light & the electromagnetic spectrum examining light 2008
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• We will investigate the following:• 1. What is light?• 2.What are some sources of light around us?• 3. What are opaque, transparent, & translucent objects?• 4. What is a light wave?
Light
• Our primary source of light is the sun.• Light travels in straight lines at a speed of
186,000miles per second.• *Light waves travel faster than sound waves!• Light energy from the sun travels through space ,
reaches earth, and some of it turns to heat energy and warms the earth’s air.
• Light from the sun also travels to the cells of green plants (producers) and is stored as energy.
• When light reaches an object, it is absorbed, reflected, or passes through it.
How many sources of Light can you list and explain?
• SUN=warms air, water, and land.• Fire=provides heat, light,
and cooking fuel.• Lightning=• Firefly=• Flashlight=• Light bulb=• Laser beams=• Optical telephone fibers=*Traffic lights=
*AIMS: Primarily Physics:Light Sources Activity
“Just Passing Through:” What happens when light strikes
glass? Or waxed paper? Or a book?
• If light travels through an object it is =transparent
• If light is blocked by an object and a dark shadow is cast it is= opaque.
• If some light passes through but not all and a light shadow is present it is=translucent.
*AIMS:Primarily Physics: Just Passing Through Activity
What happens when light hits these objects?
• Glass of water • School bus window• Notebook paper• Waxed paper• Plastic wrap• Tissue paper• Cardboard• Textbook• Hand lens…
Transparent objects:
• The windows on a school bus,• A clear empty glass,• A clear window pane,• The lenses of some eyeglasses,• Clear plastic wrap,• The glass on a clock,• A hand lens,• Colored glass…• ALL of these are transparent.
Yes, we can see through them because light passes through each of them.
Translucent objects• Thin tissue paper,• Waxed paper,• Tinted car windows,• Frosted glass,• Clouds,• All of these materials
are translucent and allow some light to pass but the light cannot be clearly seen through.
Opaque objects:
• Heavy weight paper,• Cardboard• Aluminum foil,• Mirror, bricks, buildings,• Your eyelids and hands,• Solid wood door,• All of these objects are
opaque because light cannot pass through them at all.
• They cast a dark shadow.
What is light really?Electromagnetic radiation waves
• Light waves are three dimensional.
• Light waves vibrate in all planes around a center line.
• The waves have high points called “crests.”
• Waves also have low points called “troughs.”
• *The distance from one crest to the next crest is called a “wavelength.”
• *The number of waves passing a given point in one second is called the “frequency.”
wavelength
*A Science Museum of VA:Light Science Activity
*Electromagnetic Radiation
• Electromagnetic radiation can be described as a stream of photons. Each photon traveling in a wave-like pattern, moving at the speed of light and carrying some amount of energy.
• The only difference amongst radio waves, visible light, and gamma-rays is the amount of energy of the photons. Radio waves have photons with low energies. Microwaves have a little more energy than radio waves. Gamma-rays and cosmic rays have highest energy waves and are the deadliest.
*Page info from NSTA Conference 2004
*Page info from NSAT Conference 2004
Don’t’ forget…longest waves (radio) ..to shortest waves (cosmic)
*Science Museum of VirginiaActing It Out! Activity +Scaling the Spectrum Activity ORGEMS: Invisible UniverseComparing Wave Makers I.
Radio(Longest electromagnetic waves)
• Emitted by– Astronomical Objects– Radio Station
Transmitters
• Detected by– Ground based radio
telescopes– *If you turn on a radio,, it
will convert the radio wave energy into sound energy.
Television• Shorter than radio,
also used to carry messages (pictures & sound) to our TV sets.
• *We can sense the TV waves around us with our televisions.
Microwave
• Emitted by:– Gas clouds collapsing
into stars– Microwave Ovens– Radar Stations– Cell Phones
• Detected by– Microwave Telescopes– Food (heated)– Cell phones– Radar (systems)
Infrared(Heat or Thermal)
Are you a source of infrared? YES you are!
• Emitted by– Sun and stars (Near)– TV Remote Controls– Food Warming Lights
(Thermal)– *Everything at room
temperature or above,=HEAT
• Detected by– Infrared Cameras– TVs, VCRs,– Your skin
Let’s take a look at Herschel’s Experiment
• Herschel’s Experiment– Discovered Invisible
Light– In 1800, Herschel
places his control thermometer just outside the red end of the spectrum
– Result: The outside thermometer registered the highest temperature
Conducting Hershel’s Experiment• Place a sheet of white paper
inside a cardboard box• Tape three thermometers
together and place inside box
• Cut a small notch in the top of the box and position a glass prism so that the spectrum is projected inside the box
• Arrange the thermometers so that one is just outside the red end of the spectrum, with no visible light falling on it
VisibleEach color is a different size wave.
