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Unformatted text preview: Astronomy Picture of the Day
http://antwrp.gsfc.nasa.gov/apod/astropix.html The zodiacal light Light and Radiation Light is a wave phenomenon To understand how light behaves, we need to understand the general properties of waves Waves Waves are periodic disturbances propagating through some medium. Some examples: Waves on the surface of a body of water Sound waves propagating through air Waves propagating along a vibrating string Light waves Important point: the wave disturbance travels through the medium: the medium itself does not travel with the wave Properties of waves Amplitude: the height of a wave crest A Properties of waves Wavelength: the distance between two successive wave crests (or two successive troughs) Wavelength is denoted by the Greek letter (lambda) Properties of waves Frequency: the number of wave crests passing a fixed point each second Frequency is denoted by the Greek letter (nu) Units for frequency: 1 Hertz (Hz)= 1 wave cycle per second Count how many wave crests pass a fixed marker in 1 sec A note about units One Hertz means one event (of some kind) per second In other words: the unit Hz is the same as 1/second Properties of waves
For high frequencies, use units of kilohertz (kHz): 1000 cycles per second megahertz (MHz): 1 million cycles per second gigahertz (GHz): 1 billion cycles per second Count how many wave crests pass a fixed marker in 1 sec Properties of waves Speed or velocity: how fast a wave crest is traveling Wave speed measured in meters per second velocity v Example: 2 waves moving with the same speed
Shorter wavelength, higher frequency Longer wavelength, lower frequency Properties of waves
Key relationship: (length per wave) (waves per sec) = (length per sec) = speed of wave wavelength frequency = speed = v Light Waves For the speed of light (in vacuum), we use the letter c c is a universal constant of nature c = 3108 meters per second = c Light Waves Example: Orange light has a wavelength of around 600 nm. What is its frequency? c = 3108 m/sec = 600 nm = 60010-9 m = c/ = (3108 m/sec) / (60010-9 m) = 51014 (1/sec) = 51014 Hz Units for energy and power The standard unit for energy is the Joule ( J ) This is roughly the energy you'd need to lift a 1-kilogram mass by a height of 10 centimeters from Earth's surface Power means energy produced or expended per unit time The unit of power is the Watt 1 Watt = 1 Joule per second Electric and Magnetic Fields Electric charges create "fields" around them which attract or repel other charged objects Magnetic fields exert forces on other magnetic objects But instead of individual charges, every magnetic object has both N and S poles + Electric field around a charged object N S Magnetic field around a bar magnet Electromagnetic waves By "jiggling" an electrically charged particle back and forth, we can set up oscillations in the electric field surrounding it e
jiggle the electron up and down A changing electric field creates a magnetic field, and a changing magnetic field creates an electric field An oscillating electric charge sets up a pattern of oscillating electic and magnetic fields: an electromagnetic wave Electromagnetic waves Light is an electromagnetic wave: a series of oscillating electric and magnetic fields propagating through space at speed c = 3105 km/sec Arrows represent the direction and strength of the electric and magnetic fields along the light ray The electric field and magnetic fields are directed perpendicular to each other, and perpendicular to the wave propagation direction The Visible Spectrum We see different wavelengths of light as different colors White light is a mixture of all colors Light can be dispersed into its constituent colors by a prism The spectrum of visible light Visible light wavelengths range from about 400 to 700 nm From long to short wavelengths, colors are ROYGBIV The full electromagnetic spectrum
Visible Light Microwave Ultraviolet Gamma-ray X-ray Infrared Radio TV FM AM 101 102 103 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 Wavelength (meters) Photons Light has both wave and particle properties Electromagnetic radiation comes in discrete, particle-like packets, called photons White light contains a mixture of photons of different wavelengths Photons and Energy A photon has no mass, but carries energy The energy of a photon is proportional to its frequency: E = h where h = Planck's Constant h = 6.63 10-34 J sec A 100 W light bulb emits more than 1020 photons each second! The energy of a single visible photon is tiny! Visible Light Microwave Ultraviolet Gamma-ray X-ray Infrared Radio TV FM AM 101 102 103 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 Wavelength (meters) Smaller Higher Higher Wavelength Frequency Photon Energy Larger Lower Lower The Spectrum of Light We can describe an object's spectrum with a plot of the amount of energy emitted as a function of wavelength, like this: Energy A continuous spectrum includes light over a broad range of wavelengths V I B G Y O R Wavelength Colors of Spectra A redder spectrum is one that peaks at longer wavelengths A bluer spectrum is one that peaks at shorter wavelengths Bluer spectrum Redder spectrum Energy V I B G Y Wavelength O R Black-body Radiation "Black-body": an object which absorbs all light incident on it A black body does not reflect or transmit any light The shape of the spectrum of light emitted by a black body depends only on its temperature (and not on other properties, such as its chemical composition, density, etc.) A black body is also a perfect emitter of light The energy spectrum of light emitted by a black body was first explained by Max Planck in 1900 The Spectrum of Black-Body Radiation
T = 7000 K
A black-body with higher temperature has a spectrum that peaks at shorter wavelengths T = 5800 K T = 4500 K Examples
Lava flow: the reddish color comes from black-body radiation from lava at a temperature of around 800 K Incandescent bulb: electric current heats the tungsten filament to around 2800 K Examples
Objects at around room temperature radiate in the infrared part of the electromagnetic spectrum Properties of black-body radiation If we compare 2 black-body emitters having the same size but different temperatures, the hotter black-body will: emit more radiation overall (i.e., it will be more luminous) emit more photons at any given wavelength have a spectrum peaking at shorter wavelengths have a bluer color The wavelength at which a star's spectrum peaks is a measure of its surface temperature ...
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