QUANTUM THEORY

Light as a Particle

Objectives

Define a quantum Calculate the energy of a photon Relate photon energy to wavelength and

frequency of EMR

Blackbody Radiation

It was known that as one heats an object it changes color. What colors can you name as objects heat up?

The color corresponds to the emission of EMR. As the object gets hotter it should emit greater and greater frequencies of EMR.

Classical Physics

– light, energy, heat, forces, electricity and magnetism

– Everything had been explained as far as scientists thought

A problem that classical physics could not explain was:

the mathematical relationship between the temperature of a material and the colour of light that material gives off

According to classical physics, the radiation emitted from an incandescent object is caused by the oscillation of the atoms and molecules that make it up.

The color corresponds to the emission of EMR. As the object gets hotter it should emit greater and greater frequencies of EMR.

called Ultraviolet catastrophe

What actually happened intensity increases until it reaches a

peak intensity that depends on temperature, then it drops off sharply

blackbody – a perfect absorber: an object that

completely absorbs all wavelengths of EMR that strike it (all the energy) and when heated, that energy is re-emitted as EMR

Max Planck – (early 1900’s) Planck

assumed that the energy given off by these objects had to occur in discreet units (bundles) of energy.

• EMR could only vibrate at certain discrete frequencies.

• Planck called the smallest amount of energy a quantum.

• One quantum of light is called a photon.

Quanta

Planck’s Formula the energy emitted by a vibrating atom can

be explained by the following equation

34

15

energy emitted in J or eV

frequency of the vibrating atom or frequency

of emitted radiation (/s or Hz)

Planck's constant 6.63 10

4.14 10

E hf

where

E

f

h h Js or

h eVs

The energy of a single quantum of EMR at a specific frequency or wavelength

Vibrating at some frequency

atom

of EMR is the same as the frequency of the vibrating atomf

So an EM wave is analyzed by the following equations:v c

c f

cf

cE h

To determine the energy of multiple pieces or quanta:E = nhfn = number of pieces of radiation emitted or number of quanta

Examples:

Eg) Determine the energy emitted by a photon of green light having a wavelength of 518nm.

34 8

7

19

6.63 10 (3.00 10 / )

5.18 10

3.84 10 10

hcE

Js m s

m

E J

Eg) What is the wavelength of a 6.75 eV photon?

34 8

19

7

6.63 10 (3.00 10 / )

6.75(1.60 10 )

1.84 10

hcE

hc

E

Js m s

J

m

(Recall: 1 eV = 1.60 x 10-19 J)Reading: Text: p. 704 - 709Assignment: p. 710 #1 – 6