Download Light Electromagnetic waves

Light
Electromagnetic waves
E+M Waves
• Electromagnetic waves are transverse waves that
can travel through a vacuum or uniform mediums
• Light is created by vibrations of electrons which
creates both electric and magnetic fields
• Light moves in a straight line- called the Ray
Model of light, treats light as a particle…
• Light also has Wave properties…
• Duality of Light: Light has the properties of waves
and particles
– Examples- shadows around corners, photoelectric
effect
Light Speed
o Before the 17th century people thought light
was instant
o Early experiments showed light had a finite
speed: Michelson “mountain” test
o Speed = 299,792,458 m/s
o The generally accepted value is 3.0 x 108 m/s
o Symbol = c
Source of light
• Luminous bodies:
– Objects that emits light waves
– Example: Sun, light bulb
• Illuminated bodies:
– Objects that reflect light produced from another
source
– Examples: moon, pictures
Light and Matter Interactions
• 3 things can happen when light reaches
matter
– When light passes through an object without
distortion, the object is transparent.
– When light passes through an object but is
distorted, the object is translucent.
– When light is reflected or absorbed by a material,
the object is opaque.
• When objects reflect light… we see colors…
Visible Light
• Very small portion of the E&M spectrum
• The difference between red and violet is only
about 4 nanometers (4 millionths of a meter)
Color
• Newton experimented with a beam of light
passing through a prism.
• Each color has a specific wavelength in the
E+M spectrum
Color by Addition
• Light has 3 primary colors
– Red, Blue, green
– These are the colors (wavelengths) that the
human eye can perceive (look close at a TV
screen)
• Cones- part of eye that detects color
• Adding pairs make the secondary colors
• Red + Blue = Magenta
• Red + Green = Yellow
• Blue + Green = Cyan
So why is grass green?
• What colors can your eyes detect?
– Red, green, blue
• If you see green, what is being reflected?
– Green- that is what you perceive in your eyes
• Then what color(s) is being absorbed?
– Red and blue
• If a shirt appears to be yellow to you, what
colors are being absorbed and reflected?
Subtractive Colors (paint pigments)
• Pigments are materials that absorb some light
waves and reflect others.
• The three Primary colors are yellow, magenta
and cyan (not red, yellow and blue)
– Example: an object that appears red must absorb
cyan but reflect yellow and magenta
Color is absorbed and “subtracted” from what is
seen. Therefore, you see what is left behind.
Pigments
• A student walks in with a bright yellow shirt.
– What color light is hitting the shirt?
– What colors does the shirt absorb?
– What colors does the shirt reflect? .
• A lab table appears to be black.
– What color light strikes the table?
– What color light is reflected?
– What colors of light are absorbed?
• The starts of the flag appear white.
– What color light is striking the flag?
– What color light is reflected? Absorbed?
Polarization
• Polarized light waves are light waves in which
the vibrations occur in a single plane
• The process of transforming un-polarized light
into polarized light is known as polarization
• there are 4 ways to polarize light
– Transmission
– Reflection
– Refraction
– scattering
Transmission “filtering”
• The most common way is to “filter” out waves
of a an alignment you do not want.
• Example: Polarized lenses on cameras
• Class example: think about the glasses!
Polarization by reflection
• Example: night time driving in the rain, the
headlights give a glare from the reflection on
the road.
• Light hits the road un-polarized but reflects off
as polarized- causing glare
Snell’s Law
• Refraction occurs when waves move from one
medium to another.
– Each medium has an “index of refraction” (n)- a
ratio of the speed of light in a vacuum versus the
speed of light in the material.
The equation is:
ni *(sinθi) = nR *(sinθR)
• Θ is ALWAYS measured from the normal line!
– Normal means “perpendicular”
Practice problem
• A drinking glass full of water has light reflecting off a
pencil inside of it. As the light leaves the pencil it
strikes the container edge at a 25° to the normal line.
If it enters air on the other side, what angle will it
1.0003
leave the surface at? Air
Ice
1.31
Water
1.333
Ethyl Alcohol
1.36
Plexiglas
1.51
Crown Glass
1.52
Light Flint Glass
1.58
Dense Flint Glass
1.66
Zircon
1.923
Diamond
2.417
Rutile
2.907