Chapter 2 Light and Matter

Chapter 2Light and Matter

Chapter 2Part One

Ideas in Chapter 2 Information from the Skies Waves in What? The Electromagnetic Spectrum Thermal Radiation Spectroscopy The Formation of Spectral Lines The Doppler Effect

Question 1 a) gamma rays b) infrared c) sound d) visible light e) radio Which of these is NOT a form of electromagnetic radiation?

Question 1 a) gamma rays b) infrared c) sound d) visible light e) radio Which of these is NOT a form of electromagnetic radiation? Sound comes from pressure waves; all others are types of EM radiation of different wavelengths.

2.1 Information from the Skies Electromagnetic radiation: Transmission of energy through space without physical connection through varying electric and magnetic fields Example: Light

2.1 Information from the Skies Wave motion: Transmission of energy without the physical transport of material

2.1 Information from the Skies Example: Water wave Water just moves up and down. Wave travels and can transmit energy.

2.1 Information from the Skies Frequency: Number of wave crests that pass a given point per second units of Hertz (Hz) Period: Time between passage of successive crests Relationship: Period = 1 / Frequency Wave with frequency of 1000 Hz Period = 1/1000 Hz 0.001 s or 1 ms

2.1 Information from the Skies Wavelength: Distance between successive crests Velocity: Speed at which crests move Relationship: Velocity = Wavelength / Period 1 m/s = 1 m / 1 s 3x108 m/s = 660 nm / 2.2 x10-15 s

What is the Frequency? 3x108 m/s = 660 nm / 2.2 x10-15s Period = 1 / Frequency Frequency = 1 / period 1 / 2.2x10-15 s = 4.54x1014 Hz

Question 2 a) wavelength b) frequency c) period d) amplitude e) energy The distance between successive wave crests defines the ________ of a wave.

Question 2 a) wavelength b) frequency c) period d) amplitude e) energy The distance between successive wave crests defines the ________ of a wave. Light can range from short-wavelength gamma rays to long-wavelength radio waves.

2.2 Waves in What? Diffraction: The bending of a wave around an obstacle Interference: The sum of two waves; may be larger or smaller than the original waves

2.2 Waves in What? Water waves, sound waves, and so on, travel in a medium (water, air, …). Electromagnetic waves need no medium. Created by accelerating charged particles

2.2 Waves in What? Magnetic and electric fields are inextricably intertwined. A magnetic field, such as the Earth’s shown here, exerts a force on a moving charged particle.

2.2 Waves in What? Electromagnetic waves: Oscillating electric and magnetic fields; changing electric field creates magnetic field, and vice versa

2.3 The Electromagnetic Spectrum Different colors of light are distinguished by their frequency and wavelength. The visible spectrum is only a small part of the total electromagnetic spectrum.

2.3 The Electromagnetic Spectrum Different parts of the full electromagnetic spectrum have different names, but there is no limit on possible wavelengths.

2.3 The Electromagnetic Spectrum The atmosphere is only transparent at a few wavelengths – the visible, the near infrared, and the part of the radio spectrum with frequencies higher than the AM band. This means that our atmosphere is absorbing a lot of the electromagnetic radiation impinging on it, and also that astronomy at other wavelengths must be done above the atmosphere.

Question 3 a) radius. b) mass. c) magnetic field. d) temperature. e) direction of motion. The frequency at which a star’s intensity is greatest depends directly on its

Question 3 a) radius. b) mass. c) magnetic field. d) temperature. e) direction of motion. The frequency at which a star’s intensity is greatest depends directly on its Wien’s Law means that hotter stars produce much more high- frequency light.

2.4 Thermal Radiation Blackbody spectrum: Radiation emitted by an object depending only on its temperature

The Kelvin Temperature Scale • Kelvin temperature scale: • All thermal motion ceases at 0 K. • Water freezes at 273 K and boils at 373 K.

2.4 Thermal Radiation Radiation laws: Peak wavelength is inversely proportional to temperature.The higher the temperature the shorter the wavelength

Temperature vs. Wavelength

2.4 Thermal Radiation Radiation laws: 2. Total energy emitted is proportional to fourth power of temperature.

Question 4 Rigel appears as a bright bluish star, whereas Betelgeuse appears as a bright reddish star. Rigel is ______ Betelgeuse. a) cooler than b) the same temperature as c) older than d) hotter than e) more massive than Betelgeuse The constellation ORION Rigel

Question 4 Rigel appears as a bright bluish star, whereas Betelgeuse appears as a bright reddish star. Rigel is ______ Betelgeuse. a) cooler than b) the same temperature as c) older than d) hotter than e) more massive than Betelgeuse Hotter stars look bluer in color; cooler stars look redder. The constellation ORION Rigel

Chapter 2Part Two

First exam on September 14th, 2011 Mastering Astronomy 50 unregistered students 4:00 PM to 5:30 PM Class ID MAMILLER18823

i>clickers 20 unregistered clickers 50 unregistered students Great clicker shortage of 2011

Supernova SN2011fe Brightness of a Billion Suns Coming to a Galaxy near you!

Look at the Big Dipper with a small telescope or binoculars

Spectroscopy Spectroscope: Splits light into component colors

Spectroscopy Emission lines: Single frequencies emitted by particular atoms

Spectroscopy Emission spectrum can be used to identify elements.

Spectroscopy Absorption spectrum: If a continuous spectrum passes through a cool gas, atoms of the gas will absorb the same frequencies they emit.

Spectroscopy Absorption spectrum of the Sun

Spectroscopy • Kirchhoff’s laws: • Luminous solid, liquid, or dense gas produces continuous spectrum. • Low-density hot gas produces emission spectrum. • Continuous spectrum incident on cool, thin gas produces absorption spectrum.

Spectroscopy Kirchhoff’s laws illustrated

The Formation of Spectral Lines Existence of spectral lines required new model of atom, so that only certain amounts of energy could be emitted or absorbed. Bohr model had certain allowed orbits for electron.

Bohr Model

The Formation of Spectral Lines Emission energies correspond to energy differences between allowed levels. Modern model has electron “cloud” rather than orbit.

Question 1 a) depend on its temperature. b) are identical to its absorption lines. c) depend on its density. d) are different than its absorption lines. e) depend on its intensity. The wavelengths of emission lines produced by an element

Question 1 a) depend on its temperature. b) are identical to its absorption lines. c) depend on its density. d) are different than its absorption lines. e) depend on its intensity. The wavelengths of emission lines produced by an element Elements absorb or emit the same wavelengths of light based on their electron energy levels.

Excited State 1 Excited State 2

Excitation

Chapter 2 Light and Matter