# Presentation: 305725

## What is light?

- We see light as color and brightness
- It’s actually electromagnetic radiation:
- Partly electric, partly magnetic
- Flows in straight line (radiates)

## Where does light come from?

- The Sun and stars.
- But how do they make light?
- It all starts with ATOMS
- A nucleus surrounded by electrons that orbit.
- Like the planets in the solar system, electrons stay in the same orbit, unless...

## Where does light come from(2)

- Electrons get kicked into a different orbit
- This doesn’t happen very often in solar systems, but it does in atoms
- If 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 light

- Each electron that jumps back emits one photon of light
- What color is this light?
- Depends on how big the jump between orbits was
- The bigger the jump, the higher the energy.
- The energy determines color; a blue photon has more energy than a red
- Shine all the colors together, you get white light!

## Light as particle 

- A photon is like a particle, but it has no mass
- Think 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 wave

- But sometimes light acts like a wave
- A wave has a wavelength, a speed and a frequency.
- We’ll learn more about wave behavior when we talk about polarization
- All light travels same speed (in vacuum)
- The energy goes up as frequency goes up
- Color depends on frequency
- Wavelength gets shorter as frequency goes up

## Speed of light

- Light travels at 300,000,000 meters/second
- It 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 light
- It takes 1 second for light reflected off the moon to reach the earth.

## Light and matter

- When 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.

## Reflection and color

- Remember, 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 color

- Why 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 transmission

- Transparent 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.