LIGHTTHE ELECTROMAGNETIC SPECTRUM
Mr. RockensiesRegents Physics
COLORS OF LIGHTWhen 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 electromagneticspectrum.
• 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.
Speed of light in vacuum Speed of light in other medium
Index of Refraction =
n =cv
Typical n values are on the reference tables
SNELL’S LAW
n1sinθ1 = n2sinθ2
boundary
Medium 1 n1
Medium 2 n2
θ1
θ2
θ1 = angle of incidence
θ2 = angle of reflection
Waves bending in a Medium
EFFECTS OF REFRACTION
EXAMPLES OF REFRACTIONab
c
d d
c
b
a
θ1 = θ c
θ2 = 90°
slower medium
faster medium
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°.
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
Fiber Optic 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.
EFFECTS OF REFREACTION
1. Extended Sunset
Earth
Atmosphere
Ray from the Sun
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”)
moving source
Approaching source, frequency higher (called “Blue Shift”)
( ( ( ( ( • ) ) ) ) ) ) ) ) ) )
REAL WORLD APPLICATIONS
1.Radar Guns
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
gun ( ) ) ( ) ) ( ) ) car
v
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 LIGHTWe 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.
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.
Young’s Double Slit Experiment
POLARIZATIONBecause 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!!!