Lecture #2Lecture #2

Seeing the light1/29/13

What happens to light when it What happens to light when it interacts with matter?interacts with matter?

• Reflects• Absorbed• Refracts

Changes speed

• Polarized• Diffracts

What happens to light when we What happens to light when we see?see?

TodayToday’’s topicss topics

• Learning styles

• Waves• Refraction• Diffraction / interference• Light sources• Intensity

• Homework on web site for next week

We can think about light in several We can think about light in several waysways

• Light as a wave: oscillating electromagnetic field

We can think about light in several We can think about light in several waysways

• Light as a wave: oscillating electromagnetic field

• Light as a ray: direction of wave

We can think about light in several We can think about light in several waysways

• Light as a wave: oscillating electromagnetic field

• Light as a ray: direction of wave• Light as a photon: packet of energy which

excites electrons

Light as a waveLight as a wave

• Wave characteristicsWavelengthFrequencySpeed

• Wavefront goes in one direction = rayTravels in straight line till it encounters

different material

Wavelength – distance btn Wavelength – distance btn peakspeaks

λ varies across visible spectrumλ varies across visible spectrum

• 400 nm

• 700 nm

FrequencyFrequency

• Frequency of wave depends on wavelength and speed

c= λf f = c /λ

Units make sense:

FrequencyFrequency

• Typical frequency of visible light

Huge number

So we characterize light by wavelength

Visible light is small part of the Visible light is small part of the electromagnetic spectrumelectromagnetic spectrum

Different colors correspond to Different colors correspond to different wavelengthsdifferent wavelengths

Wavelength is proportional to 1/ frequency

Speed, cSpeed, c

• Speed of light in a vacuum (outer space)3 x 108 meters / second (299,792,458 m/s)6.7 x 108 miles per hour

•Moon is 384,403 km awayTakes 1.2 s for light go from moon to earth

•Sun is 149,600,000 kmTakes light 8 min 19 s to get from sun to earth

Speed of light in other materialsSpeed of light in other materials

• Light moves slower in matter• Index of refraction = speed in

vacuum speed in

matter

• n depends on materialMore light interacts, the slower it goes

Speed of light in a material (v) Speed of light in a material (v) versus index of refraction, nversus index of refraction, n

v = c / n

water glass diamond silicon

What happens when light goes What happens when light goes from one material into another?from one material into another?

What do you think will happen to the What do you think will happen to the angle between the ray and the normal angle between the ray and the normal

as it enters the water?as it enters the water?

a. It will increase (move away from the normal)

b. It will stay the samec. It will decrease (move towards the normal)

What characteristics of the ray and/or What characteristics of the ray and/or the materials could be causing this?the materials could be causing this?

a. b. c.

Possible answers?

SnellSnell’’s law quantifies bendings law quantifies bending

θ

θ

n1 sin θn2 sin θ

n1

n2

SnellSnell’’s laws law

θ

θ

n1

n2

and so light bends in

SnellSnell’’s laws law

θ

θ

n1

n2

If go from low to high index

- light bends in towards normal

SnellSnell’’s law - in reverses law - in reverse

θ

θ

n1

n2

If go from HI to LOW index

-Light bends away from normal

-Light path is reversible

Can download simulator from Can download simulator from PhET PhET http://phet.colorado.edu/en/simulation/bending-http://phet.colorado.edu/en/simulation/bending-

lightlight

Part of homework#2 uses this simulator

Effect of changing angle and Effect of changing angle and materialsmaterials

Can use tools to measure angles, light speed and light intensity

Outcome: Objects are not where Outcome: Objects are not where they appear to bethey appear to be

Hemisphere of light above Hemisphere of light above becomes a cone belowbecomes a cone below

Archer Archer fish make fish make

an an adjustmeadjustme

ntnt

SnellSnell’’s window – see light above as a s window – see light above as a cone of light below the water cone of light below the water

Shanon Conway

How does refraction depend on How does refraction depend on wavelength?wavelength?

