Particle or Wave?• Light has been traditionally conceived of as

a wave. After all visible light is simply the section of the EM spectrum to which the human eye is sensitive.

• On the other hand, light betrays itself as having certain properties that cannot be explained by modeling it as a wave. One such property is called the Photoelectric Effect and was first fully explained by Albert Einstein in 1905.

Light as a Wave:• Dutch astronomer Christian Huygens (c. 1660)

was the first to posit significant evidence of light as a wave.– He proved that light, like other waves, spread out

(diffracted) upon encountering a sharp edge in its medium.

– He was the first to successfully measure the speed of light.

• American Physicist Albert Michelson (1880)– Observed light waves interfering constructively and

destructively– More accurately measured the speed of light

Light as a Wave: (cont’d)• Perhaps the most convincing evidence of light

as a wave is provided by British Physicist Thomas Young’s Double Slit Diffraction Experiment performed in 1801. (we will look at this in more detail later)– In the experiment Young observed that light

emitted though two thin slits appeared to behave exactly as waves ought, producing a pattern of bright and dark patches.

– What wave phenomena do you think would be responsible for this behavior?

Light as a Particle:• The particle model of light was first posited

by the ancient Greeks after their realization that light, like a thrown stone, bounced or reflected off a flat surface.

• Later this idea was dismissed for many centuries until the discovery of a phenomenon that was inexplicable by means of the wave model of light.

The Photoelectric Effect:• When light of a certain frequency is shined on

certain metals, those metals begin to kick off electrons. We can observe this by using these electrons in an electric circuit to, say, light up a light bulb. The problem arose when scientists realized that different colors (frequencies) of the light were responsible for this behavior, rather than either the intensity, or the amount of time the light was shined on the metal--as would be expected if light was simply a wave. Once the right color light was used, the metal would instantly begin kicking out electrons.

The Photoelectric Effect: (cont’d)

• Einstein resolved the issue in his 1905 paper which posited that while light had wave-like properties, it was composed of an infinite number of tiny particles called photons, each of which carried a unit of energy corresponding to the frequency of the light and Planck’s constant.– E=hf, this relationship was actually adopted

from Max Planck’s 1900 paper on Blackbody Radiation.

So WHICH is it?• Particle? Or wave? The modern model is

inconclusive and merely suggests that while sometimes light behaves as a particle, sometimes it also behaves as a wave.

Where does it come from???We do know that the source of light radiation is from emissions from the valence (outer) electron shells of unstable, highly energized atoms. When the electrons decay to lower energy states, they give up the extra energy in the form of light radiation.

The speed of light:Read in your textbook about Michelson’s spinning mirror

experiment to measure the speed of light.

c = 3.00 x 108 m/s

That’s about 671 million miles per hour!

The EM spectrum:

All EM waves are created by the vibration of charged particles at various energies.

http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html

Same diagram w/ direction of energy reversed—Don’t be fooled

• Transparency:– Transparent materials allow light to pass through in

straight lines.– The atoms in say, glass and water vibrate when they

are struck by a light beam (the light beam deposits energy into the atomic structure).

– This energy is then re-emitted in some form inside the material. For transparent materials it is usually emitted as either UV light or thermal radiation--infrared (both of which we cannot see).

– These emissions are passes from atom to atom inside the transparent material until they are ultimately re-emitted as visible light at the boundary of the medium. (CP physics--pg. 410)

• Opaqueness– Opaque materials absorb light without any re-

emission.– No light penetrates opaque materials.

Polarization• When light is modeled as a wave, it is

actually separate waves in 3-D space, one part representing a magnetic field, and one part representing an electric field.

Magnetic Component

Electric Component

• A polarizer is a material that has a very fine grating of lines running in the same direction.

• When light is passed through a polarizer, it is said to become “polarized” as it only lets through one part of the light wave—either the electric component or the magnetic component—depending on the orientation of the polarizer to the light beam.

Waves in a rope becoming polarized

This diagram seems to defy logic. How can the part of the light wave oriented perpendicularly to the polarizing surface get through, and the parallel component be blocked? It has to do with what is happening at the atomic level.

• As the light passes the atoms at the edge of the polarizing bands, the component parallel to the bands excites the electrons at the surface of those bands. This requires that the wave give up some of its energy to the polarizing material.

• As the wave passes completely through the material, all of the wave energy in the parallel orientation is lost to and absorbed by the bands in the polarizing filter.

• Thus the ONLY part of the light wave transmitted is that part perpendicular to the polarization grating.

• If you take two polarizing gratings and align them such that the lines are in the same direction, while the intensity of the light maybe reduced, all of its perpendicular wave component will pass through.

• However, if the lines in the polarizers are aligned at any angle (other than 0o or 180o) to each other, part of this perpendicular wave component is lost as the wave passes through the second polarizer.

• When the two polarizers are at 90 degrees to one another, both the electric and magnetic field components of the light waves are removed and NO light passes the second polarizer.

A final thought...Given that we see by observing the light that is reflected off the various objects around us, consider an orange…

Still thinking about it? Good. Now put the orange inside a shoebox. Close the lid. Is it still orange? Of course it is, but can you prove it without opening the box?

This analogy is very similar to the idea of “Schroedinger’s Cat.” The story goes that you put a cat in a box. How can you prove it is really there short of doing something to the box? You cannot.

What we see is dependent upon our human senses. Our human senses can be subject to our own whims, perceptions and imagination. If you wanted the orange to be purple while it was inside the box, it would be and no one could prove you wrong.