Light: An Introduction Or You are the light of the world Standard 4e Students know radio waves, light and X-rays are different wavelength bands in the spectrum of electromagnetic waves whose speed in a vacuum is approximately 3 x 108 m/s (186,000 mps)

Light in general Electromagnetic wave Transverse wave Caused by accelerating or vibrating electric charge. Brightness follows inverse square law Intensity ~ 1/d2 Doubling distance means 1/4th the brightness Dual nature of light Question: What type of wave is a light wave? What is the cause of light? How is the intensity of light related to the distance from the source? What is meant by saying light has a dual nature? Activities: 1) Show slide listing general properties of light 2) Do a few brightness calculations 3) Light acts like a wave when in a vacuum or in a gas, it acts more like a solid when interacting with dense solids like metals. So light is both wave and particle, it’s a wavicle

Types of Light: The EM Spectrum All EM waves alike Produced by accelerating electric charge Travel at 300 000 km/s in a vacuum Waves differ in Frequency Wavelength Energy Question: List all the forms of light (EM radiation) from lowest to highest frequency. How are all these types of light similar? How are they different? Indicate the direction of increasing wavelength and energy on your list. Where does sound belong on this list? Activities: Make list of EM spectrum or show slide State commonalities of EM radiation State differences and add wavelength and energy to list or point out wave length size chart and temperature chart Video: Seeing the Invisible Point out that sound does not belong on this list since it requires a medium to travel through and travels much slower than these waves in most substances

Transparent vs. Opaque Transparent Opaque Incident light is not at a natural frequency of the substance. Small forced vibrations pass the energy from molecule to molecule Energy is transmitted through the material Opaque Incident light is at the natural frequency of the substance Resonance causes large vibrations which crash one molecule against another creating heat Energy is not transmitted No sun tan in green house example or how to stay married Glass opaque to UV light Glass transparent to visible light Question: What happens to light waves incident on a surface that are at the natural frequency of the substance? How about if the incident waves are not at the natural frequency of the substance? For which case is the substance transparent to that frequency of light? Explain why you cannot get a tan in a greenhouse? Activities: Draw long wavelength light incident on glass that passes through and discuss how wave at non-natural frequency causes a forced vibration of the particles of the substance. This small vibration allows the energy to pass from one particle to another in a gulp burp fashion all the way through the material until it reaches the other side. The substance is transparent to this light. The gulping and burping takes time so light travels slower in mediums than it does in a vacuum. Speed depends on how long the gulp burp cycle takes and how close together particles are. Speed in vacuum 300 000 km/s Speed in water 75% of speed in vacuum Speed in glass 67 % of speed in vacuum Speed in diamond (densest material) 47% speed in vacuum Describe how when incident light is at or near natural frequency then resonance occurs and vibrations of particles is large resulting in particles of material hitting each other creating friction and producing heat. The energy of the light wave is quickly used up in this process and the wave is not transmitted. The material is opaque to this color of light

Speed of Light 300, 000 km/s =300 000 000 m/s =3 x 108 m/s = 186,000 mps Roemer calculation Michaelson calculation Cosmic speed limit In media Speed less than that in vacuum speed~ 1/density Speed~ temperature Question: What is the speed of light in a vacuum? How does this speed change with the density and temperature of a medium? Outline the Roemer experiment that first estimated the speed of light in 1647. Outline the Michaelson experiment for finding the speed of light in 1900-1930. What else did Michaelson find out? Activities: At speed of light car would go around earth 7.5 times each second. Fastest thing man has made moves at 1/100 th of 1% the speed of light (5 km/s for New Horizons Space Craft). Galileos discovery Outline Roemer experiment 22 minutes to travel diameter of earth’s diameter (300 000 000 km) 300 000 000 km / (22 minutes x 3600 sec/minute) ~ 214 000 km/s. Ask if any difference if used moon of saturn Outline Michaelson experiment using transparency. Discuss how rotating mirror gives time while distance was measured via trig. Talk about local history. Talk about how speed of light was found to be constant no matter the direction. 1900-1930 1647

Cosmic Distances Light year is distance light travels in one year 6 trillion miles Sun nine light minutes away Pluto 14 light hours away Nearest star 4.3 LY away Center of galaxy 30 000 LY away Next galaxy 2.52 million light years away Looking into space is looking back into time Lots of empty space out there Questions: What is a light year? How far away is the Sun, Pluto, the nearest star, the nearest galaxy? Why do astronomers say they are looking back in time when they look into space? What is most the universe made of?

Realms of the Universe Image courtesy of The Cosmic Perspective by Bennett, Donahue, Schneider, & Voit; Addison Wesley, 2002

Earth Planet where we all live Comprised primarily of rock Spherical in shape 12,700 km in diameter It would take 17 days to circumnavigate the globe driving a car at 100 km/hr At the speed of light, it would take 0.13 seconds to go all the way around Earth.

Sun Star that Earth orbits Composed primarily of hydrogen and helium gas Uses nuclear fusion in its core to generate heat and light to allow itself to resist the crushing weight of its own mass Spherical in shape 1.39 Million km in diameter

Earth & Sun The Sun’s diameter is 109 times greater than that of Earth Over 1 million Earths would fit inside the Sun’s volume Earth orbits the Sun at an average distance of 150 million kilometers. This distance is called an Astronomical Unit (AU) It would take 11,780 Earths lined up side to side to bridge the 1 AU between Earth and Sun.

The Solar System 8 planets, thousands and thousands of dwarf planets and asteroids, trillions of comets and meteoroids Mostly distributed in a disk about the Sun Sun blows a constant wind of charged gas into interplanetary space, called the Solar Wind Boundary between Solar Wind and interstellar space at 100 AU from the Sun (200 AU diameter)

The Solar Neighborhood The region of the Galaxy within about 32.6 light-years of the Sun (65 light-years diameter) is considered its neighborhood. Here stars move generally with the Sun in its orbit around the center of the Galaxy This region is inside a large bubble of hot interstellar gas called the Local Bubble. Here the gas temperature is about 1 million degrees Kelvin and the density is 1000 times less than average interstellar space. To Center of Galaxy The image is 390 light-years across. Direction of Galactic Rotation

The Milky Way Galaxy The Milky Way Galaxy is a giant disk of stars 160,000 light-years across and 1,000 light-years thick. The Sun is located at the edge of a spiral arm, 30,000 light-years from the center It takes 250 Million years for the Sun to complete one orbit There are over 100 Billion stars in the Milky Way The Spiral arms are only 5% more dense than average, and are the locations of new star formation

The Local Group Contains 3 large spiral galaxies--Milky Way, Andromeda (M31), and Triangulum (M33)—plus a few dozen dwarf galaxies with elliptical or irregular shapes. Gravitationally bound together—orbiting about a common center of mass Ellipsoidal in shape About 6.5 million light-years in diameter

The Local Supercluster A cluster of many groups and clusters of galaxies Largest cluster is the Virgo cluster containing over a thousand galaxies. Clusters and groups of galaxies are gravitationally bound together, however the clusters and groups spread away from each other as the Universe expands. The Local Supercluster gets bigger with time It has a flattened shape The Local Group is on the edge of the majority of galaxies The Local Supercluster is about 130 Million light-years across The Local Supercluster

The Universe Surveys of galaxies reveal a web-like or honeycomb structure to the Universe Great walls and filaments of matter surrounding voids containing no galaxies Probably 100 Billion galaxies in the Universe The plane of the Milky Way Galaxy obscures our view of what lies beyond. This creates the wedge-shaped gaps in all-sky galaxy surveys such as those shown here.