Pink FloydPink Floyd

Refraction differs with Refraction differs with wavelengthwavelength

Index of refraction depends on Index of refraction depends on how much light interacts with how much light interacts with

materialmaterial

Glass

SnellSnell’’s laws law

θ

θ

n1=1.00

n2 =1.50917n2 =1.51534n2 =1.52136

glass

Air

SnellSnell’’s laws law

θ

θ

n1=1.00

n2 =1.50917n2 =1.51534n2 =1.52136

glass

Air

As n2 gets bigger…

sin θ2 and θ2 get smaller

SnellSnell’’s laws law

θ

θ

n1=1.00

n2 =1.50917n2 =1.51534n2 =1.52136

glass

Air n 2 θ

1.50917 27.941.51534 27.8227.821.52136 27.70Shorter wavelength -

SnellSnell’’s laws law

θ

θ

n1=1.00

n2 =1.50917n2 =1.51534n2 =1.52136

glass

Air n 2 θ

1.50917 27.941.51534 27.8227.821.52136 27.70Shorter wavelength –

bends MORE

PhET only does what you tell itPhET only does what you tell it

Doesn’t have built in relationships of n and wavelength

Applications of SnellApplications of Snell’’s laws law

• Eye design

• Glasses design

• Seeing across interfaces

• Separating wavelengths of light

Another way to separate wavelengths Another way to separate wavelengths – Diffractive interference– Diffractive interference

Double slit – each slit becomes a point source of light

InterferenceInterference

If waves are in phase – constructive If waves are in phase – constructive interference; if they are out of phase – interference; if they are out of phase –

destructive interferencedestructive interference

Construct

Destruct

Construct

Destruct

Constructive interferenceConstructive interferenceDistance two rays travel must differ Distance two rays travel must differ

by a multiple # of whole wavelengthsby a multiple # of whole wavelengths

r = nλ

r

θD

D

r

Constructive interferenceConstructive interferenceDistance two rays travel must differ Distance two rays travel must differ

by a multiple # of whole wavelengthsby a multiple # of whole wavelengths

r = Dsinθ = nλ

r

θD

xn

L

D

r

θ

Similar triangles

θ

sinθL

xn

Constructive interference Constructive interference occurs at distance x, which is occurs at distance x, which is

given by:given by:

D

D = distance between two slitsL = distance between slits and screenx = distance between bright spots

L

x

DiffractionDiffraction

• What happens as slits get closer together?

For more closely spaced slits, D is smaller and bright bands are further apart

•What happens as wavelength gets longer?

Two slit interferenceTwo slit interference

http://www.colorado.edu/physics/2000/schroedinger/two-slit2.html

DiffractionDiffraction

• Depends on wavelength

Spots are further apart for longer λ

SimulatorSimulator

http://www.walter-fendt.de/ph14e/doubleslit.htm

Diffraction depends on Diffraction depends on wavelengthwavelength

Light sourcesLight sources

Light Light ““sourcesource””??

Light sourceLight source

Northern lightsNorthern lights

Phillipe Mousette, Quebec Canada

Biological light sourceBiological light source

Different species either make light through a luciferase reaction or have bacteria that make light and are symbionts.

Incandescent bulbIncandescent bulb

California lawmaker proposes to ban the bulb

http://sustainabledesignupdate.com/?p=115

Fluorescent Fluorescent bulbbulb

Electric discharge inside bulb causes high speed electrons to strike coating which fluoresces

Light emitting diodeLight emitting diode

LaserLaser

Sun is a high temperature light Sun is a high temperature light bulbbulb

• Temperature is around 5800 K• This produces broad spectrum

light just like an incandescent lightbulb

Solar spectrum peaks near 500 Solar spectrum peaks near 500 nmnm

Actual solar spectrumActual solar spectrum

Measuring spectral distributionMeasuring spectral distribution

• Use computerized spectrometer• Collects light• Disperses with diffraction grating• Sends to multipixel detector

Ocean Optics spectrometerOcean Optics spectrometer1. Fiber in2. Slit3. Filter4. Collimating mirror5. Diffraction grating6. Mirror7. Lens8. Detector

Spectra of different light Spectra of different light sourcessources

Relative brightness of sun and Relative brightness of sun and moonmoon

Sun light comes directly to earth

Moonlight - sun is scattered off moon and comes to earth

Light Light intensityintensity

Sun is about 105-106 brighter than a full moon

We calculated sun to be 2 x 105 brighter

1candela/m2 = 1 lux

SummarySummary

• Waves• Refraction - Snell’s law• Interference - diffraction• Light sources and spectral

distribution• Intensity

• Learning styles