1. Allen High School Physics Let there be light! 14 April 2005

2. Allen High School Physics What is light? Light is an electromagnetic waveLight is an electromagnetic wave –Some frequencies we can see (visible light), and most we can’t. –These include radio, microwaves, infrared, ultraviolet, x rays and gamma rays We can only see between 4.3 x 10 14 and 7.5 x 10 14 Hz, just like we can only hear between 20 and 20,000 HzWe can only see between 4.3 x 10 14 and 7.5 x 10 14 Hz, just like we can only hear between 20 and 20,000 Hz Light is an electromagnetic waveLight is an electromagnetic wave –Some frequencies we can see (visible light), and most we can’t. –These include radio, microwaves, infrared, ultraviolet, x rays and gamma rays We can only see between 4.3 x 10 14 and 7.5 x 10 14 Hz, just like we can only hear between 20 and 20,000 HzWe can only see between 4.3 x 10 14 and 7.5 x 10 14 Hz, just like we can only hear between 20 and 20,000 Hz

3. Allen High School Physics DefinitionDefinition Electromagnetic waveElectromagnetic wave –A transverse wave consisting of oscillating electric and magnetic fields at right angles to each other –Some examples: –http://www.colorado.edu/physics/200 0/waves_particles/ http://www.colorado.edu/physics/200 0/waves_particles/http://www.colorado.edu/physics/200 0/waves_particles/ Electromagnetic waveElectromagnetic wave –A transverse wave consisting of oscillating electric and magnetic fields at right angles to each other –Some examples: –http://www.colorado.edu/physics/200 0/waves_particles/ http://www.colorado.edu/physics/200 0/waves_particles/http://www.colorado.edu/physics/200 0/waves_particles/

4. Allen High School Physics Calculating the speed of light All wave velocities can be found with v = fAll wave velocities can be found with v = f Einstein's said the speed of light is constant value, just like the speed of sound at standard temperature and pressure.Einstein's said the speed of light is constant value, just like the speed of sound at standard temperature and pressure. So we replace v with c, andSo we replace v with c, and c = 3 x 10 8 m/s All wave velocities can be found with v = fAll wave velocities can be found with v = f Einstein's said the speed of light is constant value, just like the speed of sound at standard temperature and pressure.Einstein's said the speed of light is constant value, just like the speed of sound at standard temperature and pressure. So we replace v with c, andSo we replace v with c, and c = 3 x 10 8 m/s

5. Allen High School Physics Lets try another applet http://www.walter-fendt.de/ph14e/emwave.htmhttp://www.walter-fendt.de/ph14e/emwave.htmhttp://www.walter-fendt.de/ph14e/emwave.htm http://www.walter-fendt.de/ph14e/emwave.htmhttp://www.walter-fendt.de/ph14e/emwave.htmhttp://www.walter-fendt.de/ph14e/emwave.htm

6. Allen High School Physics Waves, rays, it’s all good Rays are a model we use to approximate the location of the source and location of the wave frontRays are a model we use to approximate the location of the source and location of the wave front Figure 14-3Figure 14-3 This is called Huygens’ Principle, named after Christian HuygensThis is called Huygens’ Principle, named after Christian Huygens We use this idea to make ray approximationsWe use this idea to make ray approximations Rays are a model we use to approximate the location of the source and location of the wave frontRays are a model we use to approximate the location of the source and location of the wave front Figure 14-3Figure 14-3 This is called Huygens’ Principle, named after Christian HuygensThis is called Huygens’ Principle, named after Christian Huygens We use this idea to make ray approximationsWe use this idea to make ray approximations

7. Allen High School Physics That’s a bright idea Take a look at figure 14-4Take a look at figure 14-4 Notice how bright the first square isNotice how bright the first square is As we move out that light is now distributed over a larger area, in fact 2 times as largeAs we move out that light is now distributed over a larger area, in fact 2 times as large The light has the same intensity over all but for a given area, say 1 m 2, it is smallerThe light has the same intensity over all but for a given area, say 1 m 2, it is smaller Take a look at figure 14-4Take a look at figure 14-4 Notice how bright the first square isNotice how bright the first square is As we move out that light is now distributed over a larger area, in fact 2 times as largeAs we move out that light is now distributed over a larger area, in fact 2 times as large The light has the same intensity over all but for a given area, say 1 m 2, it is smallerThe light has the same intensity over all but for a given area, say 1 m 2, it is smaller

8. Allen High School Physics Inverse square So light “brightness” obeys an inverse square lawSo light “brightness” obeys an inverse square law If you take a book and double your distance from the source, only ¼ of the light will hit itIf you take a book and double your distance from the source, only ¼ of the light will hit it So light “brightness” obeys an inverse square lawSo light “brightness” obeys an inverse square law If you take a book and double your distance from the source, only ¼ of the light will hit itIf you take a book and double your distance from the source, only ¼ of the light will hit it