1 Nature of Light Wave Properties Light is a self- propagating electro- magnetic wave –A time-varying electric field makes a magnetic field –A time-varying magnetic field makes an electric field Wavelength (or frequency) are related to energy Wave amplitude  brightness Angle of field lines  polarization

2 Nature of Light Particle Properties Photons have energy, but no mass. Photon flux  brightness

3 Nature of Light Properties Short wavelength, λHigh Frequency, νHigh Photon EnergyBlue Long wavelength, λ Low Frequency, νLow Photon EnergyRed Small Amplitude - FaintLow Photon Flux Faint Large Amplitude - BrightHigh Photon Flux Bright

4 What is a Spectrum Brightness or Intensity as a function of Energy May equivalently shown be shown as a function of wavelength or frequency Describes the energy distribution of the observed light, or Describes how much of the observed light has what energy value

5 Wien’s Law Wien’s law allows astronomers to determine the temperature of a star. The wavelength at which a star is brightest is related to its temperature Hotter objects radiate more strongly at shorter wavelengths Blue has a shorter wavelength than red, so hotter objects look bluer. Objects can emit radiation at many different wavelengths. The wavelength at which a star is brightest is related to its temperature. This is Wien’s Law

6 When can you use Wien’s Law? Only for objects that emit light not for those that reflect light Light emitted by hot, solid objects obey Wien’s Law Can not use with gases unless they are of a high density The Sun and other stars obey Wien’s Law since the gases they are composed of remain at a high density (at least up to the outermost layers of the star).

7 Why Learn about Atomic Structure? Knowing the structure of atoms tells us about their –chemical properties –light-emitting properties –light-absorbing properties From this information we can learn about galaxies, stars, planets, asteroids, based on the light they emit or reflect. An example of absorption spectra from many different types of stars.

8 Atomic Structure An atom is composed of a dense core called a nucleus and surrounding this nucleus one or more negatively charged electrons. The nucleus is composed of positively charged protons and neutral neutrons. The electron is also 1,800 times lighter than a proton. Protons and neutrons however weigh about the same. The electric force of attraction between the positive protons and negative electrons keeps the electrons bound to the nucleus.

9 Atomic Structure An atom is mostly empty space because the electron moves around the nucleus at such a great distance. If the proton were 1 cm wide a hydrogen atom would be larger than a football field! A chemical element is determined based on how many protons the nucleus contains (Hydrogen has 1, Carbon has 6, Oxygen has 8 protons). When two atoms with the same number of protons have different numbers of neutrons the two atoms are isotopes of one another (Carbon has 6 protons but can have 6, 7, or 8 neutrons).

10 Ions An atom normally has the same number of electrons as protons. With the same number of positive and negative charges, an atom normally has no net charge. If an atom loses or gains one or more electrons it has become ionized. With too few electrons, the atom has a net positive charge, too many electrons and it has a net negative charge.

11 Quantum Structure of an Atom An electron does not orbit a nucleus like a planet orbits the Sun. For a given atom there are only a select few orbits that an electron can occupy. This means that the orbits are quantized. Electrons may shift between these quantum levels with either the emission or absorption of a photon of electromagnetic radiation.

12 Energy Level Transitions If an electron absorbs a photon of light it can shift to a higher quantum level –Atom (or ion) gains energy If an electron emits a photon of light it can shift to a lower quantum level –Atom (or ion) loses energy The energy and wavelength of the photon in both cases depends on the energy difference between the two quantum levels.

13 Spectra and Atomic Structure Each type of atom has a unique set of wavelengths of light that it can absorb and emit –Hydrogen emits red light at 656 nm, blue light at 486 nm and other lines –Hydrogen will also absorb only red light at 656 nm, blue light at 486 and a few other lines We use this to identify the atoms present by studying the spectrum of an object

14 Emission or Absorption Spectra The brightness of an emission line or the darkness of an absorption line indicates how many atoms are absorbing or emitting that color. The number of atoms absorbing or emitting depends on the number present and on the temperature of the gas.

15 Types of Spectra Continuous Spectra - from hot, dense or solid objects. Emission Line Spectra - from hot, tenuous (thin) gas. Absorption Line Spectra - from cold, tenuous gas through which light from a hot, dense object passes.

16 Types of Spectra Continuous Spectra - atoms/ions are closely packed-outer electron orbits are distorted-more (any/all) energy transitions are allowed. Emission Line Spectra - electron drops to lower energy level and emits a photon. Absorption Line Spectra – atom (or ion) absorbs a photon; electron is raised to a higher energy state.

17 Continuous and Absorption Spectra

18 Emission Spectrum

19 Types of Spectra

20 Doppler Shift If a source of light is moving towards or away from an observer its spectral lines are shifted based on the speed and direction The faster the object is moving the greater the shift Shifts to longer wavelengths means object is moving away. Shorter, coming closer.

21 Doppler Shift Radar guns used by police to catch speeders use Doppler shift to determine speed. Astronomers refer to an increase in wavelength (object moving away) as a redshift. A decrease in wavelength (object moving closer) is called a blueshift. This technique is also used to search for planets around other stars.

22 Redshift / Blueshift

23 The Motion of Stars This star is moving away from Earth. The light from this star will appear redshifted.

24 Light and Atoms Summary 1 Wien’s Law –hot objects emit most of their light at short wavelength’s (high energy) – cool objects emit most of their light at long wavelengths (low energy) Blue stars are hot – red stars are cool. Atoms in thin gases emit or absorb radiation at discrete wavelengths (energies) –the wavelengths of the spectral lines depend on the types of atomic or ionic elements present in the gas –the strength of the spectral lines corresponds to the temperature and density of the gas. Hot solid objects or hot dense gases (stars) emit continuous spectra.

25 Light and Atoms Summary 2 Different atoms emit/absorb light at different wavelengths Observe in as many wavelengths (or energy bands) as possible to fully understand a physical system. A picture is worth a thousand words, a spectrum is worth a thousand pictures.

26 Light and Atoms Summary 3 Spectral lines from a moving source will move to shorter wavelengths (blueshift) if the source moves towards the observer, or move to longer wavelengths (redshifts) if the source moves away from the observer. The velocity of an object towards (blueshifted) or away from (redshifted) an observer can be determined by measuring the wavelengths of observed object’s spectral lines relative to a fixed reference spectrum.

27 Light and Atoms Summary 4 Light (especially spectra) gives us –Temperature –Chemical Composition What elements are present and their relative abundances –Density –Line of sight velocity