atomic physics atomic spectra lasers applications · atomic physics quantization of orbits: •...

Lecture 21

Electromagnetic waves

Atomic Spectra

Atomic Physics

Lasers

Applications

Electromagnetic Waves

•composed of electric and magnetic fields

•can be created by an oscillating charge



Q Stationary charge

showing electric field lines

Q Oscillating charge

•field lines follow charge

•EM waves created

EM waves can be created by an oscillating charge

A moving charge creates both an oscillating electric and magnetic field

Q

Charge stationary but

EM waves continue

to move away

Electromagnetic spectrum

Electromagnetic waves; unlike mechanical waves

•do not require material substance for propagation

•can travel in vacuum

Characteristic of all waves is that they can

interfere constructively or destructively

when superimposed

All EM waves (e.g. radio and X-rays) belong

to the same class: only difference is frequency


101

All EM waves travel at the same speed in vacuum c = 3.0x108 ms-1

Lightening strikes 10 km away.

(a) How long after the strike will you see the light?

(b) How long after the strike will you hear the sound?


Example

c = 3x108 m/s, s = 10 km, t = ? (a)

(b) v = 344 m/s, s = 10 km, t = ?

s = vt t = s/v = (10,000 m)/(344 m/s) = 29 s

s = vt t = s/v

t = (10,000 m)/(3x108 m/s) = 3.3x10-5 s

lightening flash almost instantaneous

–Speed of sound in air is ≈ 344 m/s


What is significant about this wavelength?

If light has a frequency of 1.94x1014 Hz

what is its wavelength?

example

c = f

c = 3x108 m/s,, = ?

= (3x108 m/s)/(1.94x1014 Hz)

f = 1.94x1014 Hz

= 1.55x10-6 m


Wave nature of light

First proof---Thomas Young 1801

Superimposed two light beams

and saw constructive and

destructive interference

Beams obtained by passing

sunlight through two closely

spaced narrow slits

Interference pattern

(bright & dark regions)

Slit widths ≈ x1

x2

x2 = x1+ n constructive interference (bright)

x2 = x1+ (n+½) destructive interference (dark)

where n is an integer

laser


Wavelength: ≈1 m Radio waves

Marconi :

Nobel Prize in 1909 for “contributions to the

development of wireless telegraphy

Microwaves • Wavelength: ≈ 1 cm

• Radar systems

• Communications

-Mobile phone networks

Applications

• Microwave ovens

Infrared radiation


http://science.hq.nasa.gov/kids/imagers/ems/infrared.html

Wavelength: ≈ 1 mm - 1 m

Cat

Uses

•Heat transfer by radiation

•Spectroscopy

•Night vision

Visible waves

• Wavelength: ≈ 400 nm - 700 nm

http://science.hq.nasa.gov/kids/imagers/ems/ir_man.html


Ultra Violet • Wavelength: ≈ 10 nm - 300 nm

Characteristics

• Reacts with the skin to cause tanning,

• sunburn, and skin cancer

• Can be used to sterilize (kills

microorganisms)

• Mostly absorbed by the ozone layer

Disinfection

•penetrates cell walls and disrupts the cell’s

genetic material, impairs reproduction

•Optimum UV wavelength range to destroy

bacteria is between 250 nm and 270 nm.


Gamma Rays

• Wavelength: ≈ 0.01 nm

Characteristics

• Produced in the nuclei of atoms

• (stars, nuclear reactors, nuclear bombs)

• Biologically hazardous

• Used in medical diagnostics and

therapeutics

X rays

Characteristics

Wavelength: ≈ 0.1 nm - 1 nm

• Biologically hazardous

• Used in medical diagnostics

and materials testing

Wilhelm Roentgen Nobel Prize in

1901 for “the discovery of x-rays”

Atomic Physics

Nature of a substance can be studied by

measuring the intensity and wavelengths of

radiation coming from it

Hot neon gas emits wavelengths that

give it a red appearance

Gold illuminated with white light appears yellow

due to wavelengths absorbed and reflected

Study of atoms and the physical principles underlying their characteristics

Atomic Spectra

Hot Gases

• Atomic spectra

• EM radiation emitted by atoms and molecules

• Presence of spectral lines - a few strongly

emitted frequencies

• Other frequencies are completely absent

Atomic Spectra

Cool solids illuminated by white light

• Object’s color is determined by absorbed

wavelengths

Hot Solids

• Emits infrared and visible light

• Spectrum is related to the

object’s temperature

Structure of the atom

Nobel Prize in 1922 for

“investigation of the structure of

atoms and of the radiation

emanating from them”

http://nobelprize.org/nobel_priz

es/chemistry/laureates/1922/

Atomic Physics

1911 – Ernest Rutherford

discovered that the nucleus

is extremely small and

dense

1913 – Bohr proposed

planetary model of the atom

based on Rutherford’s results

• Dense nucleus at center

– Nucleus made of neutrons and protons

– Has positive charge

• Electrons orbiting the nucleus

• Only certain electron orbits allowed

Atomic Physics

Planetary Model of the Atom

Energy of electron

determined by orbit

in which it resides

+ _

_

_

_

_

_

Generation of a photon:

