THE ELECTROMAGNETIC

SPECTRUM AND PLANCK’S LAW

Electromagnetic spectrum

Visible Spectrum

Only small portion of the Electromagnetic spectrum is visible to us. ~400nm – ~700nm

Before the 17th century, scientists believed that there was no such thing as the "speed of light". They thought that light could travel any distance in no time at all. Later, several attempts were made to measure that speed.

1667 Galileo Galilei: at least 10 times faster than sound 1675

Ole Roemer: 200,000 Km/sec 1728 James Bradley: 301,000 Km/s 1849 Hippolyte Louis Fizeau: 313,300 Km/s 1862 Leon Foucault 299,796 Km/s Today: 299792.458 km/s

Speed of Light

wavelength of light

λ is the Greek letter lambda and it stands for the wavelength of light.

Wavelength is defined as the distance between two successive crests of a wave.

When studying light, the most common units used for wavelength are: meter, centimeter, nanometer, and Ångström.

Keep in mind these definitions: one centimeter equals 10¯2 meter

one nanometer equals 10¯9 meterone Ångström equals 10¯8 centimeter

frequency of the light wave

ν is the Greek letter nu. It is NOT the letter v, it is the Greek letter nu!!! It stands for the frequency of the light wave.

Frequency is defined as the number of wave cycles passing a fixed reference point in one second. When studying light, the unit for frequency is called the Hertz (its symbol is Hz).

There is an important point to make about the unit on Hz. It is NOT commonly written as cycles per second (or cycles/sec), but only as sec¯1

the speed of light

c is the symbol for the speed of light, the speed at which all electromagnetic radiation moves when in a perfect vacuum

Both ways shown below are used to write the value. You need to be aware of both: 3.00 x 108 m/s3.00 x 1010 cm/s

Manipulate!

Using λν = c given wavelength, calculate

frequency given the frequency,

calculate the wavelength

In Summary

Since there are two variables (λ and ν), we can have two types of calculations: (a) given wavelength, calculate frequency and (b) given frequency, calculate wavelength.

Note that we can easily rearrange the main equation to fit these two types of problems: (a) given the wavelength, calculate the frequency; use this equation: ν = c / λ (b) given the frequency, calculate the wavelength; use this equation: λ = c / ν

Planck's law

Planck's law describes the electromagnetic radiation emitted from a black body at absolute temperature T. The Planck distribution of radiation is the unique stable distribution that can persist in thermodynamic equilibrium

Planck’s Constant

Equation Number Two: E = hν Brief historical note: It is well-known who

first wrote this equation and when it happened. Max Planck is credited with the discovery of the "quantum," the discovery of which took place in December 1900. It was he who first wrote the equation above in his announcement of the discovery of the quantum.

energy of the particular quantum of energy under study E is the energy of the particular quantum

of energy under study

Planck's Constant

h stands for a fundamental constant of nature now known as Planck's Constant

The value for Planck's Constant is 6.6260755 x 10¯34 Joule second. Please note that the unit is Joule MULTIPLIED BY second. It is not a division, both Joule and second are in the numerator.

frequency of the particular photon

ν is the frequency of the particular photon being studied

Nanometers to meters…

Please note… 1nm = 10-9m