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 EM Spectrum 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 Primary Colors 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