Electromagnetic Waves

Electromagnetic Wave Properties

Electromagnetic waves do not need a medium to propagate. They can travel in a vacuum.

The speed of all electromagnetic radiation in a vacuum is a constant.

c = 3 X 108 m/s.

Electromagnetic Spectrum

Draw out the spectrum noting trends in frequency, wavelength, wave speed and energy.There is a easy mnemonic to help you remember the order of EM radiation….

Red Martians Invade Venus Using X-ray GunsRadio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma

Wavelength…Starts Long and gets shorter towards Gamma

Frequency……Starts Low and gets higher towards Gamma

Speed is the same for all EM waves

Energy…Starts low and gets higher towards Gamma

Visible Light

ColorsAddition vs. Subtraction of Color•The perceived color of an object is due to two processes, addition and subtraction of colors.

•The addition of color is the projection of various intensities of the primary colors of light to produce the desired color.

•The subtraction of color is the absorption of specific colors by pigments. The colors that remain produce the desired color. The reflection of white light off a colored object is an example of color subtraction.

Visible Light

Primary and Secondary Colors

Light-Additive

Primary Colors: RedGreenBlue

Secondary Colors: CyanYellowMagenta

Pigments-Subtractive

Primary Colors: CyanYellowMagenta

Secondary Colors: RedGreenBlue

Visible Light

Color ExamplesStare blankly at a colored object for a minute.

Visible Light

Visible Light

Color ExamplesStare blankly at a colored object for a minute.

Visible Light

Reflection

Definition of Reflection

The turning back of a wave.

Law of ReflectionAngle of Incidence = Angle of Reflection.

i = r

or ‘theta’ is a symbol used to denote a variable angle.

(It is like ‘x’ in algebra.)

RefractionDefinition of RefractionThe bending of a wave disturbance as it passes at an angle from one medium to another due to a change in speed.

Definition of Index of Refraction The ratio of speed of light in a vacuum to its speed in a given transparent medium.   

v

cn

v

cn

v

cn

v

cn

v

cn

v

cn

Index of Refraction of Common Items

Vacuum 1.00Air 1.0002926

(we round to 1.0)

Water 1.33Common Glass 1.5Diamond 2.42

Refraction

Bending Rules• Light traveling from a less dense to more dense medium slows down. •The ray bends towards the normal,• ni < nr.• Light traveling from a more dense to less dense medium speeds up.•The ray bends away from the normal, •nr < ni.

  

Snell’s Law

ni sini = nr sinr

n = index of refraction = angle  

i = incident ray

r = refracted ray

Example #1

What is the index of refraction of a medium in which the speed of light is 1.85 X 108 m/s?

Example #2

A fisherman shines a flashlight at a fish swimming underwater. If the refracted ray makes an angle of 35.0 with normal, what is the angle of incidence? 

Total Internal Reflection

Definition of Total Internal Reflection

The complete reflection of light at the boundary of two transparent media. The effect occurs when the angle of incidence exceeds the critical angle for a light ray going from a more dense medium to a less dense medium.

Definition of Critical Angle

The minimum angle of incidence for which total internal reflection occurs. This occurs when the angle of refraction becomes 900.

Lenses

Convex LensConverging Lens

Focal PointLocation at which rays parallel to the principal axis converge to or appear to diverge from for a convex or concave lens respectively.Focal LengthDistance from the focal point to the center of the lens.Real ImageAn image formed when rays of light actually intersect at a single point.

Lenses

Concave LensDiverging Lens

Focal PointLocation at which rays parallel to the principal axis converge to or appear to diverge from for a convex or concave lens respectively.Focal LengthDistance from the focal point to the center of the lens.Virtual ImageAn image formed by light rays that only appear to intersect

Ray Diagram

Rules for Lenses

Ray Diagram

Lens Trends

Case

Converging or Convex Lens Diverging or Concave Lens

Size Orientation Image Size Orientation Image

do > 2f

do = 2f

2f > do > f

do = f

do < f

Ray Diagram

Lens TrendsConverging or Convex Lens Diverging of Concave Lens

Focal Length

Object Distance

ImageDistance

Real Image

Virtual Image

Object Height

Image Height

Real Image

Virtual Image

Lens Equation

io d

1

d

1

f

1

do= object distance

di = image distance

f = focal length

Example #4

Example #4 If an object is placed 15 cm in front of a converging lens with a focal length of 10 cm, how far from the lens should the card be placed in order to find a clear image?

Interference

InterferencePattern of constructive and destructive interference.

DiffractionThe spreading of waves into a region behind an obstacle.

PolarizationThe alignment of electromagnetic waves.

Doppler EffectA frequency shift that is the result of relative motion between the

source of sound waves and an observer.