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The Electromagnetic Spectrum
The Electromagnetic Spectrum
A Brief History of Light
1000 AD It was proposed that light consisted of tiny
particles
Newton Used this particle model to explain
reflection and refraction
Huygens 1678
Explained many properties of light by proposing light was wave-like
A Brief History of Light, cont
Young
1801
Strong support for wave theory by showing interference
Maxwell
1865
Electromagnetic waves travel at the speed of light
A Brief History of Light, final
Planck
EM radiation is quantized
Implies particles
Explained light spectrum emitted by hot objects
Einstein
Particle nature of light
Explained the photoelectric effect
The Particle Nature of Light
“Particles” of light are called photons
Each photon has a particular energy
E = h ƒ
h is Planck’s constant
h = 6.63 x 10-34 J s
Encompasses both natures of light
Interacts like a particle
Has a given frequency like a wave
Dual Nature of Light
Experiments can be devised that will display either the wave nature or the particle nature of light
In some experiments light acts as a wave and in others it acts as a particle
Nature prevents testing both qualities at the same time
The Spectrum of EM Waves
The EMSpectrum
Note the overlap between types of waves
Visible light is a small portion of the spectrum
Types are distinguished by frequency or wavelength
Notes on The EM Spectrum
Radio Waves
Used in radio and television communication systems
Microwaves
Wavelengths from about 1 mm to 30 cm
Well suited for radar systems
Microwave ovens are an application
Notes on the EM Spectrum, 2
Infrared waves Incorrectly called “heat waves”
Produced by hot objects and molecules
Readily absorbed by most materials
Visible light Part of the spectrum detected by the
human eye
Most sensitive at about 560 nm (yellow-green)
Notes on the EM Spectrum, 3
Ultraviolet light Covers about 400 nm to 0.6 nm
Sun is an important source of uv light
Most uv light from the sun is absorbed in the stratosphere by ozone
X-rays Most common source is acceleration of
high-energy electrons striking a metal target
Used as a diagnostic tool in medicine
Notes on the EM Spectrum, final
Gamma rays
Emitted by radioactive nuclei
Highly penetrating and cause serious damage when absorbed by living tissue
Looking at objects in different portions of the spectrum can produce different information
Sources of Light
Rays of light come from sources of light
Luminous Source – an object that emits its own light
Illuminated Source – an object that reflects light
Doppler Effect and EM Waves
A Doppler Effect occurs for em waves, but differs from that of sound waves For sound waves, motion relative to a
medium is most important For light waves, the medium plays no role since
the light waves do not require a medium for propagation
The speed of sound depends on its frame of reference
The speed of em waves is the same in all coordinate systems that are at rest or moving with a constant velocity with respect to each other
Luminous Flux
The rate at which light energy is emitted from a luminous source.
Quantity of light
Symbol: P
Unit: Lumen (lm)
100 W light bulb emits 1750 lm
Illuminance
The amount of illumination provided by a luminous source.
Rate at which light strikes a surface
Symbol: E
Unit: lux, lx
Equivalent to lm/m2
Point Source Illumination
How do make an object brighter?
Use a brighter light bulb
Increases the luminous flux
Move the light bulb closer to the object
Inverse-square relationship
Point Source Illumination
Color
White light is a combination of all wavelengths
Can be broken into wavelengths with a prism
Primary Colors
Red, Blue, Green
Primary colors of pigments:
Red, Yellow, Blue
Color Mixing
Combining two primary colors gives secondary colors
Red + Blue = Magenta
Green + Red = Yellow
Blue + Green = Cyan
Complimentary Colors
Two colors, when added together create white light
Primary + Secondary Color
Blue + Yellow
Red + Cyan
Green + Magenta
Doppler Equation for EM Waves
Doppler Shift
Doppler Equation, cont
Astronomers refer to a red shift when objects are moving away from the Earth since the wavelengths are shifted toward the red end of the spectrum
If an object is moving toward Earth, it has a blue shift
