Electromagnetic RadiationElectromagnetic Radiation

Electromagnetic Spectrum

LightLight

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

Notice: red light bent the least, violet the most

http://www.pact.cpes.sussex.ac.uk/~edmundjc/web-pictures/newton.gif

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 Huygens1678: 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

Direction of displacement = direction of travel

Direction of displacement is at right angle to direction of travel

Transverse Wave

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 notnot 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

Electromagnetic Spectrum

Comparing

• Long wavelength

• Slow frequency• Low energy

• Short wavelength

• Rapid frequency• High 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: Planck: –Energy consists of tiny particles that Energy consists of tiny particles that

can’t be divided into anything smallercan’t be divided into anything smaller