LIGHT –REFLECTION

AND REFRACTION

Electromagnetic Waves Magnetic field wave perpendicular to an

electric field wave

All objects emit EMWs. Temp EMW

Electromagnetic spectrum Range of all frequencies of light

Visible light is a very small portion of that entire spectrum.

http://upload.wikimedia.org/wikipedia/commons/c/cf/EM_Spectrum_Properties_edit.svg

c

Speed of Light - 3.00 x 108m/s.

= (wavelength) x (frequency)

c = ƒ

ExampleAM Radio waves

5.4 x 105 Hz1.7 x 106 Hz = ?

Visible LightPart of the EMS

humans can seeRed - 750nm (x10-9m) Purple - 380nm

Bees, Birds – UVSnakes – IR

ReflectionLight waves usually travel in straight paths

Change in substance changes direction

Opaque - does not permit lightsome light reflectedsome light absorbed as heat

ReflectionTexture affects reflectionDiffuse reflection (rough)

reflects light in many different directions,

Specular reflection (smooth)reflects light in only one direction

Smooth – variations in surface

MirrorsLight striking a mirror reflects at the

same angle that it struck the mirror

Flat Mirrorsp = q

p- objects distance to the mirror

q - distance from the mirror to the image

Virtual imageDoes not existMade by our eyes

Ray Diagrams Used to predict the location of the

image of an object

Concave Spherical MirrorsReflective surface is on the interior of a curved surface C – center of curvatureR – Radius (distance to C)f – Focal Point (1/2 R)Principal axis

any line that passes through C usually oriented with an object

http://upload.wikimedia.org/wikipedia/commons/4/43/Concave_mirror.svg

Mirror Equations1/object distance + 1/image distance = 1/focal length1/p + 1/q = 1/f

Magnification (M) = Image height/object height (h / h) - (q / p)

• M = h / h = - (q / p)

Sign of Magnification

Sign of M Orientation of Image Type of Image

+ Upright Virtual

– Inverted Real

Concave Spherical Mirror Rules

A ray traveling through C will reflect back through C

A ray traveling through (f) will reflect parallel to the PA

A ray traveling to the intersection of the PA and the mirror will reflect at the same angle below the PA.

A ray traveling parallel to PA will reflect through the focal point

Ray DiagramsDraw three rays

The image forms at the point of intersection

Examplef = 10.0cmp = 30.0cmh = 3.00cm

Convex Spherical Mirrors

Reflective surface is on the outside of the curve.

The points f and C are located behind the mirrornegative

http://upload.wikimedia.org/wikipedia/commons/4/4a/Convex_mirror1.svg

RulesA ray parallel to the PA will reflect

directly away from f.A ray towards f will reflect parallel to

the PAA ray towards C will reflect directly

away from C.A ray to the intersection of PA and

mirror will reflect at the same angle below the OA.

Trace the 3 diverging lines back through the mirror to reveal the location of the image which is always virtual

Examplef = -8.00cmp= 10.0cmh = 3cm

Parabolic MirrorsRays that hit spherical mirrors far away from the OA often reflect though other points causing fuzzy images, spherical aberration.

Telescopes use parabolic mirrors as they ALWAYS focus the rays to a single point.

RefractionSubstances that are transparent or translucent allow light to pass though them.

Changes direction of light Due to the differences in speed of light

AnalogyA good analogy for

refracting light is a lawnmower traveling from the sidewalk onto mud

Index of Refraction (n)

The ratio of the speed of light in a vacuum to the speed of light in a medium

n - c

Snell’s Lawni(sini) = nr(sinr) r = sin-1{(ni/ nr)(sini)}Example

i = 30.0⁰ni = 1.00nr = 1.52

Total Internal ReflectionIf the angle of incidence of a ray is

greater than a certain critical angle the ray will reflect rather than reflect

This principal is responsible for the properties of fiber optic cables.

Remember the lawn mower analogy…

Critical Angle

sin Θc = nr / ni

As long as nr < ni

What is the critical angle for light traveling from Diamond to Air?

Thin Lenses Converging

Diverging

f- curve of lens & index of refraction

Converging Lens Diagram1. Ray parallel to PA, refracts

through far focal point2. Ray through center of lens,

continues straight line3. Ray through near focal point,

refracts through lens, continues parallel to PA

Treat lens as though it were a flat plane.

http://en.wikipedia.org/wiki/File:Lens3.svg

Diverging Lens DiagramBecause the rays that enter a diverging lens do not intersect a virtual image is formed by tracing back the refracted rays.

Ray 1 - parallel to PA, refracts away from near f, trace back to near f.

Ray 2 - ray toward far f, refracts parallel to PA, trace back parallel to PA

Ray 3 - ray through center, continues straight, trace back toward object

http://upload.wikimedia.org/wikipedia/en/0/03/Lens4.svg

Sign Conventions for LensSign p q F

+ Near side of lens

Far side of lens

Converging Lens

– Far side of lens

Near side of lens

Diverging Lens

Converging Lens Example

p = 30.0cmf = 10.cm

Diverging Lens Example

p = 12.5cmf = -10.0cm