Download The Wave Nature of Light

Section 5.1 Light and Quantized Energy
• Compare the wave and particle natures of light.
• Define a quantum of energy, and explain how it is
related to an energy change of matter.
• Contrast continuous electromagnetic spectra and
atomic emission spectra.
Light, a form of electronic radiation, has
characteristics of both a wave and a particle.
Summary of Atomic Theory
Year
400BC
Event
Democritus proposes idea of atom
1808
Dalton develops Atomic Theory
1897
Thomson uses cathode ray to
discover electron, plum pudding model
1916
Millikan measures the mass of an e-
1919
Rutherford uses gold foil experiment
to discover nucleus, proton
The Atom and Unanswered Questions
• Recall that in Rutherford's model, the
atom’s mass is concentrated in the nucleus
and electrons move around it.
• The model doesn’t explain how the electrons
were arranged around the nucleus.
• The model doesn’t explain why negatively
charged electrons aren’t pulled into the
positively charged nucleus.
The Atom and Unanswered Questions
(cont.)
• In the early 1900s, scientists observed
certain elements emitted visible light when
heated in a flame.
• Analysis of the emitted light revealed that an
element’s chemical behavior is related to the
arrangement of the electrons in its atoms.
The Wave Nature of Light
• electromagnetic radiation,
• visible light
• a form of energy that exhibits wave-like
behavior as it travels through space
• All waves can be described by several
characteristics.
The Wave Nature of Light (cont.)
• wavelength (λ)
• the shortest distance between
equivalent points on a continuous wave
• frequency (ν)
• the number of waves that pass a given
point per second
• amplitude
• the wave’s height from the origin to a crest
The Wave Nature of Light (cont.)
The Wave Nature of Light (cont.)
• The speed of light (3.00  108 m/s) is the
product of it’s wavelength and frequency
c = λν.
The Wave Nature of Light (cont.)
• Wavelength and Frequency
– inversely related
• one gets bigger, the other gets smaller
– different frequencies of light different colors
– wide variety of frequencies (spectrum)
• Homework
– Page 140 1 – 4
– Page 166 34 - 36
The Wave Nature of Light (cont.)
• Sunlight contains a continuous range of
wavelengths and frequencies.
• A prism separates sunlight into a continuous
spectrum of colors.
• The electromagnetic spectrum includes all
forms of electromagnetic radiation.
• Gamma rays, x-rays, radio waves, microwaves
The Wave Nature of Light (cont.)
The Particle Nature of Light
• The wave model of light cannot explain all
of light’s characteristics.
• Matter can gain or lose energy only in small,
specific amounts called quanta.
• A quantum is the minimum amount of energy
that can be gained or lost by an atom.
• Planck’s constant has a value of
6.626  10–34 J ● s.
Photoelectric Effect
• An experiment designed to measure the energy
carried by an electromagnetic wave
• Shining light on alkali metals knocks electrons off
• Brightness of light has NO EFFECT on the Kinetic
Energy per electron
• Brighter light simply ejects MORE electrons!
• Red light can not eject electrons- Blue light can!
The Particle Nature of Light (cont.)
• Albert Einstein proposed in 1905 that light
has a dual nature.
• A beam of light has wavelike and particlelike
properties.
• A photon is a particle of electromagnetic
radiation with no mass that carries a quantum
of energy.
Ephoton = h
Ephoton represents energy.
h is Planck's constant.
 represents frequency.
• The energy per electron can be
calculated.
• frequency-to-energy conversion factor is
“h” (Planck's constant, 6.626×10-34 J·s)
•
E=h*ν
•ENERGY = Planck’s Constant *
frequency
• energy (E ) of electromagnetic radiation is
directly proportional to frequency () of radiation.
• Planck-Einstein Equation: E = h
• E = Energy, in units of Joules (kg·m2/s2)
(Joule is metric unit of energy)
• h = Planck’s constant (6.626 x 10-34 J·s)
(reflecting sizes of energy quanta)
•  = frequency, in units of hertz (hz, sec-1)
Atomic Emission Spectra (cont.)
• The atomic emission spectrum of an
element is the set of frequencies of the
electromagnetic waves emitted by the
atoms of the element.
• Each element’s atomic emission spectrum is
unique.
Atomic Emission Spectra
• Light in a neon sign is produced when
electricity is passed through a tube filled with
neon gas and excites the neon atoms.
• The excited atoms emit light to release
energy.
Section 5.1 Assessment
What is the smallest amount of energy
that can be gained or lost by an atom?
A. electromagnetic photon
B. beta particle
D
A
0%
C
D. wave-particle
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. quanta
Section 5.1 Assessment
What is a particle of electromagnetic
radiation with no mass called?
A. beta particle
B. alpha particle
D
A
0%
C
D. photon
A. A
B. B
C. C
0%
0%
0%
D. D
B
C. quanta
• A certain photon of light has a wavelength of 422
nm. What is the frequency of the light?
• What is the energy of a quantum of light with a
frequency of 7.39 x 1014 Hz?
• A certain red light has a wavelength of 680 nm.
What is the frequency of the light?
• A certain blue light has a frequency of 6.91 x
1014 Hz. What is the wavelength of the light?
• Page 145 8 – 14
• Page 166 46 - 49