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Light as a Wave OBJECTIVES:
• Describe the relationship between the wavelength and frequency of light.
Electrons and Light After Bohr’s model, the mystery of the
atom was the nature of the electron cloud.
The study of light – particularly the wave nature of light – played a critical role in probing the nature of the electron cloud.
Light and Energy are Linked Energy travels through space as light
waves. These Light waves are known as
Electromagnetic Radiation (EMR) EMR – defined as a form of energy
that exhibits wavelike behavior as it travels trough space.
Types of EMR Visible Light is one type of EMR Others include x-rays, microwaves,
radiowaves, gamma rays, ultraviolet waves, infrared waves.
All electromagnetic radiation travels at this same rate.
This rate is the speed of light (c) c = 3.0 x 108 m/sec
- Page 139
“R O Y G B I V”
Basic Properties of Waves Wavelength (λ- lambda) – distance
between two crests of a wave. Units are usually meters.
Frequency (f or (nu) )- the number of wave cycles that pass a given point per unit time (usually seconds) Units are sec-1 = Hertz
Parts of a wave
Wavelength
AmplitudeOrigin
Crest
Trough
Wavelength and Frequency Are inversely related
• As one goes up the other goes down.
• c = or c = f
c = speed of light = 3.0 x 108m/s
As frequency decreases, wavelength increases.
As frequency increases, the wavelength decreases.
Different frequencies of light are different colors of light.
There is a wide variety of frequencies
The whole range is called a spectrum
EM Spectrum
LOW
ENERGY
HIGH
ENERGY
R O Y G. B I V
red orange yellow green blue indigo violet
Equation: E = hf
EE = Energy, in units of Joules (kg·m = Energy, in units of Joules (kg·m22/s/s22)) (Joule is the metric unit of energy)(Joule is the metric unit of energy)
hh = Planck’s constant (6.626 x 10 = Planck’s constant (6.626 x 10-34-34 J·s) J·s)
f f = frequency, in units of hertz (hz, sec= frequency, in units of hertz (hz, sec-1-1))
The energy (E ) of electromagnetic radiation is directly proportional to the frequency () of the radiation.
Low ENERGYWaves
=Long
Wavelength=
Low Frequency
High ENERGYWaves
=Short
Wavelength=
High Frequency
Wavelength Table
Radiowaves
Microwaves
Infrared .
Ultra-violet
X-Rays
GammaRays
Low Frequency
High Frequency
Long Wavelength
Short WavelengthVisible Light
Low Energy
High Energy
Behavior of Light That Supports Wave Theory
Reflection — Waves rebound from a collision with an even substance at the same angle which they approached it.
Refraction — Waves change speed when they enter a new medium (from air to water).
Refraction explains how a prism separates the colors that make up white light.
Each color will refract (or bend) to different degrees based on its characteristic wavelength
Diffraction — Waves can interfere with other waves
They create diffraction patterns Constructive interference—occurs
when a crest meets a crest or a trough meets a trough
Destructive interference—occurs when a crest meets a trough
For light waves, you see darkness with destructive interference.
• Microwaves are used to transmit information.
Calculating Wavelength of an EM Wave
• What is the wavelength of a microwave having a frequency of 3.44 x 109 Hz?
• Solve the equation relating the speed, frequency, and wavelength of an electromagnetic wave for wavelength (λ).
Electrons in Atoms: Basic ConceptsElectrons in Atoms: Basic ConceptsElectrons in Atoms: Basic ConceptsElectrons in Atoms: Basic ConceptsTopic 9Topic 9
• Substitute c and the microwave’s frequency, , into the equation. Note that hertz is equivalent to 1/s or s–1.
Calculating Wavelength of an EM Wave
Electrons in Atoms: Basic ConceptsElectrons in Atoms: Basic ConceptsElectrons in Atoms: Basic ConceptsElectrons in Atoms: Basic ConceptsTopic 9Topic 9
Examples2) What is the frequency of red light with a
wavelength of 4.2 x 10-5 m?
3) What is the energy of the photon above?
c = λf or f = c/λ
f = 3.0 x 108m/s
4.2 x 10-5 m= 7.1 x 1012 sec-1
E = hfE = (6.626 x 10-34 J sec) (7.1 x 1012 sec-1)
E = 4.73 x 10-21 Joules
