Electromagnetic Waves

Electromagnetic WavesElectromagnetic Waves

• An Electromagnetic wave travels like a wave, but can interact with matter like a particle.

• It has a “dual nature”, behaving like a wave at times and behaving like particles (of NO mass!) at times.

• An Electromagnetic wave travels like a wave, but can interact with matter like a particle.

• It has a “dual nature”, behaving like a wave at times and behaving like particles (of NO mass!) at times.

When it is behaving like a particle, we call these massless

bundles of energy photons.

• In a pure vacuum, all electromagnetic waves travel at the speed called “c”, which we call “the speed of light”, however the average speed slows down a little when it travels through substances such as water or glass as it interacts with the molecules.

Albert Michelson is known for making an definitive measurement of the speed of light in the late 1800’s to early 1900’s, using a rotating mirror apparatus between 2 California mountains 22 miles apart. His value: 299,796 km/s. Accepted value today: 299,792 km/s He was the first American to win the Nobel Prize in physics.

c = 3 x 108 m/s

As is true of ALL waves, Velocity = wavelength x frequency

So, for electromagnetic waves,

c = f

Now, you try one….

• What is the frequency of purple light, which has a wavelength of 450 nm?

1 nanometer = 10-9 m

• What is the frequency of purple light, which has a wavelength of 450 nm?

1 nanometer = 10-9 m

Don’t just sit there! Get out a calculator and find the frequency.

c = f

c = 3 x 108 m/s 450 nm (1 nm = nanometer = 10-9 m)

What is f, the frequency?f = c ÷ f = 3 x 108 ÷ (450 x 10-9 )

f = 6.67 x 1014 Hz

How to make an electromagnetic wave:

A vibrating (accelerating!) electric charge creates a wavy electric field that produces a wavy magnetic field that produces an electric field that produces a magnetic field………

…

Electromagnetic waves are transverse waves of intertwined electrical and magnetic fields.

“Visible Light” is only a VERY small part of the ELECTROMAGNETIC SPECTRUM. It is the only part of the EM spectrum that is VISIBLE to us. The entire spectrum, in order from long wavelength to small wavelength, is

“Light”

Our color spectrum is only a very small part of the electromagnetic spectrum.

“Visible Light” is the part of the electromagnetic spectrum that is in RESONANCE with the rods

and cones in our retinas.

All electromagnetic waves are the same kind of wave.

Blue light is the same type of wave as X-rays.

Their names (microwave, infrared, radio, etc.) are based only on a particular range of

wavelengths (or frequency).

All electromagnetic waves travel at the “speed of light” in a vacuum.

Radio waves: used by all TV and Radio stations and garage door openers, have very long wavelengths.

Frequencies:AM radio stations broadcast in kHz. FM stations broadcast in MHz. TV stations broadcast in Mhz.

What is the wavelength of our public radio station, FM 90.1? (90.1 x 106 Hz) Using c = f, = ?

= 3.3 m

Microwaves have an energy that is easily absorbed by water molecules. This is why they are used in “microwave

ovens”. What happens is that when the water in your TV dinner absorbs the microwaves, it makes the water molecules

vibrate faster. The energy of the microwaves is converted into heat.

Cell phone calls, radar and “bluetooth” are all transmitted using microwaves. Some people are frightened that the

waves coming in and out of their cell phones are short enough (energetic enough) to cook their brains.

(They aren’t!)

Infrared:These waves have a

shorter wavelength than microwaves, but their

wavelength is longer than visible light.

IR waves are used in remote controls for your

TV Animals like the pit-viper have infra-red detectors so that they can find their prey in the dark.

You have been warned !!

The Visible Spectrum Roy G Biv:

Red, Orange, Yellow, Green, Blue, Indigo, and Violet are the colors of visible light.

Wavelength range from about

700 - 400 nm1 Nanometer = 10-9 m

Ultraviolet:These waves have very high energy and very short wave lengths- shorter

than visible light.

Too much exposure to UV can cause skin cancer- too little exposure and we

don’t produce enough Vitamin D!

UV is sometimes used to sterilize tools, glassware, and kill bacteria on

food.

Some animals like honey bees can see ultraviolet waves.

Even shorter wavelength:

X-Rays have so much energy and such a short wavelength that they can go

right through you.They also mess with your DNA…

that’s bad

However, they cannot get through bone as easily as they can get

through muscle.

