Ast October 2007 The Nature of Light (This topic is a little out of order; we’ll return to the Solar System later)

Ast October 2007 ELECTROMAGNETIC WAVES = “ELECTROMAGNETIC RADIATION” JAMES CLERK MAXWELL, 1864:

Ast October 2007

FREQUENCY = NUMBER OF WAVE CRESTS PER SECOND.

Ast October 2007 FREQUENCY = NUMBER OF WAVE CRESTS PER SECOND. IF A WAVE MOVES AT SPEED c, THEN FREQUENCY = c / WAVELENGTH or WAVELENGTH = c / FREQUENCY.

Ast October 2007 FREQUENCY = NUMBER OF WAVE CRESTS PER SECOND. IF A WAVE MOVES AT SPEED c, THEN FREQUENCY = c / WAVELENGTH or WAVELENGTH = c / FREQUENCY. HIGH FREQUENCY MEANS SHORT WAVELENGTH.

Ast October 2007 THE ELECTROMAGNETIC SPECTRUM GAMMA RAYS X-RAYS ULTRAVIOLET LIGHT ( UV ) VISIBLE LIGHT ( VIOLET TO RED ) INFRARED LIGHT ( IR ) MILLIMETER WAVES RADIO ( INCLUDING MICROWAVES )

Ast October 2007

1900—1905, MAX PLANCK AND ALBERT EINSTEIN: LIGHT CAN BE REGARDED AS PARTICLES AS WELL AS WAVES. “QUANTA” “PHOTONS”

Ast October —1905, MAX PLANCK AND ALBERT EINSTEIN: LIGHT CAN BE REGARDED AS PARTICLES AS WELL AS WAVES. ENERGY OF A PHOTON = h X FREQUENCY -- where h is a universal quantity called “Planck’s constant”.

Ast October 2007 HIGHER-ENERGY LOWER-ENERGY PHOTONS PHOTONS

Ast October 2007 Mention how E.M. waves / radiation affect matter, and vice-versa: continuous electric waves, vs. sudden impacts ( see next slide )

Ast October 2007

Next: SPECTRA

Ast October 2007

Three concepts to remember... CONTINUOUS SPECTRUM ABSORPTION LINES EMISSION LINES

Ast October 2007 SPECTRAL LINES DEPEND ON THE CHEMICAL ELEMENT ( atoms or molecules )

Ast October 2007 NEILS BOHR’S THEORY OF THE HYDROGEN ATOM (1913)

Ast October 2007

REAL ATOMS ARE MORE LIKE THIS: QUANTUM PHYSICS “WAVE FUNCTIONS”.

Ast October 2007 ATOMIC ENERGY LEVELS

Ast October 2007 ENERGY LEVELS OF HYDROGEN

Ast October 2007 ATOMIC ENERGY LEVELS

Ast October 2007

E.M. RADIATION EMITTED BY OPAQUE SURFACES...

Ast October 2007

HIGHER TEMPERATURE implies HIGHER AVERAGE PHOTON ENERGIES implies SHORTER WAVELENGTHS (“bluer”)

Ast October 2007 HIGHER-ENERGY LOWER-ENERGY PHOTONS PHOTONS

Ast October 2007 Some examples... “ROOM TEMPERATURE” (~ 300 °K): INFRARED.

Ast October 2007 Some examples... “ROOM TEMPERATURE” (~ 300 °K): INFRARED. 3000—10000 °K: VISIBLE LIGHT.

Ast October 2007 Some examples... “ROOM TEMPERATURE” (~ 300 °K): INFRARED. 3000—10000 °K: VISIBLE LIGHT. 40,000 °K: ULTRAVIOLET.

Ast October 2007 Some examples... “ROOM TEMPERATURE” (~ 300 °K): INFRARED. 3000—10000 °K: VISIBLE LIGHT. 40,000 °K: ULTRAVIOLET. 10 million degrees: X-RAYS.

Ast October 2007 Some examples... “ROOM TEMPERATURE” (~ 300 °K): INFRARED. 3000—10000 °K: VISIBLE LIGHT. 40,000 °K: ULTRAVIOLET. 10 million degrees: X-RAYS. ( Caution: These are the average wavelengths. )

Ast October 2007