Electromagnetic Radiation

Electromagnetic Spectrum

Light 1600’s: sunlight considered purest form of light 1665 – Isaac Newton –passed beam of sunlight through a prism beam spread out band of colors (Roy G. Biv) –rejoined the colors with 2 nd prism & got white light again

note: red light bends the least, violet the most

Newton Thought light made up of tiny particles with no mass – explains why shadows have sharp edges couldn’t explain how different colored particles 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 waves Wavelength Frequency Amplitude Velocity

Direction of displacement is at right angle to direction of travel Transverse Wave

Direction of displacement = direction of travel

Transverse vs Longitudinal Waves Transverse (ocean) Longitudinal (compression/sound)

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: –1 st determined about 1676 by Danish astronomer –speed of light = 3.0 X 10 8 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 worked out mathematical equations describing electricity & magnetism electric & magnetic fields cannot exist independently electromagnetic radiation in Maxwell’s equation moved at same speed as light! not –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? 1800: Herschel 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 could not be seen by human eye

Silver Nitrate as detector 1614: known that silver nitrate (AgNO 3 - white cmpd) darkens on exposure to sunlight 1770: Scheele soaked strips white paper in AgNO 3 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 AgNO 3 & 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

Electromagnetic Spectrum

Comparing Light LONG wavelength –red end –slow frequency –low energy SHORT wavelength –violet end –rapid frequency –high energy All electromagnetic radiation has same velocity: 3.00 x 10 8 m/sec 3.00 x 10 8 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!

Pre-Planck classical physics: –assume 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 called quanta

Explanation Pre-Planck: –energy considered to be continuous –could be broken into smaller & smaller bits indefinitely Planck:Planck: –energy consists of tiny particles that can’t be divided into anything smaller