1. 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)

2. 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

3. Types of Light: The EM Spectrum
All EM waves alike
Produced by accelerating electric charge
Travel at 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

4. 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 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

5. Speed of Light
300, 000 km/s = 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 Outline the Michaelson experiment for finding the speed of light in 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 ( km) km / (22 minutes x 3600 sec/minute) ~ 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.
1647

6. 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 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?

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

8. 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.

9. 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

10. 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.

11. 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)

12. 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

13. 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

14. 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

15. 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

16. 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.