Download File - Mrs. Hille`s FunZone

Light and the
Electromagnetic
Spectrum
Light as Energy
There is much evidence in
our world that light is a form
of energy.
Electromagnetic Spectrum
• Electromagnetic waves
include visible light and
several other types of
waves.
• Arranged in order, they
form the electromagnetic
spectrum.
Electromagnetic Spectrum
• Waves with shorter
wavelengths have higher
frequencies and greater
energies.
• Radio waves are the least
energetic; gamma waves
are the most energetic.
Radio Waves
• used in TV and radio
transmissions
• used in communications
• microwaves
Infrared Waves
• produced by the thermal
motion of atoms
• all matter emits infrared
waves
• have many commercial
uses
Visible Light Waves
•
•
•
•
•
narrow band
3.9 × 1014 to 7.7 × 1014 Hz
λ = 770 nm to 390 nm
deep red to deep violet
a continuous spectrum
Ultraviolet Waves
• greater energy and higher
frequency than visible light
• three levels
X-rays
• produced when highenergy electrons strike
atoms and suddenly
decelerate
• penetrate solid matter
• medical and industrial
diagnostics
Gamma Rays
• produced by high-energy
changes in subatomic
particles
• stopped only by very thick
or dense materials
• high doses can cause
damage to living things
Sources and
Propagation
of Light
Incandescent
• Incandescent sources are
objects that are heated until
they glow.
• The frequency and color of
the light are related to the
object’s temperature.
Gas-Discharge
• consist of a sealed glass
tube containing a gas and
fitted with electrodes
• current flowing through the
tube generates visible light
• type of gas determines
color of light
Gas-Discharge
• Fluorescent lights emit UV
which strikes phosphors on
the inside of the glass tube.
• Phosphors glow when
struck by high-energy EM
radiation.
Lasers
•
•
•
•
light at a single frequency
single, energetic EM wave
extremely intense
many practical uses, but
not suitable for area
lighting
LED’s
• light-emitting diodes
• solid-state electronic
component that emits
monochromatic light when
a small potential difference
is established across it
LED’s
• wide variety of applications
• have become practical for
illumination
• use low power and are very
efficient
Cold Light
• generate light with minimal
heat through chemical
reactions
• chemiluminescent
• bioluminescence—
produced by living things
• very efficient
The Speed of Light
• Many have tried to calculate
the speed of light.
• Galileo
• Ole Rømer
• Armand Fizeau
• Léon Foucault
• Albert Michelson
The Speed of Light
• The currently accepted
value for the speed of light
is exactly 299,792,458 m/s.
• We usually round this to
3.00 × 108 m/s.
• This is the speed of light in
a vacuum (c).
Light Waves
• Light travels outward in
concentric spherical waves.
• Light waves travel at equal
speeds through a uniform
medium.
• plane waves
• wave fronts
Light Waves
• Huygens’s principle
postulates how light waves
propagate.
• wavelets
• envelope
Light Waves
Mathematical Description
• The magnitude
of
the
E = Emax sin ωt
electric field strength (E)
B=
Bmax sin ωt
and the
magnitude
of the
magnetic
fieldfield
vector
The electric
and(B)
the
both act as
sine
magnetic
field
arewaves.
in phase.
Light Waves
Mathematical Description
• James Clerk Maxwell related
electricity, magnetism, and
light.
Reflection
and Mirrors
Ray Optics
• Light can be regarded as a
group of rays.
• Light travels in reasonably
straight lines.
• Reflection: light waves
change direction
Ray Optics
• Diffuse reflection: light
waves reflect in random
directions
• Regular or specular
reflection: light waves
reflect predictably
Ray Optics
• normal = perpendicular
• angle of incidence (θi)
• angle of reflection (θr)
Ray Optics
Law of Reflection
• The angle
incoming
of incidence
ray, the
normal,the
equals
andangle
the reflected
of
ray all lie in the same
reflection.
plane.
Albedo
• Visible-light albedo is a
ratio of the reflected light
to the incident light.
• All light is reflected:
albedo = 1.00
• All light is absorbed:
albedo = 0.00
Albedo
• geometric albedo: sun is
directly behind the
observer relative to the
observed object
• bond albedo: no regard to
the position of the sun
Plane Mirrors
• The image we “see” in a
mirror is called a virtual
image.
• In a plane mirror, it appears
that the left and right sides
are reversed.
Plane Mirrors
• By using multiple plane
mirrors at various angles,
we can see multiple
images
• 90° → 3 images
• 60° → 5 images
• 45° → 7 images
Plane Mirrors
• The number of images (n)
for a given angle θ is
determined by this formula:
360°
n=
-1
θ
Curved Mirrors
• concave mirrors
• convex mirrors
• Spherical concave mirrors
produce spherical
aberration.
• not an issue with
parabolic mirrors
Concave Mirrors
• principal focus or focal
point (F)
• distance from F to mirror
is the focal length (f)
• radius of the mirror (R) is
important for spherical
concave mirrors
Concave Mirrors
• center of a spherical mirror
(C) is the center of the
spherical surface
• line through F and C
intersects mirror at its
vertex (V); called the
principal or optical axis
Concave Mirrors
• On a spherical concave
mirror, the focus (F) is
midway between V and C.
R
f=
2
Concave Mirrors
Concave Mirrors
• object distance (dO) is the
distance of the object from
the mirror
• image distance (dI) is the
distance of the image from
the mirror
Concave Mirrors
• There are six possible cases
with the object located on
the optical axis.
• A real image is one which
can be focused on a screen.
• “in front of” the mirror
Concave Mirrors
• Case 2 (dO > R)
Concave Mirrors
• Case 4 (f < dO < R)
Concave Mirrors
• Case 3 (dO = R)
Concave Mirrors
• Case 1: “infinite” distance
from mirror
Concave Mirrors
• Case 5 (dO = f)
Concave Mirrors
• Case 6 (dO < f)
Finding Image Position
• The mirror equation:
1
1
1
+
=
dO
dI
f
• Distances behind the mirror
are assumed to be negative.
Magnification
• For all spherical mirrors, the
height of the image (HI)
relates to the height of the
object (HO) by:
HI
dI
= HO
dO
Magnification
• The magnification of the
image is the absolute value
of the image height to the
object height:
m=
HI
HO
Spherical Mirrors
• For a concave spherical
mirror, not all incoming rays
are reflected so that they
actually pass through the
focal point.
• This results in spherical
aberration.
Spherical Mirrors
• Spherical aberration is more
obvious in highly curved
spherical mirrors.
• Flatter mirrors minimize, but
do not eliminate, spherical
aberration.
Parabolic Mirrors
• A paraboloidal reflecting
surface is not subject to
spherical aberration.
• All rays parallel to the
optical axis are reflected
through the focus (F).
Parabolic Mirrors
• R = 2f
• Parabolic mirrors are often
used to project rays, with
the light source at the
principle focus.
• flashlights, spotlights,
etc.
Convex Mirrors
• produce only one type of
image
• virtual (behind mirror)
• erect
• smaller
• f = R/2 (behind mirror)
Convex Mirrors
• The mirror equation is still
valid, but f and dI are
negative numbers.
1
1
1
+
=
dO
dI
f
Convex Mirrors
• Image size can also be
computed:
HI
dI
= HO
dO