Matter and Light

Properties of Matter

  • Different phases: Bulk matter can be in the solid, liquid, or gas form. Sometimes the same substance can be in any of those three phases, depending on its temperature (and pressure); Evaporation, melting are examples of phase transitions.
  • Why? To understand, we need to make a model for what matter is made of, and what happens when we give or take away energy.
  • Atoms and molecules: The smallest units of a simple substance (an element) are atoms, those of a compound substance are molecules, or combinations of atoms held together by chemical bonds.
  • Is there a highest temperature? Probably not; A fourth phase is that of a plasma, ...


  • Heat and energy: Heat is energy and produces random motion. When the temperature rises, atoms or molecules move faster, flow more easily past each other and take up more space. Heat can travel, by conduction, convection or radiation.

  Structure of Matter
  • Structure of an atom: One model is that of a miniature "solar system", with protons (+ charge ) and neutrons in the nucleus, and electrons (– charge) orbiting around it.
  • Variation: The number of p's determines the element (see the periodic table), the number of n's the isotope of that element, the number of e's whether the atom is a neutral one or an ion. A gas can become ionized when T is in the 1000s of K.
  • The quantum model: All particles are also waves, which tell us where there is a higher probability of finding the particle. In an atom or a molecule, those waves can only fit in certain patterns, or energy levels. Particles can only be in one of those states, and jump between them.
  • Changing a nucleus: Some nuclei break up easily, but in general temperatures of millions of K are needed to produce the nuclear reactions that combine or break nuclei.
  • Other particles: Other types are produced in many reactions, like photons, neutrinos... Each type of particle has an anti-particle.
  • How many forces? So far, it boils down to four: Gravitational, electromagnetic, and the strong and weak nuclear forces.
Properties of Light
  • What kinds of light are there? We can see different colors, which we can separate by separating them into a spectrum, either with a prism or with a diffraction grating. There are more kinds of "light", that our eyes can't see, like infrared (IR) and ultraviolet (UV).
  • How does it affect matter? It can be emitted by matter, be absorbed, reflected, scattered or transmitted by it. It can heat matter, because light (radiation) carries heat-energy.
  • How fast does light move? Almost instantaneously, but actually at almost 300,000 km/sec! Usually in a straight line, but there are exceptions: Reflection, scattering as in atmosphere, bending (refraction) in a transparent material, diffraction and interference – which tell us that light is a wave!
  • What does it tell us about the source? How far it is, how it is moving, and also its temperature and what it is made of. To understand how this works, we need to know more about what light is and where it comes from.

  • Relationship: For any wave (sound, water waves, strings, ...), v = λ f.
  Nature of Light
  • Light is a wave: We know because it shows interference and diffraction. Ther waves are oscillating electric and magnetic fields, and can be produced by oscillating charges.
  • Important concepts: Period, wavelength λ, amplitude, frequency f, speed or velocity v.
  • Light is made of particles: We know because of the photoelectric effect; The particles of light are called photons, and for any of them, energy = h (frequency), where h is Planck's constant.
  • So: Light is not that different from matter, they are all both particles and waves!

Spectroscopy and Interaction with Matter
  • The whole electromagnetic spectrum: If we consider all wavelengths, the possibilities are radio waves (the longest ones), microwaves (mm's or cm's), infrared radiation (microns to mm's), visible light (between 400 and 700 nm), ultraviolet light (down to nm's), X-rays (down to 10–11 m), and gamma rays (the shortest ones).
  • What makes objects emit different types? The main factor is temperature; For a dense object that emits a continuous spectrum, it is the only factor; The hotter it is, the brighter the radiation and more energetic the photons.
  • For thin gases: You get an emission line spectrum, with wavelengths that depend on the atoms or molecules, which can emit only certain fixed amounts of energy.
  • Do all waves reach us from space? No, visible light does, many radio waves do; some IR but not much.

  Information from Light and Radiation
  • Different type of spectrum: If a dense source of light is behind a thin cooler gas, we get an absorption spectrum, like the one from the Sun.
  • Effect of distance: The brightness of the light decreases in a specific way with distance.
  • Effect of motion: There is a Doppler shift towards the red if the distance is increasing, towards the blue if it is decreasing.
  • Information on the source: We can often find out what a source of light is made of, how hot it is, how fast it is moving and spinning, and/or how far it is from us.Once reflection from other places has been taken into account!

page by luca bombelli <bombelli at>, modified 11 sep 20012