PHYS 206 Matter and Light At least 95% of the celestial information we receive is in the form of light. Therefore we need to know what light is and where it comes from.

PHYS 206 Electric and Magnetic Fields Electrical charges and magnets alter the region of space around them so that they can exert forces on distant objects. James Clerk Maxwell proposed that if a changing magnetic field can make an electric field, then a changing electric field should make a magnetic field. A consequence of this is that changing electric and magnetic fields should trigger each other and these changing fields should move at a speed equal to the speed of light. Maxwell also said that light is an electromagnetic wave.

PHYS 206 Electromagnetic Radiation Light, electricity, and magnetism are manifestations of the same thing called electromagnetic radiation. This energy also comes in many forms that are not detectable with our eyes such as infrared (IR), radio, X-rays, ultraviolet (UV), and gamma rays.

PHYS 206 Properties of EM Radiation It can travel through empty space. Other types of waves need some sort of medium to move through: water waves need liquid water and sound waves need some gas, liquid, or solid material to be heard. The speed of light is constant in space. All forms of light have the same speed of 300,000 kilometers/second in space (often abbreviated as c). The forms of light are Gamma rays, X-rays, Ultraviolet, Visible, Infrared, Radio. A wavelength of light is defined similarly to that of water waves---distance between crests or between troughs. Visible light (what your eye detects) has wavelengths Ångstroms. 1 Ångstrom = meter. Visible light is sometimes also measured in nanometers: 1 nanometer = meter. Radio wavelengths are often measured in centimeters. The abbreviation used for wavelength is the greek letter lambda.

PHYS 206 Intensity and Energy The energy of the EM radiation depends only on the wavelength (frequency), the shorter the wavelength, the higher the energy (blue is more energetic than red). The type of EM radiation produced by an object will also depend on its energy (temperature). Temperature is a measure of the random motion (or energy) of a group of particles. Higher temperature (T) means more random motion (or energy). Blue stars have a higher temperature than red stars.

PHYS 206 Refraction The direction of light propagation can be changed at the boundary of two media having different densities. This property is called refraction. To decode the information stored in light, you pass the light through a prism or diffraction grating to create a spectrum. If white light is examined, then the spectrum will be a rainbow. Different frequencies will break differently at the interface.

PHYS 206 Spectrum of visible light Continuous spectrum Discrete spectrum Emission spectrum Absorption spectrum Hot objects give thermal spectrum (continuous spectrum). White light has a continuous spectrum. Even though all the colors are present in the spectrum we can still see a different color.

PHYS 206 Temperature Dependence of Spectra All “hot” objects give out light. The shape of the spectrum depends only on the temperature of the object. As the temperature of an object increases, more light is produced at all wavelengths than when it was cooler. As the temperature of an object increases, the peak of thermal spectrum curve shifts to smaller wavelengths. When you add up all of the energy of all of the square meters on the object's surface, you get the luminosity---the total amount of energy emitted every second by the object. Luminosity is proportional to the fourth power of temperature.

PHYS 206 Discrete Spectrum Emission spectra are produced by thin gases in which the atoms do not experience many collisions (because of the low density). A continuum spectrum results when the gas pressures are higher, so that lines are broadened by collisions between the atoms until they are smeared into a continuum. An absorption spectrum occurs when light passes through a cold, dilute gas and atoms in the gas absorb at characteristic frequencies. Where do these characteristic frequencies come from?

PHYS 206 Atoms and Matter Niels Bohr provided a solution (Bohr atom). Electrons can be only found in energy orbits of a certain size (quantization). As long as the electron is in one of those special orbits, it would radiate no energy. The massive but small positively-charged protons and massive but small neutral neutrons are found in the nucleus. The small, light negatively-charged electrons move around the nucleus in certain specific orbits (energies). In a neutral atom the number of electrons = the number of protons. All atoms with the same number of protons in the nucleus are called an element.

PHYS 206 Bohr Atom Elements are divided into sub- groups called isotopes based on the number of protons AND neutrons in the nucleus. All atoms of an element with the same number of neutrons in the nucleus are of the same type of isotope. When an atom has an extra positive or negative charge, than it is called an ion. Electrons have only certain energies corresponding to particular distances from nucleus. As long as the electron is in one of those energy orbits, it will not lose or absorb any energy. The orbits closer to the nucleus have lower energy. Atoms want to be in the lowest possible energy state called the ground state.

PHYS 206 Emission Spectrum An emission line is produced by an atom in an “excited” energy state. The electron is not in as low an energy orbit as possible. In order to go to a lower energy orbit, the electron must lose energy of a certain specific amount. The energy of photon = the difference in energy of the energy orbits. The intensity depends on the density and temperature of the gas.

PHYS 206 Absorption Spectrum An absorption line is produced when a photon of just the right energy is absorbed by an atom, kicking an electron to a higher energy orbit. Other photons moving through the gas with the wrong energy will pass right on by the atoms in the thin gas. They make up the rest of the continuous spectrum you see. The more atoms undergoing a particular absorption transition, the darker the absorption line. The strength of the absorption line depends on the density and temperature.

PHYS 206 Atmospheric Windows Our atmosphere also absorps some of the radiation as well. Absorption is maximum for X-rays and UV rays, and minimum for visible and radio radiation.