Huygen’s Principle:

Electromagnetic waves can be examined using geometrical considerations instead of the relationships between the electric and magnetic fields (such as Snell’s Law).

Huygen’s Principle was the first theory describing the wave nature of light that explained reflection and refraction.

Huygen’s Principle states: “All points on a given wavefront are sources of spherical secondary waves called wavelets, which propagate outward. The new wavefront will be a surface line tangent to each of these wavelets.”

This theory has more historical significance than practical applications.

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Wavelets

Wavefront

Dispersion:

When light enters a different medium the average speed of light through that medium will change.

The speed of a wave is related to the frequency and the wavelength of the wave. If the average speed changes as it enters a new medium will the frequency, wavelength or both change with it?Wavelength - the frequency of a specific color of light is fixed (it is related to the energy the wave is transmitting), therefore the time to complete one cycle is fixed and hence the change in speed would correspond to a change in the distance between two identical points on the wave.

When white light (composed of all frequencies of the visible spectrum) enters a medium the average speed of the light decreases. If the speed is related to the wavelength of the light what will happen?

The different colors (frequencies) which have different wavelength will now travel at slightly different average speeds. The white light will be separated into its individual colors (frequencies).

The separation of white light into its constituent frequencies is called Dispersion.

Dispersion – separation of frequencies

Dispersion is the principle that governs the formation of a rainbow.Rainbow Formation:

• Sunlight incident on a raindrop is refracted and disperses the white light into its different frequencies.

• The curved shape of the back of the raindrop alters the direction of the different frequencies.

• The light is further refracted as it leaves the raindrop.

• Only one color (or a small range of colors) will reach your eye from each raindrop. The light from a large number of raindrops allows you to see the full circular arc of a rainbow.

A perfect rainbow would be a complete circle. You do not see a circle because there are no raindrops underground.

If you were in an airplane you would see a full circle.

Total Internal Reflection:When you have a light ray travel from a high index material to a low index material the light may not be able to transmit through the interface instead it is reflected.

As you increase the incident angle from 0o, you will find that for angles above a particular angle called the critical angle (qc) all light incident on the interface will be reflected back into the initial medium.

Light bends away from the normal as you go from a high index to a low index.

q2 = 90o

A group of sprinters gather at point P on a parking lot bordering a beach. They must run across the parking lot to a point Q on the beach as quickly as possible. Which pathfrom P to Q takes the least time? You should consider the relative speeds of the sprinters on the hard surface of the parking lot and on loose sand.

1. a 2. b 3. c 4. d 5. e6. All paths take the same amount of time.

Suppose the sprinters wish to get from point Q on the beach to point P on the parking lot as quickly as possible. Whichpath takes the least time?

1. a2. b3. c4. d5. e6. All paths take the same amount of time.

The observer at O views two closely spaced lines through an angled piece of plastic. To the observer, the lines appear (choose all that apply)

1. shifted to the right.2. shifted to the left.3. spaced farther apart.4. spaced closer together.5. exactly as they do without the piece of plastic.