Red the longest & violet the shortest• Emitted by
– The sun and other astronomical objects
– Laser pointers– Light bulbs
• Detected by– Cameras
(film or digital)– Human eyes– Plants (red light)– Telescopes
Color
• White light is not a single color; it is made up of a mixture of the seven colors of the rainbow.
We can demonstrate this by splitting white light with a prism:
This is how rainbows are formed: sunlight is “split up” by raindrops.
Adding colors• White light can be split up to make separate colors.
These colours can be added together again.
• The primary colors of light are red, blue and green:
Adding blue and red makes magenta (purple)
Adding blue and green makes cyan
(light blue)
Adding all three makes white
again
Adding red and green makes yellow
Seeing color• The colour an object appears depends on the colours
of light it reflects.
For example, a red book only reflects red light:
White
light
Only red light is reflected
A white hat would reflect all seven colours:
A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue):
Purple light
White
light
Using colored light
• If we look at a colored object in coloured light we see something different. For example, consider a football kit:
White
light
Shorts look blue
Shirt looks red
• In different colors of light this kit would look different:
Red
lightShirt looks red
Shorts look black
Blue
light
Shirt looks black
Shorts look blue
Some further examples:
Object Colour of lightColour object seems to be
Red socks
Red Red
Blue Black
Green Black
Blue teddy
Red Black
Blue
Green
Green camel
Red
Blue
Green
Magenta book
Red
Blue
Green
Using filters• Filters can be used to “block” out different colors of light:
Red Filter
Magenta Filter
Investigating filters
Colour of filter Colours that could be “seen”
Red
Green
Blue
Cyan
Magenta
Yellow
UltravioletSunburn / black light
• Emitted by– Tanning booths (A)– The sun (A)– Black light bulbs (B)– UV lamps
• Detected by– Space based UV
detectors– UV Cameras– Flying insects (flies)
X-ray• Emitted by
– Astronomical objects – X-ray machines– CAT scan machines– Older televisions– Radioactive minerals– Airport luggage
scanners
• Detected by– Space based X-ray
detectors– X-ray film– CCD detectors
Chandra X-ray Observatory
• Chandra is designed to observe X-rays from high energy regions of the universe, such as the remnants of exploded stars.
• The most sophisticated observatory built to date.
• Deployed by the Space Shuttle Columbia on July 23, 1999,
Chandra X-ray Observatory
*Slide from 2004 NSTA Conference
Gamma Ray(Short electromagnetic waves but more energetic)
• Emitted by– Radioactive materials– Exploding nuclear
weapons– Gamma-ray bursts– Solar flares
• Detected by• --Geiger counters
– Gamma detectors and astronomical satellites
– Medical imaging detectors
Sources of g-ray Emission• Black holes• Active Galaxies• Pulsars• Diffuse emission• Supernovae• Gamma-ray bursts• Unidentified
COSMIC Rays(The highest energy waves and the deadliest)
• Cosmic rays come from deep space and can pass through the Earth.
• A great question!• Radio waves= (Buildings to human size);• Microwaves
(Humans-beetles);• Infrared waves (Eye of a needle);Visible waves (microscopic size)!
WOW! All the rest are the size of molecules, atoms, atomic nuclei and smaller..
Reflection• Reflection from a mirror:
Incident ray
Normal
Reflected ray
Angle of incidence
Angle of reflection
Mirror
• The Law of ReflectionThe Law of Reflection
Angle of incidence = Angle of reflectionAngle of incidence = Angle of reflection
In other words, light gets reflected from a surface at ____ _____ angle it hits it.
The same !!!
Clear vs. Diffuse Reflection
• Smooth, shiny surfaces have a clear reflection:
Rough, dull surfaces have a diffuse reflection.
Diffuse reflection is when light is scattered in different directions
Refraction happens because the light slows down in the
material
(the part of the beam that arrives first slows down first)
You can experiment with effects of different incident angles and refractive indices with Figure 3. You can move the flashlight around, and change the refractive index of the top and bottom mediums. You can use the buttons to quickly enter the index for a few common mediums. Critical AngleUsing the refraction simulator, notice how the light bends toward the normal when the light enters a medium of greater refractive index, and away from the normal when entering a medium of lesser refractive index. Then notice what happens when you move the flashlight to an angle close to 90 or -90 degrees in the medium with a higher refractive index. As you approach the critical angle the refracted light approaches 90 or -90 degrees and, at the critical angle, the angle of refractions becomes 90 or -90 and the light is no longer transmitted across the medium/medium interface. For angles greater in absolute value than the critical angle, all the light is reflected. This is called total reflection.
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