Lecture 8: Waves and Particle- A Light Motif

"My heart leaps up when I behold A rainbow in the sky."
William Wordsworth-- My Heart Leaps Up

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Date: September 27, 1994
Reading Assignment: pp. 62-73

Description : introduction to waves and the wavelike nature of the electromagnetic spectrum

Objectives

understand the parts of a wave

be able to describe the experiments which show the wave-like nature of light

understand the parts of the EM spectrum \\

Lecture Outline

Slide # 1: Waves and Particles

Click here for subtitle

A Light Motif

Slide # 2: Astro 105- the story so far

Lecture 1-3

Sky, Sun and Moon

Lecture 4/5

Dead Greeks and the Scientific Method

Lecture 6/7

Copernican Revolution

Slide # 3: Upcoming Topics

Lecture 8-10

Light, the Spectrum and Atomic Physics

Lecture 11-12

Telescopes of all kinds

Lecture 13-14

the Earth

Lecture 15

the Moon

Slide # 4: How do Astronomers know anything?

Experiments are not possible

No direct contact with Celestial Objects

Slide # 5: Light

Almost everything we know comes from the analysis of ÒstarlightÓ

No information can travel faster than light

Slide # 6: The Nature of Light

Light is neither a wave or a particle

Wave-like properties

Particle-like properties

Slide # 7: Light

A special kind of radiation

Only one part of the Electromagnetic Spectrum

Slide # 8: Speed of Light

All EM radiation travels at the speed of light

No information can travel faster than 300,000 km/s

Slide # 9: Speed of Light

Light travels at a finite speed

Slide # 10: Waves

a way that energy is moved

wave patterns repeat

cycles in time

cycles in space

Slide # 11: Waves - cycles in space

Crests

Troughs

Slide # 12: Waves - cycles in space

Wavelength

Distance between two crests or troughs

Slide # 13: Waves - cycles in space

Amplitude

Height or depth of the wave

Slide # 14: Waves - cycles in time

waves and troughs pass the observer

Slide # 15: Waves - cycles in time

crests and troughs pass at regular time intervals

period is the length of time between troughs or crests

Slide # 16: Waves - period

observer times the time between two crests or two troughs

Slide # 17: Waves - frequency

observer times the number of cycles per second

Slide # 18: Units of Frequency

units of inverse time

events per unit time

normally measured in Hertz

one Hertz = one cycle / second

Slide # 19: Waves - cycles in time

Period and frequency are related

Slide # 20: Frequency and Period Slide # 21: Cycles of Space and Time

Wavelength, frequency, and velocity are related

Slide # 22: Cycles of Space and Time

velocities of waves are usually constant

frequency and wavelength are inversely proportional

Slide # 23: Why do we say light has Òwave-likeÓ properties

Diffraction

Interference

Slide # 24: Diffraction

ability of light to bend around corners

Slide # 25: Interference

ability of waves to reinforce or cancel each other

Slide # 26: Interference- reinforcing waves Slide # 27: Interference- cancelling waves Slide # 28: Interference

contructive interference- reinforcing

destructive interference- cancelling

Slide # 29: Interference

Waves of the same amplitude and wavelength can exactly cancel

Slide # 30: Experiments which demonstrate the wave-like properties of light

single slit

demonstration of diffraction

double slit

YoungÕs demonstration of interference

Slide # 31: Waves in what?

electromagnetic waves do not travel through a medium

oscillations of the electrical field

Slide # 32: Electrical Fields

All charged particles have electrical fields

electrical fields are Òinverse square lawsÓ like gravity

charged particles ÒcommunicateÓ through their electrical fields

Slide # 33: Electromagnetism

changing electrical fields produce magnetic fields

electricity and magnetism are two aspects of one phenomenon

Slide # 34: Electromagnetism

oscillating electrical fields produce electromagnetic waves

Slide # 35: Light is one form of electromagnetic energy

light is produced by oscillating electromagnetic fields

Slide # 36: Visible Light

white light is mixture of colors

red

orange

yellow

green

blue

violet

Slide # 37: Visible Light Slide # 38: Wavelengths of Visible Light

Red light = 7 x 10-5 cm

0.00007 cm

Blue light = 4 x 10-5 cm

0.00004 cm

Slide # 39: Red Light vs Blue Light

Red has a longer wavelength

Red has a lower frequency

Remember: Longer wavelengths = lower frequencies

Slide # 40: Electromagnetic Waves

wavelengths > 7 x 10-5 cm are invisible to humans

wavelengths < 4 x 10-5 cm are invisible to humans

longer and shorter wavelengths are also electromagnetic radiation

Slide # 41: The Electromagnetic Spectrum

Radio

Infrared

Visible

Ultraviolet

X-ray

Gamma-ray

Slide # 42: Radio

Wavelengths > 1 mm

Frequencies 0 to 300 Giga-Hz (3x1011Hz)

Typical Examples

AM/FM Radios

TV signals

Slide # 43: Infrared

Wavelengths between 1mm and 7 x 10-5 cm

Frequencies between 3 x 1011 Hz and 4 x 1014 Hz

Typical Examples

Heating devices

stove burners

space heaters

TV remote controls

Slide # 44: Visible

Wavelengths between 7 x 10-5 cm and 3 x 10-5 cm

Frequencies between 4 x 1014 Hz and 1 x 1015 Hz

Typical Examples

All that you see

Slide # 45: Ultraviolet

Wavelengths between 3 x 10-5 cm and 3 x 10-7 cm

Frequencies between 1 x 1015 Hz and 1 x 1017 Hz

Typical Examples

Sunlight which causes sunburn and suntan

Slide # 46: X-rays

Wavelength between 3 x 10-7 cm and 3 x 10-10 cm

Frequencies between 1 x 1017 Hz and 1 x 1020 Hz

Typical Example

Medical X-ray Machines

Slide # 47: Gamma-Rays

Wavelengths < than 3 x 10-10 cm

Frequencies > 1 x 1020 Hz

Typical Examples

Nuclear reactions

Nuclear bombs

Slide # 48: Atmospheric Blockage

some wavelengths are blocked by the atmosphere

Slide # 49: Opacity

the extent which a wavelength is blocked

opaque is the opposite of transparent

Slide # 50: Atmospheric Opacity- Why does it happen?

gases absorb and scatter light

the amount of absorption depends on

the type of molecule

the wavelength of light

Slide # 51: Atmospheric Opacity - Some Examples

Water and Oxygen absorb some radio frequencies

Carbon Dioxide absorbs some infrared frequencies

the Ozone layer blocks most Ultraviolet, X-ray and Gamma-rays

Slide # 52: Summary of Lecture 8

Waves are oscillations in space and time

Light has wavelike properties

Frequency and wavelength are mathematically related

Light is just one part of the EM spectrum

The atmosphere absorbs most of the other parts of the EM Spectrum