PARTICLE NATURE OF LIGHT

A Black Object

• Appears black because it absorbs all frequencies of light

• A black block of iron does this

WHEN IRON IS HEATED

• Might expect the energy it has absorbed would radiate back out, exhibiting all the frequencies it had absorbed

• THIS IS NOT WHAT HAPPENS!

• Specific colors come out depending on the temperature of the metal

Red Yellow White Blue-White

HOTTER

Max Planck – Germany, 1900

• “Matter can only gain or lose energy in small, specific amounts called QUANTA

Equantum = h h = Planck’s Constant

= 6.626 x 10-34 J.s

EXAMPLE PROBLEMS1. A metal is heated until it is red hot with a

shade of red corresponding to a frequency of 4.42 x 1014 Hz. What is the wavelength of this radiation?

• What is its energy in Joules?

EXAMPLE PROBLEMS2. Tiny water drops in the air disperse the

white light of the sun into a rainbow. What is the energy of the violet light that has a frequency of 7.23 x 1014 Hz?

The Photoelectric Effect• When light of a particular frequency shines

on the surface of a particular metal, electrons are emitted.

“photoelectrons”

The Photoelectric Effect• Light of lower frequency produces no

photoelectrons regardless of intensity or brightness. However, dim light of sufficient frequency produces“photoelectrons”

• Wave Theory fails to explain this!

Albert Einstein 1905• Expanded on Planck’s theory that EMR has a

Dual Nature – Wave and Particle

• Ephoton = h PHOTON – light particle or energy particle

Planck & Einstein’s resultsreveal an important

PARTICLE MODEL FOR LIGHT

Example: It is found that light with a wavelength of 486 nm is necessary to produce photoelectrons from a photoelectric cell. What is the energy of this photon?

Planck & Einstein’s resultsreveal an important

PARTICLE MODEL FOR LIGHT

Example: It is found that light with a wavelength of 486 nm is necessary to produce photoelectrons from a photoelectric cell. What is the energy of this photon?

ans 4.10 x 10-19J