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# Chapter 22probe

## on Dec 10, 2010

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Light

Light

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## Chapter 22probePresentation Transcript

• Chapter 22: Light
• What is light?
• It is an electromagnetic wave (EM wave).
• Does not require a medium to travel
• Consists of changing electric and magnetic fields (fields are forces that push and pull on another object without touching it)
• Why are EM waves really transverse waves?
• The electric and magnetic fields travel at right angles to each other (in other words they are perpendicular to one another)
• And the fields travel perpendicular to the direction of the motion
• How are EM waves produced?
• By the vibration of an electrically charged particle
• When the electron absorbs energy it will jump orbital levels
• Eventually, the e will move back down releasing a burst of energy called a photon
• The constant stream of photons create the EM waves and it is called radiation.
• How do you calculate the speed of light?
• Through a vacuum (in space- not Hoover), light travels about 300,000,000 m/s
• Light travels slightly slower through air, glass, and other types of matter
• Light travels more than 880,000 times faster than sound
• Tidbit: if you could run as fast as light, you will circle the Earth 7.5 times in 1 sec.
• Math Break
• Speed = distance / time
• Time = distance / speed
• EX: time = 382,000,000 m / 300,000,000 m/s
• Time = 1.27 s
• You try this: The earth is 150,000,000,000 m from the sun. How long does it take for the light to travel that distance?
• 500 s or 8.3 mins.
• I. Electromagnetic Radiation
• EM Spectrum
• Types of EM Radiation
• A. EM Radiation
• transverse waves produced by the motion of electrically charged particles
• does not require a medium
• speed in a vacuum = 300,000 km/s
• electric and magnetic components are perpendicular
• A. EM Radiation
• Photons
• tiny, particle-like bundles of radiation
• absorbed and released by electrons
• energy increases with wave frequency
• B. EM Spectrum long  low f low energy short  high f high energy
• C. Types of EM Radiation
• lowest energy EM radiation
• C. Types of EM Radiation
• FM - frequency modulation
• AM - amplitude modulation
• Microwaves
• penetrate food and vibrate water & fat molecules to produce thermal energy
• C. Types of EM Radiation
• Infrared Radiation (IR)
• slightly lower energy than visible light
• can raise the thermal energy of objects
• thermogram - image made by detecting IR radiation
• C. Types of EM Radiation
• Visible Light
• small part of the spectrum we can see
• ROY G. BIV - colors in order of increasing energy
R O Y G. B I V red orange yellow green blue indigo violet
• C. Types of EM Radiation
• Ultraviolet Radiation (UV)
• slightly higher energy than visible light
• Types :
• UVA - tanning, wrinkles
• UVB - sunburn, cancer
• UVC - most harmful, sterilization
• C. Types of EM Radiation
• Ultraviolet Radiation (UV)
• Ozone layer depletion = UV exposure!
• Warm-up
• Explain what properties you would use and how you would use them to explain the difference between salt and sand
Properties I would……
• C. Types of EM Radiation
• X rays
• higher energy than UV
• can penetrate soft tissue, but not bones
• C. Types of EM Radiation
• Gamma rays
• highest energy EM radiation
• emitted by radioactive atoms
• used to kill cancerous cells
• Warm-up
• Identify the medium through which the wave is traveling
• Interactions of Light
• Light can reflect, refract, be absorbed, be scattered, diffract, and interference
• Reflection
• The bouncing back of a wave when it strikes a surface
• Law of Reflection states that the angle of incidence is equal to the angle of reflection.
• 2 types: regular reflection and diffused reflection
• Types of Reflection
• A regular reflection is the striking of light rays at the same time. It is off a smooth surface. (Think of a ball bouncing off the pavement)
• A diffuse reflection is the striking of light rays at uneven time. It is off a bumpy surface. The light rays bounce in different directions. (Think of a ball bouncing off an uneven surface)
