We see light as color and brightnessIt’s actually electromagnetic radiation:

Partly electric, partly magneticFlows in straight line (radiates)

Where does light come from?The Sun and stars. But how do they make light?It all starts with ATOMSA nucleus surrounded by electrons that orbit

the electron cloud.

Where does light come from(2)Electrons get kicked into a different orbitThis doesn’t happen very often in solar

systems, but it does in atomsIf you add energy to an atom (heat it up), the

electrons will jump to bigger orbits.When atom cools, electrons jump back to

original orbits. As they jump back, they emit light, a form of

energy

Color of lightEach electron that jumps back emits one

photon of lightWhat color is this light?Depends on how big the jump between orbits

wasThe bigger the jump, the higher the energy.The energy determines color; a blue photon

has more energy than a redShine all the colors together, you get white

light!

Light as particle A photon is like a particle, but it has no massThink of a photon as a grain of sand.We see so many photons at the same time it’s

like seeing all the sand on a beach; we don’t notice the single grains

When light hits film in a film camera, it acts like photons.

Light as a waveBut sometimes light acts like a waveA wave has a wavelength, a speed and a

frequency.We’ll learn more about wave behavior when

we talk about polarizationAll light travels same speed (in vacuum)The energy goes up as frequency goes upColor depends on frequencyWavelength gets shorter as frequency goes up

Speed of lightLight travels at 300,000,000 meters/secondIt takes 8 minutes for a light wave (or a

photon) to travel from the sun to the earth.We see the moon because it reflects the sun’s

lightIt takes 1 second for light reflected off the

moon to reach the earth.

Light and matterWhen light hits something (air, glass, a

green wall, a black dress), it may be:Transmitted (if the thing is transparent)Reflected or scattered (off mirror or

raindrops)Absorbed (off a black velvet dress)Often it’s some combination. Take a simple

piece of paper: you can see some light through, white reflects, black print absorbs.

      

The waves can pass through the object

The waves can be absorbed by the object.

The waves can be reflected off the object.

The waves can be scattered off the object.

The waves can be refracted through the object.

Reflection and colorRemember, white light contains all colors

(a rainbow or prism separates them so we can see this)

Why does a green wall look green in the sunshine?

Why does it look different when it’s in the shade?

Well, in the dark, it’s black. No light reflects off it.

A green wall reflects only green light; it absorbs all the other colors.

Absorption and colorWhy is a black car hotter than a white car in

the summer?Remember light is energy. Heat is another

form of energy.A white car reflects all wavelengths of light.A black car absorbs all wavelengths of light,

absorbing the energy and turning it to heat.

Light transmissionTransparent materials transmit light, like

windows.Remember all light has same speed in

vacuum?Different frequencies have different speeds

in transparent materials – that causes a prism to separate the colors.

Colored glass or plastic only transmits the color that it is; it absorbs or reflects the other colors.

Using Light to Study the Stars

Astronomers collect energy from the stars with a telescopeVisible lightInfrared lightRadio waves, etc.

Each atom has a special pattern of light frequencies like a fingerprint

The fingerprint of frequencies will be shifted if the star is moving away or toward us (like the sound of a freight train)

The temperature of the Star can be determined from the color of the star

Here are two pictures of the ring nebula. One in visible light one in infrared light…