Light and Atoms
Properties of Light We can come to understand the composition of distant bodies by analyzing the light they emit This analysis can tell us about the composition as well as the temperature There are limits to what we can learn from the ground here on Earth. Why?
Properties of Light Radiant Energy: travels through empty space without a need for a direct physical link Travels at 299,792.5 km/s = the constant c (the upper limit to all motion) Can circle the Earth in 1/7 of a second The speed of light is reduced when it passes through transparent materials like glass, water and gases Different colors of light are slowed down differently (blue travels more slowly than red)
The Nature of Light We have a dual model An electromagnetic wave A stream of particles called photons Electricity and magnetism fluctuate back and forth allowing the wave to propagate itself an electric field creates a magnetic disturbance which in turn creates a new magnetic field The wave model does not work to explain the different behavior of light
Electromagnetic Waves
The Nature of Light Particles of light are called photons Some properties of light are better explained by the particle (photon) model We will use mostly the wave model
Particle Model
Light and Color The visible part of the electromagnetic spectrum is what we can see with our eyes The color of the light is determined by the wavelength = λ (lambda) the distance between wave crests Deep red = 7 X 10-7 m or 700 nanometers Violet = 4 X 10 7 m or 400 nanometers
Light and Color The shorter wavelengths tend towards the blue (Carry the most energy) The longer wavelengths tend towards the red We usually express these light wavelengths in nanometers (nm)
Electromagnetic Spectrum
Frequency Frequency: the number of wave crests passing a given point in 1 second Measured in hertz = ν (nu) λ ν = c
White Light Some light seems to have no color White light is a mixture of all colors a blend of all the wavelengths of visible light Newton passed white light through a prism which split the white light into all the colors of the spectrum He also recombined all the colors by passing them through a lens and reconstituted the white light
Infrared Infrared just beyond the red Infrared discovered by Sir William Herschel Infrared wavelength is longer than visible light
Ultraviolet Shorter wavelength than the visible Discovered in 1801 by J. Ritten
Radio Waves Predicted by James Clerk Maxwell in the mid-1800s. Produced experimentally by Heinrich Hertz in 1888 Discovered coming from the cosmos by Karl Jansky in 1930s
Radio Waves Range in length from a few millimeters to hundreds of meters Communications Radar Microwave ovens Radio telescopes SETI
X-rays Discovered by William Roentgen in 1895 Detected in space in 1940 Shorter wavelengths than visible light Help detect black holes
Gamma Rays and Region between Regions not well explored Infrared and Radio Waves Both of these areas are blocked by Earth s atmosphere making it difficult to study them form Earth
Energy carried by EM waves Different wavelengths carry different amounts of energy E = hc speed of light c and h are constant λ An inverse proportion if wavelength increases energy decreases if wavelength decreases energy increases (why UV light gives you sunburn and IR does not)
Wien s Law The wavelength at which a body radiates most strongly is inversely proportional to the body s temperature (hotter bodies radiate more strongly at shorter wavelengths) Using this law we can now measure how hot an object is simply from the color of the light it radiates most strongly This law is fairly accurate for most stars and planets
Using Wien s Law If we know the wavelength of the strongest radiation from a body we can determine its temperature T (K) = 3 X 10 6 / λ What is the surface temperature of a star that emits light the strongest at 300 nm? 3000000 300 = 10,000 K
Black Bodies Black Body: an object that absorbs all of the radiation falling upon it It reflects no light They radiate more efficiently than any other type of body Very few objects are perfect black bodies Most objects we study in space are close enough to black bodies to obey Wien s law with little error
The Structure of Atoms
Atomic Structure
Formation of a spectrum Spectroscopy breaking light down into its component parts Each atom has a spectral signature of certain amounts of light present at each wavelength Electrons moving from one orbital to another produce different kinds of light When electrons fall from a higher energy level to a lower energy level they give off light Sodium = Yellow light, Strontium = red light, Copper = green light are some examples
Emission spectrum
The Doppler Shift
Absorption in the Atmosphere Gases in the atmosphere affect the flow of heat and light Very little visible light is absorbed Infrared and UV are strongly absorbed by carbon dioxide and water X-rays and gamma rays are strongly absorbed by oxygen and nitrogen No EM radiation of wavelengths shorter than 300 nm reach the Earth
Light in the Atmosphere Refraction and Dispersion Refraction distorts the Sun s shape when it rises and sets Also makes the stars twinkle Dispersion make the flashing colors seen in twinkling stars
The Moon Illusion Why does the moon appear larger sometimes Not fully understood, but it is an optical illusion based on what our mind perceives the relationship is to the objects in the foreground
Twinkling Stars Called scintillation Caused by differences in the air density through which the starlight is passing due to subtle temperature differences in the atmosphere
Atmospheric Scattering Creates the blue color of the sky during daylight If there were not atmosphere to scatter the light the daytime sky would be black Atmosphere hardly affects the red wavelengths of light Blue scatters strongly Large particles scatter light evenly clouds are white Small molecules (like nitrogen and oxygen) scatter shorter wavelengths like blue