A “Light” Day of Notes.

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A “Light” Day of Notes

Maxwell’s Equations J. C. Maxwell developed 4 equations which govern all of electricity and magnetism. These 4 equations make light work. You do not need to know these.

Maxwell’s Equations

Spectrum Just like sound is classified according to its pitch, there are different kinds of light, classified by their frequency. The speed of light is constant.* Therefore, a certain frequency of light will always have the same wavelength. We can also classify light by wavelength.

Electromagnetic Spectrum

Radio Lowest Frequency EM waves AM (up to ~1 MHz) FM (90~100 MHz)
Shortwave (in between) VHF (TV) UHF (TV) Cell Phone reception Radar

Microwave (-wave) The Gigahertz (GHz) range Wavelength in cm.
Microwave ovens operate at 2.45 GHz This is a special frequency to cause resonance in water

Infrared (IR) Wavelength in “micrometers” (m)
Most commonly referred to as heat Hot objects radiate their thermal energy away. Up to a few hundred degrees, the radiant energy is in the IR range If we could see IR light, we would all shine brightly

More Infrared

Visible The light we can see A tiny portion of the EM spectrum
Red ( = 700 nm) Orange Yellow Green Blue Indigo Violet ( = 450 nm)

Ultraviolet (UV) Wavelength as low as 10 nm Used to kill germs
Black Lights The part of sunlight that causes sunburn and skin cancer Partially blocked by the ozone layer

X-Ray These can be useful, but also dangerous
Wavelength between 10 nm and 10 pm That is from 10 billionths of a meter to 10 trillionths of a meter

Gamma Ray ( ray)  < 10 pm High energy Dangerous
Remember Gamma radiation from last semester

Practice What is the wavelength of AM radio wave 960 kHz?
f = 960,000 Hz c = 3 × 108 m/s  = c/f  = 3 × 108 m/s ÷ 960,000 Hz  = 312 m

Practice What is the frequency of a “shortwave” radio wave 120 meters long?  = 120 m c = 3 × 108 m/s f = c/ f = 3 × 108 m/s ÷ 120 m f = 2,500,000 Hz = 2,500 kHz = 2.5 MHz

Practice What is the wavelength of an FM radio wave? These waves are approximately 100 MHz (100 × 106 Hz). f = 100,000,000 Hz c = 3 × 108 m/s  = c/f  = 3 × 108 m/s ÷ 100,000,000 Hz  = 3 m

Practice What is the wavelength of a microwave in a microwave oven? These waves are 2.45 GHz (2.45 × 109 Hz). f = 2,450,000,000 Hz c = 3 × 108 m/s  = c/f  = 3 × 108 m/s ÷ 2,450,000,000 Hz  = 0.12 m = 12 cm

Conclusion -- Quiz Wed. (4/29)
Electromagnetic Spectrum Radio Microwave Infrared Visible Roy G Biv Ultraviolet X Ray Gamma Ray Visible Spectrum Red Orange Yellow Green Blue Indigo Violet

Assignment Chapter 12 vocabulary