L 35 Modern Physics [1]

Introduction- quantum physicsParticles of light PHOTONSThe photoelectric effect

Photocells & intrusion detection devicesThe Bohr atom

emission & absorption of radiationLASERS

ScannersCD readers

Modern Physics- Introduction

“Modern” – 20th CenturyBy the end of the 19th century it seemed that all the laws of physics were knownHowever, there were a few problems where classical physics (pre-20th century) didn’t seem to work.It became obvious that Newton’s laws could not explain atomic level phenomena

ATOMS and classical physics

According to the laws of mechanics and electricity and magnetism, an orbiting electron in an atom should continually radiate away energy as electromagnetic waves.Very quickly the electron would loose all of its energy and there would be no atoms!

Newton’s Laws have flaws!

Newton’s laws, which were so successful in allowing us to understand the behavior of big objects such as the motions of the planets, failed when pushed to explain atomic size phenomena.The discovery of the laws of atomic physics led to every important 20th

century discovery that have transformed our lives, the electronic revolution.

The photoelectric effect- photons

When light shines on a metal surface, electrons pop outPhotoelectrons are only emitted if the wavelength of the light is shorter than some maximum value, no matter how intense the light is.

LIGHT

Metal plate

photoelectrons

Details of a photocell

Photocells used as a safety device

The child interrupts the beam stopping the current

Photoelectric effect defies a classical explanation

According to classical physics, if the intensity of the light is strong enough, enough energy should be absorbed by the electrons to make them pop outThe wavelength of the light should not make a difference.What is going on ? ? ?

Einstein explains the PE effect, receives Nobel Prize in 1921

A radical idea was needed to explain the photoelectric effect.Light is an electromagnetic wave, but when it interacts with matter (the metal surface) it behaves like a particle, a light particle called a photon.A beam of light is thought of as a beam of photons.

PE – explanation continued

The energy of a photon depends on the wavelength or frequency of the lightRecall that speed of light

= wavelength (λ) x frequency (f)Photon energy

E = Planck’s constant x frequency = h fShorter wavelength (higher f) photons have a higher energy

PE – explanation continued

A certain amount of energy is required to make an electron pop out of a metalAn photoelectron is emitted if it absorbs a photon from the light beam that has enough energy (high enough frequency)No matter how many photons hit the electron, if they don’t have the right frequency the electron doesn’t get out

Blue and red photons -example

How much energy does a photon of wavelength = 350 nm (nanometers) have compared to a photon of wavelength = 700 nm? Solution: The shorter wavelength photon has the higher frequency. The 350 nm photon has twice the frequency as the 700 nm photon. Therefore, the 350 nm photon has twice the energy as the 700 nm photon.

The quantum concept

The photon concept is a radial departure from classical thinking.In classical physics, energy can come in any amountsIn modern physics, energy is QUANTIZED comes in definite packets photons of energy h f.In the PE effect energy is absorbed by the electrons only in discreet amounts

Video recorders anddigital cameras

A CCD (charge coupled device) can be used to capture photographicImages using thephotoelectric effect.http://money.howstuffworks.com/camcorder2.htm

THE LASER: a product of 20th Century Physics

Light

Amplification by

Stimulated

Emission of

Radiation.

Niels Bohr and his five sons

The quantum concept and the Bohr Atom

Niels Bohr, a Danish physicist, used the quantum concept to explain the nature of the atom.Recall that the orbiting electrons, according to classical ideas, should very quickly radiate away all of its energyIf this were so, then we would observe that atoms emit light over a continuous range of wavelengths (colors) NOT SO!

Line spectra of atoms

Line spectra are like atomic fingerprints. Forensic scientists use line spectra to identify substances.

The Bohr Atom• The electrons move in

certain allowed, “stationary” orbits or states in which then do not radiate.

• The electron in a high energy state can make a transition to a lower energy state by emitting a photon whose energy was the difference in energies of the two states, hf = Ei - Ef

+

EfEi

Nucleus

The orbits farther fromthe nucleus are higher

energy states thanthe closer ones

Line spectra of atomic hydrogen

The Bohr model was successful in determiningWhere all the spectral lines of H should be.

Emission and Absorption

When an electron jumps from a high energy state to a low energy state it emits a photon emission spectrumAn electron in a low energy state can absorb a photon and move up to a high energy state absorption spectrum

Emission Absorption

+ +

transition to alower energy state

transition to ahigher energy state