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Light, Reflection, and Refraction
Chapters 14 and 15
Electromagnetic Waves
• An electromagnetic wave is composed of a
magnetic field wave perpendicular to an electric
field wave
• All objects that are not at absolute zero emit
EMWs.
• The hotter the object the more waves they emit.
• The electromagnetic spectrum is composed of a
range of wavelengths and frequencies that range
from radio waves to gamma waves.
• Visible light is a very small portion of that entire
spectrum.
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c
• The speed of an electromagnetic wave in a
vacuum is 3.00 x 108m/s.
• It is equal to the product of the wavelength
and the frequency
• c = ƒ
• Sample Problem 14A
Visible Light
• Visible Light is the part of the EMS that we
can see
• Ranges from the color red with a
wavelength of 700nm (x10-9m) to the color
purple with a wavelength of 400nm.
Reflection
• Light waves usually travel in straight paths.
• When a light wave encounters a different
substance it changes direction.
• When it encounters a substance that does
not permit light to travel through it, opaque,
some of the light will be reflected.
• Usually a portion of the light is absorbed.
Reflection (cont)
• The texture of the opaque object’s surface affects
how it reflects light.
• A rough object reflects light in many different
directions, diffuse reflection
• A smooth object reflects light in only one
direction, specular reflection
• A surface is considered smooth if variations are
smaller than the size of the wavelengths being
reflected.
• It is difficult to make objects smooth enough to
reflect X-rays and Gamma Rays.
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Mirrors
• Mirrors are smooth surfaces that reflect
nearly all of the light they encounter.
• Light that strikes a mirror at an angle from
the normal line reflects at the same angle
away from the normal line
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Flat Mirrors
• Flat mirrors are the simplest form of mirror
where the objects distance to the mirror, p,
is equal to the distance from the mirror to
the image, q.
• The image appears to be located behind the
mirror and is considered to be a virtual
image as the object would not appear on a
screen.
Ray Diagrams
• Ray diagrams are used to predict the location of
the image of an object.
• To make a ray diagram for a flat mirror choose a
point on the object and draw a ray toward the
mirror at a perpendicular angle. This ray would
reflect back on itself.
• Then draw a ray at an angle toward the mirror and
draw the reflection of that ray.
• Trace back both of the reflected rays through the
mirror, where they intersect, place the image.
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Concave Spherical Mirrors
• Concave spherical mirrors are those who
reflective surface is on the interior of a
curved surface that has a radius R to the
center of curvature C.
• The optical axis is any line that passes
through C and is usually oriented with an
object.
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Concave Spherical Mirror Rules
• A ray traveling through C will reflect back through
C. (only if object is beyond C)
• A ray traveling through the focal point f, halfway
between C and the surface of the mirror, will
reflect parallel to the OA
• A ray traveling to the intersection of the OA and
the mirror will reflect at the same angle below the
OA.
• A ray traveling parallel to OA will reflect through
the focal point
Ray Diagrams
• Using any of the two rules you must draw two
rays, the object occurs at the point of intersection.
• We will draw several ray diagrams to determine
the image produced by an object that is
– Beyond C
– Between C and f
– Between f and mirror
Convex Spherical Mirrors
• Convex spherical mirrors are those where
the reflective surface is on the outside of the
curve.
• The points f and C are located behind the
mirror
• Convex spherical mirrors have rules as well.
Rules
• A ray parallel to the OA will reflect directly away
from f.
• A ray heading towards f will reflect parallel to the
OA
• A ray heading towards C will reflect directly away
from C.
• A ray heading toward intersection of OA 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
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Equations
• While ray tracing gives us a good idea of the
location of an object it is always best to verify
with math.
• If p is the object’s distance and q is the image
distance then…
• 1/p + 1/q = 1/f
• The magnification of the object can been
calculated using the equation…
• M = -(p/q)
• Sample Problem 14C
Parabolic Mirrors
• Rays 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.
Refraction
• Substances that are transparent or translucent
allow light to pass though them.
• When light passes from one
transparent/translucent substance to another it
changes direction.
• This change is due to the slight differences in
speed that light travels in the new substance.
• This is called refraction.
Analogy
• A good analogy for refracting light is a
lawnmower traveling from the sidewalk
onto grass.
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Index of Refraction
• The ratio of the speed of light in a vacuum
to the speed of light in a medium is that
medium’s index of refraction. (n)
• The higher the index of refraction, the
slower light travels through a medium.
• Refraction causes objects to appear at
locations they are not at.
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Snell’s Law
• Snell’s Law relates the indices of refraction
as well as the angle away from the normal
line (angle of incidence) to determine the
angle of refraction.
• n1(sini) = n2(sinr)
• r = sin-1{(n1/ n2)(sini)}
• Sample Problem 15A
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Total Internal Reflection
• If the angle of incidence of a ray is very
large(close to 90º) the ray will reflect rather
than refract.
• This principal is responsible for the
properties of fiber optic cables.
• Remember the lawn mower analogy…
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Thin Lenses
• Refraction is the property that allows us to
manipulate an object’s image using a lens.
• We will be working with converging and
diverging lenses.
• Just like with mirrors, we will need to follow rules
to draw ray diagrams to predict the location of an
image.
• Thin lenses also have focal points, these points are
determined not only by the curve of the lens but
the index of refraction of the lens as well.
• A lens has two focal points, one on either side.
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Converging Lens Diagram
• Draw one ray parallel to OA, refracts
through focal point.
• Draw one ray through center of lens,
continues straight through.
• Draw one ray through focal point, refracts
through lens, travels parallel to OA.
• Image located at intersection of rays.
• Treat lens as though it were a flat plane.
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Diverging Lens Diagram
• Because 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 OA, refracts away from f, trace
back to f.
• Ray 2 - ray toward f, refracts parallel to OA, trace
back parallel to OA
• Ray 3 - ray through center, continues straight,
trace back toward object
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Equations
• You can use the same equations for curved mirrors
with lenses
• If p is the objects distance and q is the image
distance then…
• 1/p + 1/q = 1/f
• The magnification of the object can been
calculated using the equation…
• M = -(p/q)
• Sample problem 15B