Lecture #2 Seeing the light 1/29/13

What happens to light when it interacts with matter? Reflects Absorbed Refracts Changes speed Polarized Diffracts

What happens to light when we see?

Today’s topics Learning styles Waves Refraction Diffraction / interference Light sources Intensity Homework on web site for next week

We can think about light in several ways Light as a wave: oscillating electromagnetic field

We can think about light in several ways Light as a wave: oscillating electromagnetic field Light as a ray: direction of wave

We can think about light in several ways 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 wave Wave characteristics Wavelength Frequency Speed Wavefront goes in one direction = ray Travels in straight line till it encounters different material

Wavelength – distance btn peaks λ varies across visible spectrum 400 nm 700 nm

Frequency Frequency of wave depends on wavelength and speed c= λ  f f = c /  Units make sense:

Frequency Typical frequency of visible light Huge number So we characterize light by wavelength

Visible light is small part of the electromagnetic spectrum

Different colors correspond to different wavelengths Wavelength is proportional to 1/ frequency

Speed, c Speed of light in a vacuum (outer space) 3 x 10 8 meters / second (299,792,458 m/s) 6.7 x 10 8 miles per hour Moon is 384,403 km away Takes 1.2 s for light go from moon to earth Sun is 149,600,000 km Takes light 8 min 19 s to get from sun to earth

Speed of light in other materials Light moves slower in matter Index of refraction = speed in vacuum speed in matter n depends on material More light interacts, the slower it goes

Speed of light in a material (v) versus index of refraction, n v = c / n water glass diamond silicon

What happens when light goes from one material into another?

What do you think will happen to the angle between the ray and the normal as it enters the water? a.It will increase (move away from the normal) b.It will stay the same c.It will decrease (move towards the normal)

What characteristics of the ray and/or the materials could be causing this? a. b. c. Possible answers?

Snell’s law quantifies bending   n 1 sin θ   n 2 sin θ  n1n1 n2n2

Snell’s law   n1n1 n2n2 and so light bends in

Snell’s law   n1n1 n2n2 If go from low to high index - light bends in towards normal

Snell’s law - in reverse   n1n1 n2n2 If go from HI to LOW index -Light bends away from normal -Light path is reversible

Can download simulator from PhET Part of homework#2 uses this simulator

Effect of changing angle and materials Can use tools to measure angles, light speed and light intensity

Outcome: Objects are not where they appear to be

Hemisphere of light above becomes a cone below

Archer fish make an adjustment

Snell’s window – see light above as a cone of light below the water Shanon Conway

How does refraction depend on wavelength?

Pink Floyd

Refraction differs with wavelength

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

Snell’s law   n 1 =1.00 n 2 = n 2 = n 2 = glass Air

Snell’s law   n 1 =1.00 n 2 = n 2 = n 2 = glass Air As n 2 gets bigger… sin  2 and  2 get smaller

Snell’s law   n 1 =1.00 n 2 = n 2 = n 2 = glass Air n 2 θ  Shorter wavelength -

Snell’s law   n 1 =1.00 n 2 = n 2 = n 2 = glass Air n 2 θ  Shorter wavelength – bends MORE

PhET only does what you tell it Doesn’t have built in relationships of n and wavelength

Applications of Snell’s law Eye design Glasses design Seeing across interfaces Separating wavelengths of light

Another way to separate wavelengths – Diffractive interference Double slit – each slit becomes a point source of light

Interference

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

Constructive interference Distance two rays travel must differ by a multiple # of whole wavelengths r = n  r  D D r

Constructive interference Distance two rays travel must differ by a multiple # of whole wavelengths r = Dsin  = nλ r  D xnxn L D r  Similar triangles  sin   L xnxn

Constructive interference occurs at distance x, which is given by: D D = distance between two slits L = distance between slits and screen x = distance between bright spots L x

Diffraction 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 interference

Diffraction Depends on wavelength Spots are further apart for longer 

Simulator

Diffraction depends on wavelength

Light sources

Light “source”?

Light source

Northern lights Phillipe Mousette, Quebec Canada

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

Incandescent bulb California lawmaker proposes to ban the bulb

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

Light emitting diode

Laser

Sun is a high temperature light bulb Temperature is around 5800 K This produces broad spectrum light just like an incandescent lightbulb

Solar spectrum peaks near 500 nm

Actual solar spectrum

Measuring spectral distribution Use computerized spectrometer Collects light Disperses with diffraction grating Sends to multipixel detector

Ocean Optics spectrometer 1. Fiber in 2. Slit 3. Filter 4. Collimating mirror 5. Diffraction grating 6. Mirror 7. Lens 8. Detector

Spectra of different light sources

Relative brightness of sun and moon Sun light comes directly to earth Moonlight - sun is scattered off moon and comes to earth

Light intensity Sun is about brighter than a full moon We calculated sun to be 2 x 10 5 brighter 1candela/m 2 = 1 lux

Summary Waves Refraction - Snell’s law Interference - diffraction Light sources and spectral distribution Intensity Learning styles