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Electromagnetic Waves and Light EM Waves and the speed of light The EM Spectrum Reflection Refraction Wave Nature of Light Visible Light.

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Presentation on theme: "Electromagnetic Waves and Light EM Waves and the speed of light The EM Spectrum Reflection Refraction Wave Nature of Light Visible Light."— Presentation transcript:

1 Electromagnetic Waves and Light EM Waves and the speed of light The EM Spectrum Reflection Refraction Wave Nature of Light Visible Light

2 What is Light? The ancient Greeks thought that light was a particle Later scientists believed that light traveled in waves Einstein showed that light does have particle characteristics So which is it: wave or particle? The scientific community now agrees that light can be thought of as both a wave and a particle, because it shows behaviors of both..

3 Light as a Particle Seems to travel in a straight line Light seems to bounce off objects like a small particle would do Experiments show that its quantized –(that is, at very, very low levels, it can be described by integer units, like we can do with parts of an atom) –The elementary particle is called a photon

4 How is it a wave? A water wave, sent through a hole, will curve around the edges-- diffraction Same deal for light Lots of other ways Whether we think of it as a wave or a particle depends on the situation

5 They start with an accelerating charge.. Light is an Electromagnetic Wave Visible light is just one example of Electromagnetic Waves Accelerating Charge Electric Field Magnetic Field They all move at the same speed in a vacuum …that produces transverse electric and magnetic field waves Visible Light

6 What are electromagnetic waves? Accelerating Charge Electric Field Magnetic Field The changing electric field produces a changing magnetic field perpendicular to it The two waves travel together as one Electromagnetic Wave Since its a field wave, NOT mechanical, it can travel in a vacuum

7 History of the Speed of Light Ancient Greeks believed the speed of light to be infinitely fast. 1675: Olaus Roemer estimated the speed of light using observations of Jupiters moons In 1880, Albert Michelson crafted an ingenious experiment using rotating mirrors to measure the speed of light more precisely.

8 Roemers Observation How long is a moon of Jupiter hidden? Jupiter moving away: moon hidden longer Jupiter moving nearer: moon hidden shorter Difference indicates time for light to get to Earth

9 Michelsons experiment Spin the octagonal mirror, emit light pulses Change the speed at which the mirror spins At a certain rate, youll be able to see the light pulses At that rate, the mirror is turning 1/8 of a rotation in the time the light takes to go 2*35km Calculate the speed of light

10 Speed of Light 300,000,000 m/s in a vacuum (3 x 10 8 m/s) We use the symbol c = 3 x 10 8 Slower, but about the same for air Used for ALL EM Waves

11 Using the Speed of Light Just like with sound, if we know how fast something is traveling, we can use it to determine distance For light, velocity is c, so c = d/t

12 2007 Light Years A light year is a unit of distance Its the distance light travels in one year 1 l.y. = 3 x 10 8 m/s x 3.1 x 10 8 sec/yr = 9.43 x m light year

13 Electromagnetic Waves and Light EM Waves and the speed of light The EM Spectrum Reflection Refraction Wave Nature of Light Visible Light

14 EM Spectrum Spectrum Allocation Chart When we talk about light, we talk about this tiny slice of the spectrum

15 Types of EM Waves Radio waves Microwaves Infrared Visible Light Ultraviolet X-rays Gamma Rays These waves are all EM waves, all travel at speed c The differ in their frequencies (and therefore, their wavelengths and energies)

16 EM Spectrum Spectrum Allocation Chart Radio and microwaves: primarily produced by electronic equipment and antennas Infrared– heat! Produced by vibrating molecules Visible light: electrons changing state Gamma rays: radioactive decay EM Waves are created by accelerating electric charge

17 Calculating Wavelength In a vacuum, we know that EM waves travel at v = c = 3 x 10 8 The wave equation: v = f λ So we can relate frequency to wavelength For a radio wave with f = 2 x 10 7 Hz, λ = 3 x 10 8 / 2 x 10 7 = 15 m For green light with f = 6 x Hz, λ = 3 x 10 8 / 6 x = 5 x m

18 Energy Remember: higher frequency, higher energy So gamma rays have incredible energy Radio waves much, much less

19 Key Skills Know c Use c to relate distance travelled and time Use wave equation to relate frequency and wavelength


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