allen high school physics let there be light! 14 april 2005

Allen High School PhysicsAllen High School Physics

Let there be light!Let there be light!Let there be light!Let there be light!

14 April 200514 April 2005


What is light?What is light?What is light?What is light?

• Light is an electromagnetic waveLight is an electromagnetic wave– Some frequencies we can see (visible Some frequencies we can see (visible

light), and most we can’t.light), and most we can’t.– These include radio, microwaves, These include radio, microwaves,

infrared, ultraviolet, x rays and infrared, ultraviolet, x rays and gamma raysgamma rays

• We can only see between 4.3 x 10We can only see between 4.3 x 101414 and 7.5 x 10and 7.5 x 101414 Hz, just like we can Hz, just like we can only hear between 20 and 20,000 only hear between 20 and 20,000 HzHz

• Light is an electromagnetic waveLight is an electromagnetic wave– Some frequencies we can see (visible Some frequencies we can see (visible

light), and most we can’t.light), and most we can’t.– These include radio, microwaves, These include radio, microwaves,

infrared, ultraviolet, x rays and infrared, ultraviolet, x rays and gamma raysgamma rays

• We can only see between 4.3 x 10We can only see between 4.3 x 101414 and 7.5 x 10and 7.5 x 101414 Hz, just like we can Hz, just like we can only hear between 20 and 20,000 only hear between 20 and 20,000 HzHz


DefinitionDefinitionDefinitionDefinition

• Electromagnetic wave Electromagnetic wave – A transverse wave consisting of A transverse wave consisting of

oscillating electric and magnetic oscillating electric and magnetic fields at right angles to each otherfields at right angles to each other

– Some examples:Some examples:– http://www.colorado.edu/physics/2http://www.colorado.edu/physics/2

000/waves_particles/000/waves_particles/

• Electromagnetic wave Electromagnetic wave – A transverse wave consisting of A transverse wave consisting of

oscillating electric and magnetic oscillating electric and magnetic fields at right angles to each otherfields at right angles to each other

– Some examples:Some examples:– http://www.colorado.edu/physics/2http://www.colorado.edu/physics/2

000/waves_particles/000/waves_particles/


Calculating the speed of Calculating the speed of lightlight

Calculating the speed of Calculating the speed of lightlight

• All wave velocities can be found All wave velocities can be found with v = fwith v = f

• Einstein's said the speed of light Einstein's said the speed of light is constant value, just like the is constant value, just like the speed of sound at standard speed of sound at standard temperature and pressure.temperature and pressure.

• So we replace v with c, and So we replace v with c, and

c = 3 x 10c = 3 x 108 8 m/sm/s

• All wave velocities can be found All wave velocities can be found with v = fwith v = f

• Einstein's said the speed of light Einstein's said the speed of light is constant value, just like the is constant value, just like the speed of sound at standard speed of sound at standard temperature and pressure.temperature and pressure.

• So we replace v with c, and So we replace v with c, and

c = 3 x 10c = 3 x 108 8 m/sm/s


Lets try another appletLets try another appletLets try another appletLets try another applet

• http://www.walter-fendt.de/ph14e/emwave.hhttp://www.walter-fendt.de/ph14e/emwave.htmtm

• http://www.walter-fendt.de/ph14e/emwave.hhttp://www.walter-fendt.de/ph14e/emwave.htmtm


Waves, rays, it’s all goodWaves, rays, it’s all goodWaves, rays, it’s all goodWaves, rays, it’s all good

• Rays are a model we use to Rays are a model we use to approximate the location of the approximate the location of the source and location of the wave source and location of the wave frontfront

• Figure 14-3Figure 14-3• This is called Huygens’ Principle, This is called Huygens’ Principle,

named after Christian Huygensnamed after Christian Huygens• We use this idea to make We use this idea to make ray ray

approximationsapproximations

• Rays are a model we use to Rays are a model we use to approximate the location of the approximate the location of the source and location of the wave source and location of the wave frontfront

• Figure 14-3Figure 14-3• This is called Huygens’ Principle, This is called Huygens’ Principle,

named after Christian Huygensnamed after Christian Huygens• We use this idea to make We use this idea to make ray ray

approximationsapproximations


That’s a bright ideaThat’s a bright ideaThat’s a bright ideaThat’s a bright idea

• Take a look at figure 14-4Take a look at figure 14-4• Notice how bright the first square Notice how bright the first square

isis• As we move out that light is now As we move out that light is now

distributed over a larger area, in distributed over a larger area, in fact 2 times as largefact 2 times as large

• The light has the same intensity The light has the same intensity over all but for a given area, say 1 over all but for a given area, say 1 mm22, it is smaller, it is smaller

• Take a look at figure 14-4Take a look at figure 14-4• Notice how bright the first square Notice how bright the first square

isis• As we move out that light is now As we move out that light is now

distributed over a larger area, in distributed over a larger area, in fact 2 times as largefact 2 times as large

• The light has the same intensity The light has the same intensity over all but for a given area, say 1 over all but for a given area, say 1 mm22, it is smaller, it is smaller


Inverse squareInverse squareInverse squareInverse square

• So light “brightness” obeys an So light “brightness” obeys an inverse square lawinverse square law

• If you take a book and double If you take a book and double your distance from the source, your distance from the source, only ¼ of the light will hit itonly ¼ of the light will hit it

• So light “brightness” obeys an So light “brightness” obeys an inverse square lawinverse square law

• If you take a book and double If you take a book and double your distance from the source, your distance from the source, only ¼ of the light will hit itonly ¼ of the light will hit it

allen high school physics let there be light! 14 april 2005

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