LIGHT AND ATOMSChapter 4

WHAT IS LIGHT?

Radiation Waves Particles Part of the visual spectrum of frequencies

and wavelengths

LIGHT IS A WAVE

Light is a wave, but this is only half of the story.

If light is a wave then it has wavelength It also has a frequency Cycles per second or

(Hertz) It has also a speed in space: 300,000,000

meters per second or 186,000 miles per second

WAVE PATTERN

length waveone

MAGNETIC AND ELECTRIC

Light is combination of magnetic and electric waves perpendicular to each other propagating in the same direction at right angles

ELECTROMAGNETIC WAVEfc

FREQUENCY RANGE OF VISIBLE LIGHT

meters 101 1nm

nm 700or 107

nm 400or 104

9-

7

7

meters

to

meters

Short wave length high frequencyLonger wavelength lower frequency

Note there are 1 million nanometers in a millimeter. One Nm is 1 x 10^-9

Light can bend into its individual wavelengthswhen it travels through a prism.

Light will travel at different speeds through glass,

thus bending light into different wave lengths and frequencies.

THOMAS YOUNG’S DOUBLE SLIT EXPERIMENT

The double-slit experiment, sometimes called Young's experiment (after Young's interference experiment), is a demonstration that matter and energy can display characteristics of both waves and particles, and demonstrates the fundamentally probabilistic nature of quantum mechanical phenomena.

DUALITY OF LIGHT

Light is a wave and at the same time a particle

Photons are masslessParticles or bundles of light energy. They travel at c.

THE DOUBLE SLIT EXPERIMENT

Photon gun

Wave pattern

Interference patternOf a wave.

QUANTUM FREAKINESS

Observed No wave

photons

QUANTUM FREAKINESS

Not observed Forms wave pattern

photons

http://video.mit.edu/watch/thomas-youngs-double-slit-experiment-8432/

ATOMS

Composed of protons , neutrons, and electrons.

Periodic chart. The atomic number is always the number of protons the atom has.

For example Hydrogen with atomic number of 1 has 1 proton, it could have a proton and a neutron too. Protons are positively charged.

Neutrons have no charge

BORON

Atomic Number

Atomic Weight

NUCLEUS

…so the nucleus of the atom is composed of protons and neutrons.

Orbiting around the nucleus are electrons. Much smaller than the protons and they are negatively charged.

CHARGE AND MASS

Constituent Symbol Charge Mass Electrons e- 1 9.1 x 10-28 g Protons p+ 1836 times the

electron mass Neutrons n “0” charge, approximately

the mass that of p+

PHOTONS

Photons are energy packets or light particles that are absorbed or emitted from an atom when the atom is exposed to an energy field such magnetic , heat, or radiation

They have no mass. They travel at the speed of light.

Middle 20th century depiction of an atom

Current depiction of an atom today

Atoms can have negative or positive charges. This is caused by a missing electronThus creating imbalance. If another element need a electron the twoElement will share and becomes a molecule. A water molecule.

Silicon Atoms – using a scanning tunneling microscope1 x 10 ^-10 meters

GENERATION OF LIGHT BY ATOMSChapter 4 Part II

LIGHT

How is light generated?

To answer that question we need to look at the atom.

P+

P+

N

Energy Levels

n=3

n=4

n=2

n=1

e-

e-

Loses energy. Drops to lower levelEmits photon.

raises to higher levelabsorbs photon. Gains more energy.

e-

e-

ABSORPTION AND EMISSION OF LIGHT

Emission. Energy is released when an electron drops

from an upper level to a lower. Absorbtion.

If the atom’s energy is increased, the electron moves outward from inner orbital.

When an electron emits a photon, the frequency is related to the energy level.

Energy Photon and electronAbsorbs- gains energyEmits –loses energyBut emits photons

n = 3

n = 4

n = 2

n = 1

n = 5

n = 6

ultraviolet

Absorption:The energy of the light has to match the energy Difference of the two orbitals to be absorbed.

Emission Lines

IDENTIFYING ATOMS BY THEIR LIGHT

Energy difference defines wavelength.

It can be calculated from the wavelength. For instance in a hydrogen atom dropping

from 3 to 2 orbital will produce a bright red color 656 nm.

hc

E

ABSORPTION

When an atom can absorb light because of a certain energy difference at an orbital.

We can determine the element by looking at its spectra.

The spectra is like a rainbow. From violet to red, colors missing are the signatures on that element.

ABSORPTION IN THE ATMOSPHERE

DOPPLER SHIFT

DOPPLER SHIFT

The wavelength of light emitted by a moving object is shifted. This effect is called the doppler shift. If the object is coming toward you, the light is shifted toward shorter wavelengths, blue shifted.

If the object is going away from you, the light is shifted toward longer wavelengths, red shifted.

The amount of shift is bigger if the emitting object is moving faster.

We don't normally notice this for light. But it is easy to observe for sound: Blue shifted = higher frequency = higher pitch.

Red shifted = lower frequency = lower pitch.

DOPPLER SHIFT

The amount of shift is bigger if the emitting object is moving faster.

Thus if we know the original wavelength, we can deduce the speed.

For a spectral line from an element that we can identify, we do know the wavelength.

Thus for many astronomical objects we can tell Is it coming toward us? Is it going away? If so, how fast is it moving toward us or away from us? But note that this method does not tell us how fast it is

moving sideways.

END CHAPTER 4