Light is a WAVE

5.2.1 EM Spectrum & EM Waves

EM Waves• Charged objects generate

electric fields.– When they move, the field changes.

• Changing electric fields produce MAGNETIC FIELDS.

• Changing magnetic fields produce ELECTRIC FIELDS.

• The E-fields and B-fields are perpendicular to each other.

• Waving a charge back and forth generates an EM WAVE.

Require a medium

Do not require a medium

The Speed of Light (well, EM Waves really)• EM Waves travel at 3.0 x 108 m/s (in a vacuum).

• They are slower in denser mediums.

• Speed of a medium depends on INDEX OF REFRACTION.

• The closer the index is to 1.0 the faster the medium.

Equation

v

cn

Index of Refraction Speed of Lightin vacuum

Speed of Lightin medium

What is the speed of light in a medium with an index of refraction of 2.0?

n = c / vv = c / n

v = (3 x 108 m/s) / (2.0)

v= 1.5 x 108 m/s

Example

EM Wave Types

An EM Wave’s type depends on its frequency/wavelength

103 10-2 10-5 10-7 10-8 10-10 10-12Wavelength (m)

Freq (Hz)

Temperature of bodies emitting the wavelength 1 K 100 K 10000 K 10 million K

104 108 1012 1015 1016 1018 1020

About the size of…

Building Human Honey Bee Pinpoint Protozoan Molecules Atoms Atomic Nuclei

Radio Microwave Infrared Visible Ultraviolet X-ray Gamma Ray

All objects emit EM radiation

Practice1. The constellation of Orion is a

highly recognizable arrangement of stars. There are two very distinct stars in this constellation, Betelgeuse which is a red star and Rigel which is a bluish star. The color red has a wavelength of about 750 nm and the color blue has a wavelength of about 400 nm.

a. Which color has the higher frequency?

b. Which star is hotter, Betelgeuse or Rigel?

BLUE

RIGEL

Practice2. An EM wave is traveling through outer space with a

frequency of 8.0 x 1013 hertz.a. What is the wavelength of this wave?

v = fλ λ= v / f

λ = (3.0 x 108 m/s) / (8.0 x 1013 Hz)

λ = (3.8 x 10-6 m)

b. What type of wave is this?

Infrared

Practice3. Listed below are the properties of several EM Waves.

State whether the property given is a wavelength or frequency, then state which type of EM Wave it is.

a. 500000 hertz _______________ ______________

b. 5.0 x 10-9 meter _______________ ______________

c. 3.0 x 1013 hertz _______________ ______________

d. 4.5 x 1014 hertz _______________ ______________

e. 200 meters _______________ ______________

frequency

wavelength

wavelength

frequency

frequency

radio

X ray / UV

infrared (IR)

visible light (red)

radio

End of 5.2.1