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Physics Mrs. Tobler
What is Light?
• Debated by scientists–Dual “Personality”
•Acts like transverse waves •Acts like it is made of particles –Photons (packets of energy)
• Light is a form of energy
Light as a Wave• Light travels as a transverse wave
– Made up of Electric and Magnetic Fields moving perpendicular to one another
• Light is a range of frequencies of electromagnetic waves– A.K.A. Electromagnetic Radiation
Properties of Electromagnetic Waves
• Travel at 3.0 x 108 m/s in a vacuum (empty space)
• Light year = distance light travels in one year (about 6 trillion miles)– The light from a star 4 light years away
traveled for 4 years before reaching your eyes
– Light from our own sun takes 8 minutes to reach Earth
– When you look at the stars you are looking into the past
• Transmit energy via Radiation– Don’t need a medium
• Slow down when it enters a medium.– Diamond slows it down to 124 m/s
Visible Light
• Remember “ROY G BIV”• Which has a longer wavelength
Orange or Blue?– Orange
Complete the Spectrum
Uses for Electromagnetic Waves
• Radio Waves– Transmit TV, AM &
FM– Satellite radio– Cell phones– RADAR for
communication and police
• Microwaves– Cook food by
exciting water molecules to produce heat
• Infrared– Keep food warm (like at McDonald’s or
Luby’s)– Night vision
• Ultraviolet– Suntans, skin cancer– Sterilize goggles and other items– Fluorescence (Black Light)
• X-rays– For looking at bones such as teeth or broken
arm• Gamma
– To kill off cancer cells– Can also damage healthy cells
Uses Continued
Why can’t we see all electromagnetic waves?
• Human eyes contain rods and cones in the retina that are sensitive to only a small range of frequencies
• We see only reflected or emitted light– Reflected = waves bounce off of a
surface back to our eyes– Emitted = object produces light at
frequencies our eyes can detect
What color is white light?• Isaac Newton
– Discovered white light consists of various colors of light in 1666 using prisms
• Made of the 3 Primary Colors of Light– Red– Blue– Green
How we See Color• Our eyes detect the color
(frequency) of light that is being Reflected off of a surface.– If an object appears blue that
means blue light is bouncing off of the object and the other colors of light are absorbed
• What happens to the light if the object appears white?
• What if it appears black?
More Fun with Color• Primary colors of Pigments
• Magenta• Cyan• Yellow
– Color printers use these same colors
• Color Filters– If an object reflects red light when
exposed to white light, then when exposed to green light the object will appear almost black because there is NO red light to reflect.
Behaviors of Light• All electromagnetic waves
experience the following wave behaviors similar to mechanical waves– Reflection– Refraction– Diffraction– Interference
• Constructive• Destructive
– Polarization
Words to Know• Ray = beam of electromagnetic radiation
(light)• Normal = imaginary line that is
perpendicular to the surface of an object• Incident Ray = emitted or incoming light
ray• Reflected Ray = light ray that bounces
off a surface; outgoing wave• Angle of Incidence = angle at which the
incoming light ray hits a surface• Angle of Reflection = angle at which
reflected light ray leaves the surface
Reflection of Light• Occurs when a wave reaches a
boundary between 2 different mediums which causes– The wave to bounce back into the 1st
medium (totally reflected)– Or some of the wave bounces back and
some is absorbed into the 2nd boundary (partially reflected).
• Depends on how rigid the boundary is – Very rigid then most of the wave is reflected – Less rigid then only some of the wave is
reflected
Law of Reflection• When a wave
hits a boundary the angle of incidence equals the angle of reflection.– Also true when
playing wall ball, basketball and tennis
Complete the Reflection Diagram
Diffraction• Bending of a wave around a
barrier, such as an obstacle or the edges of an opening.– Amount depends on the size of
the wavelength compared with the size of the obstruction or opening.•Wide opening = less bending •Narrow openings = more bending
• Thomas Young’s Double Slit Experiment
Diffraction Gratings
• A series of closely spaced parallel slits or grooves
• Used to separate colors of light by interference.– Used instead of a prism– Found in objects used to reflect
various colors of light •Back of CDs & DVDs•Bird feathers
Total internal Reflection• At a certain angle (critical angle)—
no ray will emerge into the air above the surface.– For water it is 48 which does not
allow the ray to enter the air, it is only reflected inside the water.
– Other Examples•Glass (43)•Mirrors (90-95)•Diamonds (24.6)
Review Refraction• Bending of a wave as it enters
a new medium due to a change in velocity.– Angle of Incidence is NOT EQUAL
to the Angle of Refraction• Refraction of Sound
•Even though speed of sound is greater in warm air; sound waves bend away from the ground which doesn’t allow it to carry as well.
• In cold air sound bends toward the ground and sound carries well.
Refraction of Light• Less dense to More
dense medium – Speed of light slows
down – Refracted Ray bends
toward the normal• air to glass
• More dense to Less dense medium – Speed increases – Ray bends away from the
normal.• water to air
• As it leaves the new medium it goes back to the original angle.
Complete the Refraction Diagram
Observing objects in Water
Refraction of Light
Causes white light to separate into the
primary colors as it passes through
the water droplets
How are Rainbows Produced?
• Water droplets act like prisms• Each color of light is refracted
at a different angle due to its frequency– Violet has the highest
frequency and is refracted the most.
– Red has the lowest frequency and is refracted the least.
• Sun has to be behind the observer
• Circular rainbows can be seen from airplanes
Atmospheric Refraction
• Mirage – caused by change in air temperature resulting in light refraction– Road appears
to be wet•Actually reflection of the sky
Why is the Sky Blue?• Light from the
sun gets scattered as it collides/passes through molecules in the atmosphere– Blue light is
scattered the most because of its short wavelength
Index of Refraction• Unique to the substance
– Based on density
• Ratio between the speed of light in a vacuum to the speed in a particular material
v
cn
n = index of refraction
c = speed of light in vacuum
(3x108 m/s)
v = velocity of light in material
Example Index of Refractions
Material Refractive Index
Air 1.0003
Water 1.333
Glycerin 1.473
Immersion Oil 1.515
Crown Glass 1.520
Flint Glass 1.656
Zircon 1.920
Diamond 2.417
Lead Sulfide 3.910
Polarization of Light• The aligning of vibrations in an
electromagnetic wave, usually by filtering out waves of other directions.
• Polarized Filter– Only allows waves through that are
parallel to the slits
• Occurs only with transverse waves – Not all light is polarized
How Polarization Works
Fiber Optics• A.K.A. Optical fibers or light
pipes• Thin glass fibers
– Use total internal reflection • Developed for communication
and data transmission– Replacing electric circuits and
microwave links • Thousands of miles of it in our
phone lines
Polarized Sunglasses.
• Only allow light that is in the same direction as its polarized axis to pass.– If polarization axis of 2 filters are at right
angles to each other then the light can’t pass through.
Traditional 3-D Glasses
• The red and blue lenses filter the two projected images allowing only one image to enter each eye.
• Brain combines to images to look 3-D
Modern 3-D Glasses
• Uses polarized filters instead of color
• Allow only one of the images intoeach eye because each lens has a different polarization.