Course Info

http://www.colorado.edu/physics/phys1230/phys1230_fa12/

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Office hoursInstructor contact info

Dr. Ellen Keister

Map to Help Room (G2B90)

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Chapter 1

• Light rays– Laser beam– Ray propagation

• Waves– Rope waves

• Wave medium• Wave propagation

– Other waves– Wave properties– Electromagnetic waves

• Light waves (EM waves)– Speed of light– Wavelength and

frequency– Amplitude

• The Electromagnetic Spectrum– Visible light– Radio waves– Microwaves– Xrays

Light Rays• Rays are a convenient descriptive tool• Indicate the direction the light is traveling

Light Rays

• We only see light when a ray enters our eye

Light Rays: Bending

• Light rays bend when they pass from one material to another

• This is the principle behind lenses (chapter 3)

Air

Glass orwater

Light Rays: Reflection

Mirror

Specular reflection Diffuse reflection

Waves• A wave is a propagating disturbance or oscillation

• The medium does not move very far, but the wave disturbance does

Rope Waves• Disturbance has some speed, but rope segments

do NOT move in direction of wave

• Rope segments move up and down, not in the same direction as the disturbance moves

Rays, Waves and Wavefronts• The The wavefrontwavefront is defined as a curve or surface is defined as a curve or surface

perpendicular (at a 90perpendicular (at a 90oo angle) to all the rays angle) to all the rays• OR, rays are drawn at right angles to the OR, rays are drawn at right angles to the

wavefrontwavefront

Wavefront ExamplesLet’s draw wavefronts for these rays:Let’s draw wavefronts for these rays:

What type of light source might produce these rays?What type of light source might produce these rays?

A laserA laser Two light bulbsTwo light bulbs

Wave Properties

Aperiodic waveAperiodic wave

Periodic wavePeriodic wave

Periodic wavePeriodic wave

Periodic Wave Properties (Vocabulary)

Wavelength (λ)Amplitude

Ray: Direction of wave motion

Periodic Wave Properties (Vocabulary)

• Frequency (f) is the number of oscillations per Frequency (f) is the number of oscillations per second at a fixed point in spacesecond at a fixed point in space

• Period (T) is the inverse of frequency, is the time it Period (T) is the inverse of frequency, is the time it takes for one wavelength to pass a fixed pointtakes for one wavelength to pass a fixed point

Ray: Direction of wave motion

Clicker Question: Wavelength

What is the wavelength of the red wave?

A)1m

B)2m

C)3m

meters

Clicker Question: Period

meters

What is the period of What is the period of the red wave?the red wave?

A)A)2 milliseconds 2 milliseconds

B)B)2 seconds 2 seconds

C)C)2 minutes2 minutes

Periodic Wave Properties: Speed

For periodic waves, we can identify a speed (v)

speed = distance/time

speed = wavelength/period

speed = Wavelength x frequency

v = λf

Light Waves: Properties• What do these properties mean for light waves?

• The amplitude of a light wave is related to its brightness

• Light waves are special. They have a constant speed over all colors, over billions of years, and over all brightness.

We use the letter ‘ c ’ to denote the speed of light in a vacuum

Light Waves: PropertiesRecall:

v = λffor light waves we use

c = λfand ‘c’ never changes.Thus the wavelength and the frequency of light waves have a fixed relationship:

λ = c/f or f = c/λ

Wavelength and Color

• The wavelength or frequency of light waves is related to the colors we perceive when the waves enter our eyes

• The wavelengths of light that our eyes and brain “see” are in the 400-700 nanometer (nm) range.

• One nanometer is 10-9 meters

Clicker Question: Wavelengths• The wavelength of green

light is around 500 nm. How many wavelengths of green light fit into one centimeter (0.4 inches, about a fingertip)?

a) 20 thousandb) 50 thousandc) Two milliond) Two billione) 5 billion

Wavelength and Color

Light with wavelength of 650 nmappears red when it enters a viewers eye

Light with wavelength of 520 nmappears green when it enters a viewers eye

Light with wavelength of 470 nmappears blue when it enters a viewers eye

Clicker questions• Which of the light waves has

the longest wavelength?• Which of the light waves is

brightest?• Which of the light waves has

the highest speed in empty space?

a) b) c)d) They all have the same speed

a)

b)

c)

Clicker question

• What does the viewer see when the wave at left with wavelength 650 nm goes by him?a) Redb) Bluec) Greend) Whitee) Nothing

What happens when two or more waves with different wavelengths reach your eye?

Light with both wavelengths 650 nm and 520 nm appears yellow when it enters a viewers eye

Light with only wavelength 580 nm also appears yellow when it enters a viewers eye

What is white light?

Light which is a mixture of 650, 520 and 470 nm wavelengths (and possibly more wavelengths) appears white when it reaches your eye

No single wavelength (mono-chromatic) wave appears white when it reaches your eye!

White Light through a Prism

A prism spreads out the over- lapping wavelengths in white light into different spatial locations where they can be seen as colors.

400 nm400 nm

700 nm700 nm