A “Light” Day of Notes

Maxwell’s EquationsJ. 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.

t

BE

E

t

EJB

B

0

000

0

SpectrumJust 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

RadioLowest 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)

rangeWavelength in cm.Microwave ovens

operate at 2.45 GHzThis is a special

frequency to cause resonance in water

Infrared (IR)Wavelength in

“micrometers” (m)Most commonly referred

to as heatHot 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

VisibleThe light we can seeA tiny portion of the EM

spectrum

Red ( = 700 nm)OrangeYellowGreenBlueIndigoViolet ( = 450 nm)

Ultraviolet (UV)Wavelength as low as 10

nm

Used to kill germsBlack LightsThe part of sunlight that

causes sunburn and skin cancer

Partially blocked by the ozone layer

X-RayThese can be useful, but

also dangerousWavelength between 10

nm and 10 pmThat is from 10 billionths of a meter to 10 trillionths of a meter

Gamma Ray ( ray) < 10 pm

High energyDangerousRemember Gamma

radiation from last semester

PracticeWhat is the wavelength of AM radio wave

960 kHz?f = 960,000 Hzc = 3 × 108 m/s = c/f = 3 × 108 m/s ÷ 960,000 Hz = 312 m

PracticeWhat is the frequency of a “shortwave” radio

wave 120 meters long? = 120 mc = 3 × 108 m/sf = c/f = 3 × 108 m/s ÷ 120 mf = 2,500,000 Hz = 2,500 kHz = 2.5 MHz

PracticeWhat is the wavelength of an FM radio wave?

These waves are approximately 100 MHz (100 × 106 Hz).f = 100,000,000 Hzc = 3 × 108 m/s = c/f = 3 × 108 m/s ÷ 100,000,000 Hz = 3 m

PracticeWhat 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 Hzc = 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

RadioMicrowaveInfraredVisible

Roy G BivUltravioletX RayGamma Ray

Visible Spectrum

RedOrangeYellowGreenBlueIndigoViolet