Cool Things Light Does

It moves at about 300,000,000 m/sec!

Speed of Light, c

Roemer’s First Measurement of c (1676)

So, What is Light?

Light consists of a varying electric and magnetic field

Different Wavelengths Lead To:

Radio Waves

• Astronomy• Communication – AM, Shortwave, TV, FM

Microwave

• Radar (Airport, Police, Weather Stations)• Cooking• Cell Phones

Infrared

• TV remotes/remote keyboard• Heating/Drying• Night Vision

Visible Light

• Photography• Photosynthesis

Ultraviolet

• Sterilize Food and Surfaces• Run Solar Cells• Set Dental Fillings

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Xray

• Bones Pictures

Roentgen’s wife’s hand• Wilhelm Conrad Röntgen (German:

[ˈvɪlhɛlm ˈʁœntɡən]; 27 March 1845 – 10 February 1923) was a German physicist, who, on 8 November 1895, produced and detected electromagnetic radiation in a wavelength range today that was known as X-rays or Röntgen rays, an achievement that earned him the first Nobel Prize in Physics in 1901.[1] In honour of his accomplishments, in 2004 the International Union of Pure and Applied Chemistry (IUPAC) named element 111, roentgenium, a radioactive element with multiple unstable isotopes, after him.

• http://en.wikipedia.org/wiki/Wilhelm_R%C3%B6ntgen

Gamma Radiation

• Medical Imaging• Cancer Treatment• (A form of nuclear radiation)

Polarized Light

Light Waves are Transverse

A Physical Model

Two Polaroid Filters

Test Sunglasses in the Store

Three Processes To Polarize Light

1. Polaroid Filters Let Only One Plane Through

2. Calcite – double refraction

3. Reflected Light May Be Polarized

Vertical Glare from Vertical Surfaces

Adding a Filter to a Camera

Horizontal Glare from Horizontal Surfaces

Which way should sun glasses be made?

In Summary: Polarized Light• Polarized light lies along the same plane as that of the

vibrating electron that produced it.– A vertically vibrating electron emits light that is vertically

polarized.

– A horizontally vibrating electron emits horizontally polarized light.

• Common light sources emit light that is not polarized because the electrons vibrate in random directions.

• Light that is not polarized can be polarized by a polarizing filter.

Polarized Light• Light that reflects at glancing angles

from nonmetallic surfaces vibrates mainly in the plane of the reflecting surface.– Which pair of glasses would best

block glare from the road?

• Light can also be polarized by refraction.– the mineral calcite refracts incident

light into two different paths.– Both refracted light beams are

polarized - one in a direction parallel to the surface and the other in a direction perpendicular to the surface.

Polarized Light

• Light will pass through a pair of polarizing filters when their polarizing axes are aligned, but not when they are crossed at right angles.

Polarized Light and 3-D Viewing

• Vision in 3 dimensions depends on the fact that each eye views a scene from a slightly different angle.

• Slide shows or movies achieve a 3-D effect by projecting a pair of views that are polarized at right angles to each other.

• Polarizing eyeglasses ensure that each eye sees a separate picture.

Human Body

Light and Transparent Materials• How a material responds to light depends on the natural frequency of

its electrons and on the frequency of the incident light.– Visible light vibrates at more than 100 trillion times per second (1014 hertz).– The natural frequencies of the electrons within a material depend on how

strongly they are attached to the nucleus.

• Transparent materials allow light to pass through in straight lines.– Glass, water– Electrons in glass have a natural frequency that matches the frequency of u.v.

light– u.v. light causes resonance to occur.– Large oscillations cause collisions between atoms that dissipate energy as heat.– Lower frequency light causes forced vibrations of smaller amplitude– the energy of vibration is reemitted as transmitted light.– A series of absorption and reemission passes the light through the material.– The frequency of the reemitted light is identical to the incident light.– A slight time delay caused by the chain of absorption and reemission results in a

decreased speed of light as it moves through the medium.

Light and Opaque Materials

• Opaque materials absorb light without reemission.– Vibrations are converted into random kinetic energy.

• Metals are shiny– They have outer electrons that are free to move

between atoms.– Vibrations of outer electrons are not passed from atom

to atom– instead, light is reemitted

Optical Illusions

Black and White Spots Move

There are no vertical or horizontal lines.

Lines seem to taper

Center Circle is Same Size

Table Tops are Same Shape

Same Size

What is Light? A review

ReflectionOne obvious property of light is that it reflects off of surfaces. Among other things, this gives rise to the images we see in mirrors.

