Get a magnifying glass and focus the suns 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
2 Wave Nature of Light
We saw how Maxwells 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!
3 Wave Nature of Light
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
4 Wave Nature of Light
Why would we think that light is a wave
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
5 Wave Nature of Light
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.
6 Wave Nature of Light Huygens 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! 7 Wave Nature of Light
See how the wavelets at the edges let the light bend around corners!!!
As the slit gets smaller the bending is more obvious.
8 The Wave Nature of Light
The law of refraction is explained by Huygens Principle
The little wavelets move slower in medium 2 than in medium 1. Doing the tangent shows how the wave fronts bend. 9 The Wave Nature of Light 10 The Wave Nature of Light
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
11 Youngs Double Slit 12 Youngs Double Slit 13 Youngs Double Slit 14 Youngs Double Slit 15 Youngs Double Slit 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. 16 Dispersion 17 Dispersion
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