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# Lecture 19 - 24.1-24.4 - PowerPoint PPT Presentation

Wave Nature of Light. How do you cook an ant on the sidewalk? Get a magnifying glass and focus the sun’s rays on the ant and the ant gets hot! If the ant heats up, the light must have supplied the energy! How did it do this?. Wave Nature of Light.

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Presentation Transcript

How do you cook an ant on the sidewalk?

Get a magnifying glass and focus the sun’s rays on the ant and the ant gets hot!

If the ant heats up, the light must have supplied the energy!

How did it do this?

We saw how Maxwell’s Equations predicted that alternating electric field produced a magnetic field and an alternating magnetic field produced an electric field

The theory predicted electromagnetic waves!

These waves must transport the energy!

Last fall we saw how water waves could carry energy

They make a leaf bob up and down on the surface of a lake when waves roll by

So, we know that waves can carry energy

We just finished looking at a “ray model” of light to explain reflection and refraction

Why would we think that light is a wave?

DIFFRACTION!!!

Remember we saw that water waves diffracted around corners when the wavelengths were comparable to the size of the obstacle

To see light diffraction, we have to have some small obstacles

Back in the 1600s, Huygens developed an approach that is very helpful

“Every point on a wave front can be considered as a source of tiny wavelets that spread out in the forward direction at the speed of the wave itself. The new wave front is the envelope of all the wavelets - the tangent to all of them.

Huygen’s Principle shows how the wave front moves from position AB to position CD.

We just create a lot of new little sources and let each one generate a new wave. The combination forms a new wave front!

See how the wavelets at the edges let the light bend around corners!!!

As the slit gets smaller, the bending is more obvious.

The law of refraction is explained by Huygen’s Principle

The little wavelets move slower in medium 2 than in medium 1. Doing the tangent shows how the wave fronts bend.

The second big argument for light being a wave is the phenomenon of interference

We looked at wave interference last fall

We decided that you just add the amplitudes of the two waves that occupy the same region of space

The sources have to be coherent, that is they have the same phase relationship. The wave peaks must start out in sync at the two slits.