Light and ColorLight and Color

What is Light? What is Light?

• A wave of Electromagnetic Radiation

• Light behaves just like any other wave

• Reflects, interferes, oscillates with a certain period/frequency, has a wavelength, etc

Analogies to SoundAnalogies to Sound

• Sound Amplitude

• Sound Volume/Loudness

• Light Amplitude

• Brightness

FrequencyFrequency

• Sound Frequency

• Pitch

• Light Frequency

• Color

Pure Tone Sources?Pure Tone Sources?

• Sound?

• Tuning Forks

• Light?

• Lasers

Discrete SourcesDiscrete Sources

• Discrete sources of sound?

• Instruments (guitar strings, piano keys)

• Discrete sources of light?

• Atoms

EnergyEnergy

• Sound Energy

• Depends upon energy input by source

• Represented by amplitude

• Light Energy

• Also dependent upon energy input by source

• Represented by frequency

SpeedSpeed

• Speed of Sound

• 340 m/s, depending on the weather

• Speed of Light (c)

• 298000000 m/s, regardless of the weather

• We’ll use 3.00E8 m/s

Variations in SpeedVariations in Speed

• For sound, change the medium

• For light, change the medium (material, material’s properties)

MiragesMirages

Interference EffectsInterference Effects

• What happens when sound interferes?

• Beats, Bose Noise Cancellation Headphones

• What happens when light interferes?

• Bright and dark regions of light

Types of LightTypes of Light

• Light comes in a variety of flavors beyond what we can see

• Visible

• Infrared (wireless technology, predators)

• Ultraviolet, UV (black lights, cancer)

• Gamma Ray (cosmic sources)

• X Ray (body imaging)

• Radio (radio signals)

• Microwave (cell phone signals, cooking)

• Each type of light has a specific frequency and wavelength

• The combination of all types yields the electromagnetic spectrum

How do you “make” light?How do you “make” light?

• Electron Transitions

• Objects with Temperature Radiate Light

• http://phet.colorado.edu/new/simulations/sims.php?sim=Radio_Waves_and_Electromagnetic_Fields

““Black” Body RadiationBlack” Body Radiation

• Any object with a temperature emits light• Think about metal that gets extremely hot• Why can’t we see ourselves in the dark?• We emit IR radiation• The higher the temperature, the more

energetic the light (lower wavelength)

Gamma RaysGamma Rays

• Dying, spinning stars

• Gamma rays do not result in super powers

• They result in death

How We SeeHow We See

Seeing the World as It WasSeeing the World as It Was

• When you look at any object, you are looking back in time

• Significant impacts over large distances

How Do We See?How Do We See?

• Why do we only see “visible” light, while other animals see UV and IR light?

• The photo detecting cells in our eyes are sensitive to these specific frequencies

• In fact, the color sensitive cells (cones) are specifically sensitive to red, green, and blue

• Three primary colors

Light ColorLight Color

• Which light property determines color?

• Frequency/wavelength

• What we think of as red, blue, or yellow is simply electromagnetic radiation with a specific wavelength

““White” LightWhite” Light

• Run white light from a light bulb through a prism and something interesting happens

• You can a rainbow

• Why?

• Add all the colors in the rainbow together and you get white light

• We learned with sound, that you can add sounds of various frequencies together to get new sounds

• You can do the same thing with light color

Color by ReflectionColor by Reflection

• Imagine you see someone in the class wearing a red shirt

• Why does the shirt appear red?

Color FiltersColor Filters

• Shine white light through a color filter and you get colored light

• Why?

• The filter absorbs all frequencies but the filter frequency

Color additionColor addition

• What happens when you combine colors?

• http://phet.colorado.edu/new/simulations/sims.php?sim=Color_Vision

Ink?Ink?

• If you add together red, blue, and green light, you get white light

• If you add together red, blue, and green paint, you get a mess

• Why?

PigmentsPigments

• Why does a particular shirt look green?

• Why is it warmer when wearing a dark shirt on a sunny day?

• The pigments in fabrics absorb light of all colors except what you see

• When you mix various colors of paint, what happens?

Wave or Particle?Wave or Particle?

• 20th century experiments revealed that light sometimes behaves like a particle

• We call these particles “photons”