Announcements Tests are almost graded! If you haven’t started your first project, now would be a good time (due 13 October). Now is the BEST time to start.

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Presentation transcript:

Announcements Tests are almost graded! If you haven’t started your first project, now would be a good time (due 13 October). Now is the BEST time to start the Phases of the Moon project! Pick up Homework 5.

The Physics of Light 25 September 2006

Today: The speed of light Digression on energy

How fast does light travel? With today’s fast electronics, it’s easy to measure the time for a pulse of light to cross an ordinary room (billionths of a second). First measurement of the speed of light was by an astronomer: Olaus Roemer, Jupiter’s moons seem to orbit faster when earth is moving toward Jupiter, slower when earth is moving away.

How fast does light travel? 16 minutes Light travel time Jupiter’s moons appear to get 8 minutes ahead of schedule when we’re close, 8 minutes behind schedule when we’re on the far side. So light must take 8 minutes to travel 1 AU. (Size of 1 AU was measured around 1700, by triangulating Mars, Venus from 2 locations on earth, with aid of telescopes.) Earth Jupiter

Speed of Light 300,000,000 meters per second (m/s) or 3 x 10 8 m/s or 300,000 km/s Example: How long does it take light to reach you from the screen/dome? speed = distance time time = distance speed = 6 m 3 x 10 8 m/s = 2 x 10 –8 s = s (Be sure that you know how to use your calculator!)

How to quantify brightness? Light carries ENERGY. What is energy? I don’t know.

Types of energy Motion (“kinetic”) Gravitational Elastic Thermal Chemical Nuclear Electrical Radiant (light) Energy can be converted from one type to another, but cannot be created or destroyed. The total amount of energy in the universe never changes.

Units of energy 1 joule (official scientific unit; apple lifted 1 meter ) 1 Calorie (food) = 4200 joules (1 kg water, up 1ºC) 1 Jelly Donut = 250 Calories = 10 6 joules ($0.59) 1 BTU = 1050 joules (1 lb water, up 1ºF) 1 kilowatt-hour = 3.6 million joules (7 cents) 1 gallon of gasoline provides 30,000 Calories ($3.00) Typical American diet per day = 10 J.D.

Power = Rate of energy conversion = energy / time Power is measured in watts: 1 watt = 1 joule / second (1 kilowatt = 1000 watts; 1 horsepower = 750 watts; you convert energy at a rate of about 100 watts.) The “brightness” of a light source is really its power, measured in watts. The “intensity” of light striking a surface is measured in watts per square meter. Example: The power of our sun is about 4 x watts; the intensity of direct sunlight at earth’s surface is about 1000 watts per square meter.