Mr. Rockensies
Regents Physics
LIGHT
THE ELECTROMAGNETIC SPECTRUM
COLORS OF LIGHT
When light is seen as a single color, it is considered monochromatic
When light has more than one color, it is referred to as polychromatic
White light is the result of at least 3 colors, and usually is full-spectrum
• Full spectrum means that it has all
wavelengths of the electromagnetic
spectrum.
• Our Sun emits wavelengths across the
entire spectrum.
COHERENT LIGHT
Coherent Light is monochromatic and all pulses are in
phase
Monochromatic
Coherent
REFRACTION
Light is slower than c=3.00x108m/s in all media besides
vacuum and air.
The degree of bending depends on how much the light slows
down.
Index of Refraction = Speed of light in vacuum
Speed of light in other medium
n=c
v
Typical n values are on
the reference tables
SNELL’S LAW
n1sinθ1 = n2sinθ2
θ1 = angle of
incidence
θ1
Medium 1
n1
Medium 2
n2
boundary
θ2
θ2 = angle of
reflection
Waves bending in a Medium
EFFECTS OF REFRACTION
EXAMPLES OF REFRACTION
b
a
slower medium
c
d
d
c
In case “c” the refracted
ray is right along the
boundary – “critical” case.
At this point, θ1 is called θc
– the critical angle and θ2
is 90°.
b
θ2 = 90°
a
faster medium
Find the n value
FIBER OPTICS
Based on laws of refraction, a fiber optic cable is
designed so that any light that enters cannot leave until
reaching an exit at the other end of the cable
We call this phenomena “total internal reflection”, and
it occurs when the incident angle is greater than the
critical angle – resulting in a reflected ray.
Fiber Optic Cable
EFFECTS OF REFREACTION
1. Extended Sunset
Ray from the Sun
Earth
Atmosphere
Refracted Ray due to
the atmosphere, allows
us to see the Sun after
it has set.
EFFECTS OF REFRACTION
2. Dispersion of Colors
• in non-dispersive media (vacuum & clean air) all colors have the same speed
and bend the same amount
•in dispersive media (everything else) each color has a slightly different speed
and index of refraction
•Red is the fastest, has the smallest index of refraction, and bends the
least.
DOPPLER EFFECT FOR LIGHT
(
(
(
(
(
Receding Source,
frequency appears
lower (called “Red
Shift”)
• ) ) ) ) ) ) ) ) ) ) Approaching
source,
frequency
moving source
higher (called
“Blue Shift”)
REAL WORLD APPLICATIONS
v
1. Radar Guns
gun
()
)
()
)
(
)
)
car
reflected waves are shifted to red by receding
object. By comparing the reflected waves to the
original waves, the gun calculates the object’s
speed
2. Doppler weather radar – same as radar gun ~ reflecting
radio waves off of air masses/clouds.
3. Astronomy – stars are made of Hydrogen + Helium which
have characteristic bright line spectra at known frequencies:
Shifted lines indicate that a star is either approaching or
receding from us
When we observe light from other galaxies, all
lines are red shifted – What would this imply?
All galaxies are moving away from us – therefore
the universe is expanding
DIFFRACTION OF LIGHT
We had said that when waves diffract they bend around
corners:
Light will exhibit this same
behavior. As a result of
this bending, we can see
spaces of light and dark
when light passes through
two or more openings that
are placed near each
other.
Young’s Double Slit Experiment
If light passes through a boundary with two or
more openings, we will see bands of light and dark
appear on a surface beyond that barrier.
POLARIZATION
Because light is a
transverse wave, it
can become
polarized.
Light can also
become polarized by
reflecting off of a
shiny surface –
“glare”
TIPS FOR THE REGENTS
•Remember, if light passes from a faster medium into a slower
medium (i.e. air into anything else), the refracted ray should be
bent towards the normal. If light passes from a slow medium into
a faster one (i.e. diamond into anything else) then the refracted
ray should bend away from the normal.
•When trying to find the critical angle, always remember that θ2
MUST be 90°, and therefore sin θ2 = 1.
•You can find the velocity of light in ANY medium as long as you
have its n value (index of refraction). Use n=c/v !
•When measuring the angle of incidence/refraction ALWAYS
MEASURE FROM THE NORMAL!!
•When a wave moves from one medium into another, its speed
and WAVELENGTH change, not frequency (the color stays the
same)
•PRACTICE, PRACTICE, PRACTICE!!!