Key Concepts: Lecture 19: Light Light: wave-like behavior Light: particle-like behavior Light: Interaction with matter - Kirchoff s Laws The Wave Nature of Electro-Magnetic Radiation Visible light is just one form of electromagnetic radiation Light often behaves like a wave Diffraction: waves bend around obstacles Interference: waves add their amplitudes together Recall: The Importance of Light We can not hope to visit objects outside the solar system in our lifetime. The easiest way we can learn about distant objects is by studying the light they emit. Using information in their light, we can measure compositions, temperatures, distances, masses and ages of astronomical objects. As light travels at a finite speed, by looking further and further away we look back further and further into the past. Recall: Wave Nature of Electro-Magnetic Radiation All EM radiation is composed of two waves that move together An electric field and a magnetic field The fields are at 90 degrees to each other It needs no medium to be transmitted through, i.e. it can travel through a vacuum Energy is carried by the wave
Recall: Properties of Waves A wave is described by four properties The wavelength (λ) - units of length The amplitude of the wave The speed of the wave (c) - units of speed length/time The frequency of the wave (f) - units of 1/time Three of these properties are interrelated: c = f x λ Light Also Behaves Like A Particle! When light interacts with matter, it often behaves like a particle A particle of light is called a Photon First suggested by Max Planck (1899) The energy of a particle of light (photon) depends only on its frequency h is a constant known as Planck s constant Photoelectric effect - Einstein (1905) Certain metals emit electrons when exposed to light. Basis of exposure meters, night vision goggles etc... This only happens when a high enough frequency (i.e. short enough wavelength) light is used, because a certain amount of energy is needed to free each electron from the metal and that energy must come from a single photon of high enough energy. Lots of low energy photons don t work. E = h f Recall: Wave-Particle Duality EM spectrum extends from radio to gamma rays The Earth s atmosphere only transmits certain frequencies Light sometimes behaves like a particle, sometimes like a wave. Traditional particles, e.g. electrons, protons, bullets, when moving also sometimes behave like waves, but the effects are only noticeable on scales of the wavelength, which are typically minuscule and too small to see in everyday life. Quantum mechanics describes the behavior of these wave-particles. All objects have this behavior, but in everyday life we don t notice it. particle-like wave-like
Wave-Particle Duality Video http://www.youtube.com/watch?v=dfpeprq7ogc The Interaction of Light with Matter The possible energies which an electron can have are quantized: only certain particular energies are allowed. Electrons and photons can exchange energy An atom can absorb a photon by moving an electron to a higher energy state An atom can emit a photon by an electron moving to a lower energy state The change in the energy of the electron must exactly equal the energy of the photon The Interaction of Light with Matter Matter is composed of atoms Atoms have nuclei composed of Protons (+ve electric charge) Neutrons (neutral) The nuclei are surrounded by a cloud of Electrons (-ve electric charge) The number of protons determines what element it is. To be electrically neutral the number of electrons must equal the number of protons Light typically interacts with matter by interacting with the electrons around atoms The Stair Case Analogy You can think of the levels of an atom as being like a flight of stairs To step up to a higher level requires enough energy to lift you one or more stairs When you come back down the stairs you give up the energy you got by going up the stairs Demo...
Finger Printing the Elements Each type of atom (i.e. different elements like Hydrogen, Helium, Carbon, Oxygen, etc) has a unique configuration of electrons Each type of atom has a unique set of possible energy levels Each type of atom emits and absorbs photons with a unique pattern of energies Kirchhoff s Laws Gustav Kirchoff derived three laws to explain how matter and light interact He noted that three types of spectra arise under different conditions Continuous spectra Emission line spectra Absorption line spectra Continuous spectrum Emission line spectrum Absorption line spectrum Spectra of Atoms The Generation of Light All dense matter with a temperature above absolute zero emits light because of vibrating electric charges (electrons) The spectrum is the amount (intensity) of light at different frequencies (or wavelengths) The peak of the spectrum depends on the temperature: hotter temperature leads to peak at higher frequencies (shorter wavelengths) This is called Blackbody Radiation which is an example of a continuous spectrum frequency
Wien s Law Wien s law tells you at what wavelength (or frequency) the blackbody spectrum peaks (has the maximum value) Hotter objects emit light that peaks at shorter wavelengths (higher frequencies & higher energy photons) Colder objects emit a spectrum that peaks at longer wavelengths (lower frequencies & lower energy photons) λ max = 0.29cm T (K) Kirchhoff s 1 st Law A hot and opaque medium emits a continuous spectrum, because the atoms are very close to each other which smears out the energy levels. This is the blackbody spectrum Use of Wien s Law We can use Wien s law to find the temperatures of distant bodies we can never visit, by measuring the wavelength (or the frequency) at which the spectrum peaks. Kirchhoff s 2 nd Law A hot, transparent, low-density gas produces a spectrum of bright emission lines. The number and colors of the lines depend on which elements are present. Because the atoms are quite far apart the energies of the photons are not smeared out. The emission lines are due to electrons moving from higher to lower energy states
Kirchhoff s 3 rd Law If a continuous spectrum passes through a transparent gas at a lower temperature, the cooler gas will cause the appearance of dark absorption lines, whose colors and number will depend on the elements present in the gas. This is because particular energies are absorbed by the cool gas (electrons are raised to higher energy levels). Then when the electrons fall down and new photons emitted, these photons can go in any direction: most do not go through the slit. An absorption spectrum is produced when light passes through a cold gas Summary of Kirchoff s Laws