Electromagnetic Waves & Interference of light

The discovery of electromagnetic waves, led us to the understanding of visible light (the light we see in all the colors of the rainbow) as a type of electromagnetic wave. Other types of EM waves exist, for example radio waves that you can use to listen to music in your car or to “look” at the far reaches of the cosmos (top right). Infrared waves are emitted by warm blooded animals. A “thermogram” (bottom right), which uses infrared radiation, can help us identify areas with irregular temperature variations, as in the inflammation near the abdomen seen in the image.

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Producing an electromagnetic wave

A traveling electromagnetic wave produced by an ac generator attached to an antenna.

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E and B field direction when V is positive.

Positive charges are in the top. Current flows up. RHR2 the B field is into the page. In general:

E and B will be perpendicular to each other.

They are both perpendicular to the direction of propagation.

Point fingers along E Curl fingers in the direction of B Thumb will point in propagation

direction.

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An electromagnetic wave.

Electric and Magnetic Fields are perpendicular. Oscillating E field produces a B field and Oscillating B field produces an E field…

Wave only needs the E and B fields, it can travel in vacuum!

Speed of electromagnetic waves:

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0 0

13.0 10 /c m s

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Light as an electromagnetic wave

c=constant c=f, so larger frequency means smaller

wavelength Visible light wavelengths

Roses are 700nm, Violets are 400nm… What are the frequencies?

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Electromagnetic Spectrum

Radio waves f=106-109 Hz Alternating Current

(AC) circuits in metal antennas

Molecules and accelerated electrons in space.

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Electromagnetic Spectrum

Microwaves f=109-1012 Hz Long distance phone

conversations (cell phones).

Cooking (absorption of microwaves by water molecules).

RADAR Highest frequency

produces by purely electronic devices.

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Electromagnetic Spectrum

Infrared f=1012-4.3x1014 Hz Can feel as heat on

skin. Some animals, such as

the pit viper, can detect infrared light.

Allows to see warm blooded animals in the dark.

Emitted by warm bodies: produced by vibrations and rotations of molecules.

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Electromagnetic Spectrum

Visible f=4.3x1014-7.5x1014 Hz Light that we can see. Each frequency is

interpreted by our nervous system as a different color.

Produced by electrons changing their positions within an atom

Quantum mechanics, atomic level.

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Electromagnetic Spectrum

Ultraviolet f=7.5x1014-1017 Hz Skin is sensitive to UV

Sun tans (in low doses) Skin cancer (in high doses)

Sun produces UV light, Ozone in stratosphere absorbs most of it

Reducing ozone in the stratosphere can have unwelcome consequences!

Bees are sensitive to UV, and flowers look different in UV.

The flowers help the bees to “bullseye” the pollen!

UV absorbing chemicals in plants, help bees and fend off herbivores.

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Electromagnetic Spectrum

X-rays f=1017-1020 Hz Generated by the

rapid deceleration of high-speed electrons.

Can pass through soft tissue

Do not pass through denser material, such as bone.

Can used as “anti-smuggling” diagnostic.

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Electromagnetic Spectrum

-rays f>1020 Generated by rearrangement

of protons and neutrons in nucleus of an atom.

Also, generated by collision of particle of matter with particle of antimatter:

annihilation! produces radiation, no

“particle” with mass is left. Supernova explosions Gamma-ray bursts

Some of the most violent events in the universe

STILL DON’T KNOW THEIR SOURCE.

Highly penetrating and destructive to cells.

Used to treat cancer Used to kill microorganisms in

food.

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Superposition

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Interference: Two antennae

Point P0: midway between the antennas the waves travel the

same distance they interfere

constructively. At point P1 the distance

l2 is greater than the distance l1 by one wavelength; P1 is also a point of

constructive interference.

At Q1 the distance l2 is greater than the distance l1 by half a wavelength waves interfere

destructively

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Young’s Two Slit Experiment

The first screen produces a small source of light that illuminates the two slits, S1 and S2.

After passing through these slits the light spreads out into an interference pattern alternating bright and dark

fringes on a distant screen.

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Two slits act as two circular sources

According to Huygens’s principle, each of the two slits in Young’s experiment acts as a source of light waves propagating outward in all forward directions.

Light from the two sources can overlap Result: Interference pattern.

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Pathlength from each slit to a screen

Light propagating from two slits to a distant screen along parallel paths;

Paths make an angle relative to the normal to the slits.

The difference in path length: d sin d is the slit

separation.

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Bright and dark fringes

Bright fringes Path difference is a

“whole number” of wavelengths.

Dark fringes Path difference is a

“whole number plus a half” wavelength.

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Example 35.1 Two slit interference

Two slits with a separation of 0.200 mm create an interference pattern on a screen 1.00 m away.

If the third bright fringe above the central fringe is a linear distance of 9.49 from it, what is the wavelength of light used in the experiment?

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Example 35.2 Broadcast Pattern

A radio station operates at a frequency of 1500 KHz.

Two identical vertical dipole antennae 400 m apart are used, and they oscillate in phase.

For distances greater than 400 m, in what direction is the intensity greatest in the resulting radiation pattern?