Light Basics and Geometrical Optics


This text is meant to accompany class discussions. It is not everything there is to know about uniform circular motion. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations. See the links below.

In the 1670's, Christiaan Huygens, proposed light traveled as a wave. If light travels as a wave it have wave like characteristics. And it does.

"Christiaan Huygens" by Caspar Netscher,Museum HofwijckVoorburg Public Domain, Accesses 02/15/2016.

If the shadow of an object is far away from the object, then the edges will be fuzzy.

More proof:

In 1899 Lord Rayleigh (pronounced ray-ley) explained that the sky is blue because the water molecules in the air are the correct size to scatter the blue wavelengths of light the most. This gives the sky its blue appearance. At sunset and sunrise the sun's rays travel through more of the Earth's atmosphere than at midday. At these times the blue light is completely scattered and the remaining parts of the light spectrum begin to scatter

Sunsets and sunrises are red because of the blue and green is scattered "out" and the longer wavelength red is left.

image: accessed 02/15/2016

Newton comes along about 15 years later and says light is a particle.

Portrait of Isaac Newton in 1689 (age 46) by Godfrey Kneller By Sir Godfrey Kneller -, Public Domain, Accessed 02/15/2016

His evidence? Light reflects off of smooth surfaces at an angle equal to the incident angle. (This is the law of reflection.)

But some light bounces of the boundary of a smooth water surface while other light particles pass into the water. Newton's description was that the particles are confused and have a "fit." (By fit he means a temper-tantrum.) Not a good explanation.

Thomas Young comes along and does his famous double slit experiment and the scientific community believes light is wave... again. The scientific community is split until 1905 when Einstein says light is a wave or a particle depending on how you measure it. When light is traveling it behaves like a wave. When it impacts a surface that does not allow it to penetrate, it behaves like a particle. (This is a general simplification.)


The Duality of Light


Albert Einsten settles the argument in 1904 about the true nature of light. He postulates that light is both a wave and particle. If you measure the wave properties you destroy the particle properties of light. If you measure the particle properties of light, you destroy the wave properties of light.This is the accepted view of light today.

Lite Light Facts

Light travels at 186,000 miles/s. That's 3.00 x 108 m/s. (Memorize the speed of light as 3.00 x 108 m/s.) If you could bounce a beam of light around the Earth's equator, in 1 second it would circle the globe 7 1/2 times.

Light from the Sun takes 8.5 minutes to reach the Earth. This means a couple of things.

      1. When you look at the Sun, you are seeing where the Sun was 8.5 minutes ago and not where it is now.
      2. If the Sun were to stop burning, now, you would not know for 8.5 minutes.
      3. Because gravity travels at the speed of light, if the Sun magically disappeared, the gravitational pull from the Sun would not cease for 8.5 minutes.

When you look in the night's sky, the stars you see are so far away that many of them do not exist today. Instead, you are looking at what they looked like thousands, millions, or billions of years ago. The night sky is like a time machine looking into the past.

Image accessed 02/15/2016


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by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)