• Law of Reflection
•
• Absorption
• The transfer of light energy to particles of matter
• the frequency of the incoming light wave is at or near the energy levels of the electrons in the matter. The electrons will absorb the energy of the light wave and change their energy state.
• Scattering
• The release of light energy by particles of matter that have absorbed energy
• Refraction
• The bending of light wave as it passes as an angle from one medium to the next
• Bends due to the change of speed of light through the different media
• Creates the rainbow, optical illusions
• B. Refraction
• Refraction depends on…
• speed of light in the medium
• wavelength of the light - shorter wavelengths (blue) bend more
• B. Refraction
• Example:
View explanation.
• Optical Illusions
• How your brain interprets the light waves traveling
• Follow the pink dots and they remain pink; stare at the black center and they become green
•
• Face or flowers
• Do you see the man’s head?
• Do you see a musician or girl’s face?
• How many people you see in the picture?
• Find the man’s face
• Is this picture moving?
• Horse on a rock?
• Rabbit or Duck?
• Can you count the black dots?
• Diffraction
• Light can diffract.
• However, it can only diffract around smaller objects.
• The outside of the light gets fuzzy when diffracting.
• C. Diffraction
• Diffraction
• bending of waves around a barrier
• longer wavelengths (red) bend more - opposite of refraction
• Interference
• Does happen
• Can see constructive interference
• Destructive interference is very rare
• D. Interference
• Interference
• constructive  brighter light
• destructive  dimmer light
• E. Cool Applications!
• Fiber Optics
• Total Internal Reflection
• when all light is reflected back into the denser medium
• E. Cool Applications!
• The “Broken Pencil”
• refraction
View animation and explanation of the “Broken Pencil.”
• E. Cool Applications!
• Rainbows
• refraction-reflection-refraction
• E. Cool Applications!
• Diffraction Gratings
• glass or plastic made up of many tiny parallel slits
• may also be reflective
• spectroscopes, reflective rainbow stickers, CD surfaces
• E. Cool Applications!
• Thin Films - Bubbles & Oil Slicks
• interference results from double reflection
• E. Cool Applications!
• Blue Sky & Red Sunsets
• NOON
• less atmosphere
• less scattering
• blue sky, yellow sun
• SUNSET
• more atmosphere
• more scattering
• orange-red sky & sun
• Molecules in atmosphere scatter light rays.
• Shorter wavelengths (blue, violet) are scattered more easily.
• Light and Color
• Light and Matter
• Seeing Colors
• Mixing Colors
• A. Light and Matter
• Opaque
• absorbs or reflects all light
• Transparent
• allows light to pass through completely
• Translucent
• allows some light to pass through
• B. Seeing Colors
• White light
• contains all visible colors - ROY G. BIV
• In white light, an object…
• reflects the color you see
• absorbs all other colors
REFLECTS ALL COLORS ABSORBS ALL COLORS
• B. Seeing Colors
• The retina contains…
• Rods - dim light, black & white
• Cones - color
• red - absorb red & yellow
• green - absorb yellow & green
• blue - absorb blue & violet
Stimulates red & green cones Stimulates all cones
• B. Seeing Colors
• Color Blindness
• one or more sets of cones does not function properly
Test for red-green color blindness.
• C. Mixing Colors
• Primary light colors
• red, green, blue
• combine to form white light
View Java Applet on primary light colors .
• EX : computer RGBs
• C. Mixing Colors
• Filter
• transparent material that absorbs all light colors except the filter color
View Java Applet on filters .
• C. Mixing Colors
• Pigment
• colored material that absorbs and reflects different colors
• Primary pigment colors
• cyan, magenta, yellow
• subtractive colors
• combine to form black
• EX : color ink cartridges
• C. Mixing Colors Light Pigment When mixing pigments, the color of the mixture is the color of light that both pigments reflect.
•
• Negative Afterimage - One set of cones gets tired, and the remaining cones produce an image in the complimentary color.