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What we call light is a small portion of the electromagnetic spectrum All the different colors are electromagnetic waves with different wave lengths Wave speed = frequency x wavelength EM radiation is alternating electric and magnetic fields. Light Waves. Remember our field induction rules

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Presentation Transcript
light waves
What we call light is a small portion of the electromagnetic spectrum

All the different colors are electromagnetic waves with different wave lengths

Wave speed = frequency x wavelength

EM radiation is alternating electric and magnetic fields

Light Waves
electromagnetic waves
Remember our field induction rules

Changing magnetic/electric field induces electric/magnetic field

Electromagnetic Waves
colors
Lowest frequencies of visible light are red

Highest frequencies are violet

The order is red, orange, yellow, green, blue, violet

Colors
producing em waves
Need to vibrate charged particles

We shake electrons in an antenna to make radio and TV waves

We shake electrons in a magnetron to make microwaves

We shake electrons in atoms to make light (this is a bit of a simplification which we will correct later this semester

Producing EM Waves
light and materials
Electromagnetic Waves carry energy

When light strikes a material, it vibrates the lightweight electrons

Light and Materials
light and materials8
If the natural vibration frequency of the electrons is close to the frequency of the incoming light wave, the electron will be set into big vibrations (resonance)

It will absorb some energy from the EM wave and will vibrate with larger amplitude

It can smack into the electrons in neighboring atoms and vibrate them too

Light and Materials
light and materials9
This cuts down on the intensity of the light

If all the light is absorbed by shaking electrons we say the material is opaque

If the light frequency doesn’t come close to the natural frequencies of the electrons, they shake a bit, but just re-emit the energy

Materials that can’t hold the energy are transparent

Light and Materials
light and materials10
There is a delay in transparent materials between the time the electrons start shaking and then re-emit the energy

This means that the light travels more slowly in these materials!!!

We will use this fact later to do interesting things with lenses

Light and Materials
light and materials11
Light and Materials

See why the light travels more slowly in a transparent material than in a vacuum?

light and materials12
Infrared EM waves shake entire atoms or molecules in materials

Microwave EM waves effect the rotation of atoms about each other

Light and Materials

+

Remember our discussion of water and how it has a positive end by the hydrogen atoms and a negative end by the oxygen

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light and materials13
Things that are opaque absorb the energy carried by the light wave

This means that they heat up

Metals are opaque and shiny

The free electrons close to the surface shake and re-emit the energy back out of the material

They are reflectors!!!

Light and Materials
light and air
Our atmosphere is transparent to visible light, some ultraviolet light and some infrared light

The ozone layer absorbs a lot of ultraviolet light

The ozone molecule is O3 and breaks into a regular oxygen molecule O2 and an oxygen atom

Light and Air
colors15
Colored objects are a result of selective reflection and absorption of light

Sunlight is a mixture of colors

We call this mixture white light

When white light strikes a red object, the red frequencies are reflected and the other frequencies are absorbed

Colors
colors17
White objects reflect all frequencies

Black objects absorb all frequencies

Objects can only reflect the colors of light that shine on them

If you shine blue light on a red object it will look black

Colors
colors18
We can also consider shining light through semi-transparent objects

Filters used on the lights in a theater are materials that absorb some frequencies and let other frequencies pass through

Red filters transmit red light through the material while other frequencies are absorbed

Colors
mixing colors
Our eyes and brains are very complicated color processing sensors and computers

See the discussion on page 289 in the text

Artists have learned how to mix colors to make pleasing combinations and to create a huge array of different colors

All based on what the sensors in our eyes respond to

Mixing Colors
visual response21
Visual Response

Red + Blue = Magenta

Red + Green = Yellow

Blue + Green = Cyan

The way your computer screen makes colors. And your TV set does the same thing.

visual response22
Visual Response

Red + Blue = Magenta

Red + Green = Yellow

Blue + Green = Cyan

Magenta + Green = White

Cyan + Red = White

Yellow + Blue = White

selective scattering
This is why the sky is blue!

The higher frequencies are scattered more by the molecules in the atmosphere

Reds and oranges just pass through, so the sky appears blue

Selective Scattering
selective scattering24
Selective Scattering

Sunsets are red!