 So our journey through spectroscopy begins about 150 million kilometers in that direction, the Sun, which is showering us with, well, at the moment far too many photons, as you can see, and they will travel across space, they will hit the upper atmosphere, they will dodge various molecules from ozone to carbon dioxide and carbon monoxide and so on, and land somewhere here. Now what happens next depends on a number of different factors, but we're mostly going to be talking about frequency and energy, and that brings us to our first equation, Planck's relation. We'll unpack this a little more as the course goes on, but I first want to draw your attention to how this is, like many physics equations, simply two values linked by a proportionality constant. The same is true of famous equations like E equals MC squared, and other thermodynamic equations like the ideal gas law. This makes the Planck relation less of an equation for something and more of a definition, and a key relationship between two physical properties. This equation relates energy E to frequency nu, and the proportionality constant between them is H, Planck's constant, named after Max Planck, who first proposed that energy is quantized and related to frequency this way. This equation tells us the energy of a photon of light. In many respects, saying that a photon carries with it a specific frequency is the same as saying that it carries with it a specific energy. They're the same thing. So properties related to frequency, like wavelength, are also a measure of the energy of the photon. The nature of this photon is a question for the quantum mechanics course. So for now we're just going to blindly accept that a photon has a frequency and wavelength, which means the electric and magnetic fields are oscillating back and forth a certain number of times per second, while also accepting that the photon is a single discrete particle that can't be broken down with a set position and momentum. So when it comes to visible light and ultraviolet light, the energy of a photon is enough to start moving electrons around, and if you can move electrons around, that's a chemical change. From quantum mechanics, we know that electrons are bound to discrete states that we call orbitals, a bonding and anti-bonding orbital for instance. It takes a set amount of energy to move an electron from one orbital to another. Only this much energy delivered in a single burst will do it. If this photon has the right frequency and therefore the right energy, it penetrates deep into the cells of the leaf, and into the chloroplast where it's absorbed by a magnesium-containing porphyrin called chlorophyll, and kick-starts the light-dependent photosynthesis reaction. But if it's the wrong frequency, it's too much energy or too little energy, it bounces off and into our eye. And if it's not the right frequency or energy for photosynthesis, it might just be the right frequency to stimulate the green cone cells in our eyes. And that means that colour, this thing that we take for granted around us every day, is one of the first bits of evidence for quantum mechanics, and is itself a very crude but effective form of electronic spectroscopy.