Electromagnetic Radiation
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Explore the fascinating journey of light from the 1600s to modern physics. This overview discusses Isaac Newton's particle theory of light, the wave theory proposed by Christian Huygens, and the eventual unification of light with electricity and magnetism through Maxwell's equations. Learn about the electromagnetic spectrum, the discovery of infrared and ultraviolet radiation, and how these advances paved the way for understanding electromagnetic radiation, including radio waves and X-rays. This rich history illustrates how our perception of light has evolved.
Electromagnetic Radiation
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Presentation Transcript
Light 1600’s – sunlight considered purest form of light • 1665 – Isaac Newton • passed a beam of sunlight through a prism • beam spread out • band of colors (Roy G. Biv) • rejoined the colors with 2nd prism & got white light again
Newton Thought light made up of tiny particles with no mass • explains why shadows have sharp edges • Couldn’t explain how particles of different colors were different or why were refracted differently by prism • Couldn’t explain why 2 beams of light didn’t affect each other when crossed • particles of light should collide off each other
Christian Huygens 1678: suggested light composed of waves • explained why 2 beams of light could cross each other without being disturbed • explained refraction in prism: - different colors have different wavelengths • people were used to waves in water • water waves move around an obstruction • waves couldn’t explain shadows with sharp edges
Wave Vocabulary • Transverse and Longitudinal • Wavelength • Frequency • Amplitude • Velocity
Transverse Wave Direction of displacement is at right angle to direction of travel
Wavelength vs. Frequency • WAVELENGTH = distance light travels to complete 1 cycle • FREQUENCY = number of cycles completed in 1 second
Wave Velocity • speed of wave in direction is traveling • Velocity = wavelength x frequency Velocity =
Light: Particles vs. Waves Huygens had better argument, but Newton was more famous • people went with Newton's theory: Light was particles! • Speed of light 1st determined about 1676 by Danish astronomer • speed of light = 3.0 X 108 m/sec
Calculating Wavelengths of Light • visible light waves have lengths ~ 1/20,000 cm • Red a little longer, violet a little shorter • short wavelength explains why light cast sharp shadows despite being waves • waves can only bend around obstacles that are about same length • cannot bend around anything substantially longer than itself
The Ether • Water waves move in water • Sound waves move in air • Light waves move through vacuum • Gravity, Electricity, & Magnetism also felt across vacuum • people couldn't accept this: • postulated subtle form of matter called ether • not easily detectable • “Ether” idea held until 1900
Maxwell • 1864-1873 worked out equations describing electricity & magnetism • electric & magnetic fields cannot exist independently • electromagnetic radiation in Maxwell’s equation moved at same speed as light! • could not be coincidence!
Light that can’t be seen? • Light was electromagnetic radiation! • Maxwell’s equations unified electricity, magnetism, & light • But visible light only accounts for a fraction of Maxwell’s equations • ? other frequencies and wavelengths
Heat & Light: Separate or same? Herschel – 1800 studied spectrum with thermometer - found highest temperature at red end & coolest at violet end • placed thermometer beyond red & temp was higher there than anywhere in visible spectrum • discovered Infared rays, which we cannot see
Infrared Radiation • By 1850: infrared rays were demonstrated to have all the properties of light • except they could not be seen by eye
Silver Nitrate as detector • 1614: knew that silver nitrate (white cmpd: AgNO3) darkens on exposure to sunlight • 1770: Scheele soaked strips of white paper in AgNO3 solutions and placed them in different parts of spectrum • darkened least quickly in red and fastest in violet • ? the first photographs
Discovery of ultraviolet! • After Herschel discovered infrared using thermometers: • 1801: Ritter repeated Scheele’s experiment with paper soaked in AgNO3 & put strips beyond violet • strips darkened even quicker than in violet light • discovered ultraviolet light!
Radio Waves • 1888: Hertz used oscillating electric current to emit electromagnetic radiation • had detector that could move around to map the electromagnetic wave & determine its length • found radio waves far beyond infrared radiation • have wavelengths from cm to km
X-Rays • 1895: Roentgen discovered that his cathode ray tube was emitting radiation = X-rays
Short wavelength Rapid frequency High energy Comparing • Long wavelength • Slow frequency • Low energy All electromagnetic radiation has same velocity: 3.00 x 108 m/sec 3.00 X 108 m/sec =
Heat Flow • Objects hotter than surroundings lose heat as electromagnetic radiation • higher the temp, the more intense the radiation • Hot objects glow! • glow different colors at different temperatures!
Ultraviolet Catastrophe • Classical physics - Assume that every wavelength has equal chance of being radiated • Classical wave model could not explain why different colors were emitted at different temperatures
Planck • shorter the wavelength, the less chance it has to be emitted! • Matter can gain or release energy only in very small increments = quanta
What physical explanation goes with Planck’s assumption? • Pre-Planck: • Energy was considered to be continuous • could be broken into smaller & smaller bits indefinitely • Planck: • Energy consists of tiny particles that can’t be divided into anything smaller