This is because your bones contain so much Calcium.

The very shortest, highest frequency:

Gamma raysThese are nasty ones. They have very high energy and will even go through metals if

they are not thick layers.

Some radioactive materials produce gamma rays (on Earth) as do super nova

explosions and pulsars (out in space). Gamma rays and X-Rays can cause

cancer, but gamma rays can also be used to destroy cancer cells: this is radiation

therapy.

Light and Colors

Light vs Pigments

• If light passes through a substance, the substance is said to be

Light vs Pigments

• If light passes through a substance, the substance is said to be transparent.

Light vs Pigments

• If light passes through a substance, the substance is said to be transparent.

• If light partially passes through a substance, it is said to be

Light vs Pigments

• If light passes through a substance, the substance is said to be transparent

• If light partially passes through a substance, it is said to be translucent.

Light vs Pigments

• If light passes through a substance, the substance is said to be transparent.

• If light partially passes through a substance, it is said to be translucent.

• If light will not pass through a substance, it is said to be

Light vs Pigments

• If light passes through a substance, the substance is said to be transparent.

• If light partially passes through a substance, it is said to be translucent.

• If light will not pass through a substance, it is said to be opaque.

• An object that produces light is

• An object that produces light is

LUMINOUS

• An object that produces light is

LUMINOUS

An object that reflects light is

• An object that produces light is

LUMINOUS

An object that reflects light is

ILLUMINATED

Isaac Newton was the first to show that white light consists of every color light mixed together. He sent white light through a prism, which produced a rainbow of colors and then through a second prism, where it recombined to produce white light again. All the colors, on atop the other, combine to produce white light.

• The Primary colors of VISIBLE LIGHT

• The three primary colors of light are red, green, and blue.

• Adding two primary colors of light produces the secondary colors of light.

• Green plus blue produces cyan.

• Blue plus red produces magenta.

• Red plus green produces yellow.

• Red plus green plus blue produces white.

Two colors of light are complementary if they produce white when added together. The

complementary pairs are: • Green and magenta

• Red and cyan

• Blue and yellow

Pigments are chemicals that are used to produce different colors of paint or dyes.

The primary pigments absorb ONE primary color and reflect the other two primary colors back to our eyes.

The primary pigments are the same as the secondary colors.

• Cyan pigment absorbs red and reflects back blue and green.

• Magenta pigment absorbs green and reflects blue and red.

• Yellow pigment absorbs blue and reflects red and green.

The sky is blue because the atmospheric molecules are in resonance with blue light.

The sky is blue because the atmospheric molecules are in resonance with blue light. When white light comes from the sun, the blue waves are absorbed and reemitted in all directions by the air molecules.

When white light comes from the sun, the blue waves are absorbed and reemitted in all directions by the air molecules.

So the sky will look blue and the sun will have “lost” a little of its blue waves so that it will look slightly yellow (red and green).

So the sky will look blue and the sun will have “lost” a little of its blue waves so that it will look slightly yellow (red and green).

This is called “Rayleigh scattering”.This is called “Rayleigh scattering”.

The sunrises and sunsets look yellow and orange and red because the white light from the sun has to pass through so much more of the atmosphere to reach our eyes as we look towards the sun at the horizon. Because of that, virtually all of the blue light will have been scattered by the air molecules, leaving only red and green for us to see.

The oceans look blue-green because the ocean waters (with algae and other microorganisms and impurities) absorb red light and only reflect back to our eyes the blue and green of the spectrum.

Green Flash

Polarization

• If the vibrations of a wave are back and forth in one plane, the wave is said to be polarized.

• A single vibrating electron produces a single polarized wave.

• But a light source usually has millions of electrons vibrating in all directions so that the waves are not polarized.

Polarization

• A polarizer is a type of material that allows only waves vibrating in one plane (its polarizing axis) to pass through, so that it produces polarized light waves.

Polarization

• What if light goes through two polarizing filters?

• If their polarizing axis are aligned, light will pass through both filters. However, if the polarizing axis are perpendicular, no light will pass through.

• Materials that rotate the orientation of light (the plane in which it is vibrating) as it passes through are said to be optically active.

• If an optically active substance is placed between 2 crossed polarizing filters, some light will pass completely through since the polarized waves from the first filter will have been rotated before they strike the second filter.