sun at sunset we see red and orange because the blue light has been scattered away from the line of sight. Blue is scattered more because it travels as small and short waves.

That’s enough about light, next week is all about the sun.


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