• Electrons elevated to a higher orbit when atom absorbs energy

• Electron falls back to lower orbit due to attractive forces from positively charged nucleus

• Energy absorbed (difference in energy between two levels) is emitted

• Can be emitted as a photon of EM radiation

Atomic Physics Atomic Spectra Explained

+

+ -

-

Emitted

photon

Excited atom -

Generation of a photon:

absorption

Ground

state

excited

state

photon

Energy Levels

Difference in energies

• Energy absorbed or emitted when

electron changes orbit

Atomic Physics

Quantization of orbits:

• Only certain electrons orbits allowed

Energy of emitted photon

i fE E E

i fE E E hf

where h is Planck’s constant = 6.6x10-34J.sec

and f is the frequency of the EM radiation

Energy (Joules) = qV

Charge on an electron = 1.6 x10-19 C

1eV = 1.6 x10-19 C x 1volt =1.6 x 10-19 Joules

Energy (electron volt) = eV

Units of energy

1eV =1.6 x 10-19 Joules

Ground

state

excited

state

photon

Energy = 0.0 eV

Energy = 7.4 eV

Exercise

From the energy level diagram below calculate

the frequency and wavelength of the photons

emitted and identify the type of radiation

i fE E E hf h = 6.63*10-34 Js

7.4 0.0 7.4E eV eV eV

Convert to Joules

19 197.4 7.4 1.6 10 11.84 10eV J J 19

15

34

11.84 101.8 10

6.63 10

Jf Hz

Js

c f

8 17

15

3 101.66 10 166

1.8 10

c msm nm

f Hz

uv light

• Can return directly

• Can return in a series of smaller steps

• Fluorescence

– Different energy emitted than was absorbed

Direct

De-excitation

Absorption

Fluorescence

Atom: Excited state

The state of an atom that has absorbed energy

Excited atoms eventually de-excite

– Absorbed energy is re-emitted

– Typically emitted as a photon

– Atom returns to the ground state

Substance identification

• Shine a UV light on minerals

• Certain minerals fluoresce

• Emit visible light

• Colour of light emitted indicates material

Fluorescence

Applications

Fluorescent light bulb

• Filled with gas

• Current passed through the gas

• Atoms of gas are excited

• Atoms de-excite by emission of UV radiation

• Fluorescent material coated on inside of tube

absorbs UV radiation and emits visible light

Laser

Atomic transitions

E2

E3

E1

E4

Electron energy levels, allowed states

Ground state

Excited states

E1

E0

After excitation

hf Photon energy E=hf

(energy absorbed)

Atom: excited state

E0

E1

E0

E

Before excitation

Atom: ground state

electron

E1

E0

E

Before de-excitation

Atom: excited state E1

E0

hf = E

After de-excitation

Atom: ground state

Spontaneous emission (10-8sec)

Stimulated emission

E1

E0

E

before

hf = E

Atom: excited state

E1

E0

hf

after

Atom: ground state

hf

Laser

Atomic transitions

Excited atom returns to ground state and

hence emits a 2nd photon of the same energy

Both photons are in phase and have the

same energy (colour) (wavelength)

Both photons can stimulate other atoms to emit

photons that in turn stimulate the emission of

more photons.

Population inversion

Ordinarily more atoms in the ground state

than excited state so there is a net absorption

of energy

However if there are more atoms in the excited

state than the ground state a net emission will

take place

≈100%

reflectivity

Mirror Mirror

Energy input

≈98%

reflectivity

Laser

Laser

Acronym: Light Amplification by Stimulated

Emission of Radiation

Laser

Typical Characteristics

•Collimated beam (uni-directional)

•Single wavelength in the uv, visible or infrared

•Intense beam

Applications

•Check-out scanners

•CD $ DVD players

•Pointers

•Printers

•Eye surgery (reshaping cornea)

•Cuts tissue (burns tumours)

•Cuts metal

•Cuts patterns (many layers of cloth at once)

•Telecommunications (sent down optical fibres)

•Dentistry

Laser Dental Applications

Laser Drill

•Replace turbine drill

•Preparation for fillings

•Eliminate local anesthetic injection

•Capable of killing bacteria located in a cavity

•No vibration

Not suitable for removing amalgam fillings

Laser beam preferentially absorbed by decayed

tissue because of large water content

compared with healthy enamel

Laser: (Er: YAG) Wavelength 2940nm

light of this wavelength highly absorbed by water

no increase in pulp temperature

Result:

•selective ablation of decay,

•Conservation of healthy tooth

Consequence of Water fluoridation

Harder enamel

Good resistance to decay

Early detection of cavities more

difficult

Laser

Dental Applications

Near-infrared laser-induced reflected

fluorescence can detect early sub-surface decay

Early detection of caries

Optical Coherence Tomography (OCT)

use infrared laser light

High resolution (m) 3D images

View inside of teeth and gums in real time

Laser aided teeth whitening

Laser light used to activate and accelerate

bleaching process

Oxidizing agents such as hydrogen peroxide or

carbamide peroxide

Teeth whitening

Laser

Dental Applications

Reshape gum tissue (reduce prominence)

Restorative materials rapidly cured (set)

atomic physics atomic spectra lasers applications · atomic physics quantization of orbits: •...

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