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 LIGHTCoherent Light is monochromatic and all pulses are in phase

Monochromatic Coherent

REFRACTIONLight 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 LAWn1sinθ1 = n2sinθ2

boundaryMedium 1 n1Medium 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 OPTICSBased 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 REFREACTION1. 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 REFRACTION2. 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 APPLICATIONS1.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!!!