RefractionLight refracts, which means that it bends when passing from one medium to another. When light enters a more dense medium from one that is less dense, it bends towards a line normal to the boundary between the two media.

DiffractionAnother property that light exhibits is that it diffracts, which loosely speaking means it bends around the corner when it passes through an opening.

Interference

Waves Seem to Work

The properties of light we have described - reflection, refraction, diffraction, and interference - can all be explained in terms of light viewed as a wave. The success of these descriptions of the properties of light was a triumph of the wave picture, and by the 1850s this model of light was the generally accepted one.

Light as a Wave Also ExplainsDoppler Shift

Doppler Shift in Light Waves

If source approaches, light appears bluer than it is.

If source recedes, light appears redder than it is.

A Fly in the Ointment

The Photoelectric Effect

Why is this surprising?

How can electrons get free of their material?

What is the energy used for?

If light behaves like other wave phenomena, what effect would changing the color and brightness of the light be “expected” to have on the ejected electrons (sometimes called “photoelectrons”)?

Do you understand and agree with these predictions?

When the experiments are carried out, the results DO NOT agree with the predictions. In fact, they are nearly the opposite.

What should a scientist do when the results of an experiment are not as expected?

Unexpected results can cause a researcher to tweak an existing model or scrap it altogether in favor of a completely different idea.

Perhaps light is not behaving as a wave at all. Instead it behaves in this experiment as small independent “bits” of energy with discrete boundaries called photons.

Particle Bits!

Photons

Photons

Photons

Photons

Photons Make Sense!

• Red light is used in photographic darkrooms because it is not energetic enough to break the halogen-silver bond in black and white films

• Ultraviolet light causes sunburn but visible light does not because UV photons are more energetic

• Our eyes detect color because photons of different energies trigger different chemical reactions in retina cells

Light is Packets of Energy Called Photons

Paraffin Photometer

So, What is Light?

Light consists of a varying electric and magnetic field

Polarized Light

1. Polarizing Filters

2. Calcite – double refraction

3. Reflection from Non-metal Surface

Which way should sun glasses be made?

Cool Thing About Light

It can be thought of as both a particle and a wave, so called “particle-wave duality”

Lower energy (longer wavelength) light acts predominately like a wave

High energy (shorter wavelength) light acts predominately like a particle

Cool Things Light Can Tell Us

It can tell us what you are made out of

It can tell us if you are moving toward or away from us

It can tell us how far away you are or (if we already know that) how energetic you are

It can tell us your temperature

Kinds of Spectra

Spectral Lines

Lines from excited sodium gas in the laboratory

Spectral Lines in the Sun

1/R2 Falloff

Intensity of light falls off as we move away from the source

Light at a Distance

• Objective:

Your detector in orbit around Earth has

measured a certain amount of energy from

the direction of a faraway source.

Your job is to determine how much energy

the source actually emitted. Assume the source emits energy equally in all directions.

Think About It!• A light emits equally in

all directions. • What does this mean

about the amount of light you will measure in any given square cm as you move further and further away from the light source?

• At r1, the light per unit area, L1 = L/4(r1)2. •And at r2, the light per unit area, L2 = L/4(r2)2.

• Solving each equation for L gives us

L= L1 x 4(r1)2 = L2 x 4(r2)2.

Think of it in terms of a ratio... the amount of light per unit area at r2 relative to the amount of light per unit area at r1 is then

L2/L1 = (r1)2/(r2)2.

Add the Mathematics!

Think About What This Means

If r1 is 5 cm and r2 is 10 cm, then there is 1/4 as much light per square cm at r1 as at r2. The distance changes by a factor of 2, but the amount of light per square cm changes by a factor of 4.

• What if r1 was 5 and r2 was 50?• How much less light per cm2 do you have there?

Conclusion•We say that the intensity, or amount of light per square cm, changes as 1/distance squared (i.e., 1/r2) away from the source.

•How does this help us to achieve our Objective? If we measure X amount of energy per square cm in our detector, then we know that the source must have emitted energy equal to 4r2 times X!

Blackbody Radiation

A blackbody is an object which totally absorbs all

radiation that falls on it

Any hot body (blackbodies included) radiates light over

the whole spectrum of frequencies

The spectrum depends on both frequency and

temperature

Spectrum of a Blackbody

Fun With Lenses and Mirrors

Convex Lenses

Concave Lens