Module 2: Basic Theory of Light. 

We are going to start this lesson as we will begin most of the lessons in this theory class: with a worksheet.  The link to this module's work sheet is on the assignment page.  Click on your BACK button when you are ready to return there.   As will be true for every lesson in this course, you will not receive credit for this week's lesson until you submit this work sheet to your instructor.  If you have a printer, print it. If not, copy it down on a fresh piece of notebook paper.

As you read through this lesson and the text book, write out the answers/definitions on the worksheets. It would be good if you write them in your own words instead of copying the text. If you are not sure of any of your answers contact your instructor, who will tell you if your answers are correct.

History of the Theory of Light

ANCIENT GREEKS

As far back as we have records, people have wondered what light is and how sight works. We now know that animals and insects see differently than we do. The ancient Greeks had several possible explanations for light: that it is a type of wave, that it is made up of particles, that the eyes send out tentacles that grasp the object, that the tentacles and the particles collide in midair. Of one thing they were certain: in order to understand sight, they would have to understand light.

17th CENTURY

In the 1600's the discussion of the nature of light became heated. Sir Isaac Newton believed that light had a particle nature: he called the particles corpuscles, and stated that they were very minute particles traveling at a tremendous speed. Christian Huygens believed that light had a wave nature, and that it was propagated by ether, which filled all of space.

There were two major reasons why Huygen's wave theories were ignored, in spite of some good theoretical work:

  1. Newton's reputation was so strong that anyone disagreeing with him was not believed.
  2. Wave theories require the presence of diffraction, and no one could demonstrate that light could be diffracted.
Refraction could be explained by particle theory if it could be proved that light speeded up when it entered a transparent material from air; but Newton admitted that if anyone could prove that light slowed down in other materials, then the particle theory failed. No one proved what the speed of light did until the mid-1800's. No one showed that diffraction was present until the 1800's, either.

19th-20th CENTURIES

In the early 1800's Thomas Young resurrected the wave theory by demonstrating the principles of constructive and destructive interference in terms of light. In the mid 1800's a French Physicist, Jean Foucault, proved that light slows down, rather than speeding up, when traveling from a material such as air into a denser medium such as water. This effectively disproved the particle theory, and the wave theory was resurrected. Then, in the late 1800's, James Maxell proposed the electro-magnetic spectrum, (textbook, page 10) and Hertz showed that visible light is a portion of this spectrum.

The final step to our current understanding of the nature of light came in the early 1900's when Max Plank developed quantum theory, which proposed extremely small units of matter called photons which contain small units of energy measured in quanta. Albert Einstein expanded on this theory, as did R.A. Millikin.

The result of centuries of theories and proofs and arguments is that light is a portion of the electro-magnetic spectrum. (Textbook, page 10) Sometimes its nature can best be described using small packets of energy/matter called photons, and at other times its nature can best be described using wave models. We say that visible light has a DUALISTIC nature.
 
 


 
 

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