Nature of Light

Daily Challenge, 11/17
What is LIGHT?
The Electromagnetic Spectrum
Electromagnetic Waves
• Electromagnetic waves vary depending on
frequency and wavelength.
• All electromagnetic waves move at the
speed of light. The speed of light, c, in a
vacuum equals
c = 3.00  108 m/s
• Wave Speed Equation
c = fl
speed of light = frequency  wavelength
Different Views of Light over Time
Corpuscular Theory (Newton)
particle explanations
Wave Theory (Huygens)
wave explanations
Electromagnetic Theory
energy transfer by waves
Quantum Theory
energy transfer in “packages”
LIGHT: Wave or Particle?
Rectilinear Propagation
explained if light travels explained if light travels
faster in water that in air
faster in air that in water
Diffraction (~1800)
not explained
not explained
not explained
Photoelectric effect
The Photoelectric Effect
When a metallic surface is exposed to
electromagnetic radiation that is above a
threshold frequency (which is specific to the type
of surface and material), electrons are “kicked
off” the metal and current is produced. No
electrons are emitted for radiation with a
frequency below that of the threshold frequency.
See for more
What happens when you
viewed yourself at different
distances on either side of a
spoon? Why?
Daily Challenge, 11/18
What are some
characteristics of a
reflected image?
Reflection of Light
the turning back of a wave meeting the boundary of a medium
is the ratio of the light reflected from a surface to the light
falling on a surface, commonly expressed as a percentage
Regular, specular reflection – scattering is negligible
Diffuse reflection – scattering of light is significant
(rays not parallel)
Laws of Reflection
1st – angle of incidence equals angle of reflection, i = r
2nd – incident & reflected rays & normal are all in a plane
Mirror Terminology
C = center of curvature R = radius of curvature
f = focal length
red dot = principal focus
R = 2f
Principal axis = goes through C & principal focus
Reflected Images
Real images
formed by converging rays of light passing through a
real image point
appear upside-down
produced by concave mirrors when object is further
away than F
Virtual images
formed by rays of light appearing to diverge from
unreal image point
appear right-side-up, but are inverted left to right
produced by plane & convex mirrors, concave mirrors
when object is closer than F
Geometric Image Construction
Images Formed by Mirrors
Concave Mirrors
• virtual or real images, depends on object location with respect to F
object at infinite distance
• “image” is a point at F
object at finite distance beyond C
• image is real, inverted,
• reduced, between C and F
object at C
• image is real, inverted, at C
object between C and F
• image is real, inverted,
• enlarged, beyond C
object at F
• image is not formed,
• reflected rays are all parallel
object between F and mirror
• image is virtual, erect, enlarged
See Page 460 in the text for
pictures of these situations
Images Formed by Mirrors
Convex Mirrors
• always virtual, erect images of reduced size
Object-Image Relationships
Mirror Equation
1/f = 1/p + 1/q
Heights & magnification M = h’ / h = q / p
where f = focal length of mirror
p = distance of the object from the mirror
q = distance of the image from the mirror
(negative value means image is virtual)
M = magnification (# times bigger image)
h’ = height of the image
h = height of the object
Sign conventions
+ f = concave mirror
- f = convex mirror
p = always positive
+q = real image
- q = virtual image
Example Problem
A 5.00 cm arrow stands at the 0.0-cm
mark of a meter stick. At the 50.0-cm
mark is a convex mirror whose radius
of curvature is 45.0 cm. How far from
the mirror is the image? How tall is it?
MiniLab, 11/19
How far away would a 50-cm tall
mirror have to be before a 2-m tall
person could see themselves in it?
(This challenging question can be solved
with “thought” experiments, real experiments,
ray diagrams, or the mirror equation!)
Daily Challenge, 11/20
All electromagnetic energy
travels at the speed of light.
WHY is short-wavelength
electromagnetic radiation
“high energy” and long
wavelength electromagnetic
radiation “low energy”?
Light Colors
Primary Colors of Light
red, green, blue
mixing all 3 makes white light
Complimentary Colors of Light
any two colors that form white light when
combined (cyan-red, yellow-blue, green-magenta)
Primary Pigments (reflect light)
cyan, magenta, yellow
compliments of primary light colors
Dispersion & the Colors of Light
White light passing through a prism is
separated into a visible solar spectrum
consisting of red, orange, yellow, green, blue,
and violet (elementary) colors.
Object Color
opaque – color seen depends on the
frequency of the light reflected (white reflects all)
transparent/translucent – color seen
depends on the frequency of the light transmitted
Umbra –
full shadow
Penumbra –
partial shadow
Daily Challenge, 11/23
MINILAB: Use the optical bench to create
and view real images with a concave mirror.
Check your observations to be sure that
they verify the “6 cases” of images
produced by concave mirrors.
Compare these images to those created by
a convex mirror.
For credit, each person must make some
detailed notes and/or sketches of your setup
and observations.
Daily Challenge, 11/24
How far from a concave mirror, of
focal length 6.0 cm, does a candle
have to be placed to look like it is
burning on both ends?
Daily Challenge, 11/30
An Electrician’s Nightmare
Five wires appear the following colors under sunlight:
1 white 2 black 3 red 4 green 5 yellow
If an electrician must work under a cyan light, what
color will each wire appear to be?
If the electrician works in sunlight, but wears
sunglasses with a magenta tint, what color will each
wire appear to be?
Daily Challenge, 12/1
What’s the difference
between a luminous object
and an illuminated object?
Daily Challenge, 12/2
White spotlights often show thin colored
fringes around the edge of the white light.
Why? Explain what is happening and the
pattern of light you would expect to see.
the quantitative study of light
Luminous Intensity, I
► is the light energy produced per time per area
► measured with a photometer
(Bunsen, Joly, Photoelectric, Spherical)
►measured in candelas (cd)
The candela is the luminous intensity, in a given
direction, of a source that emits monochromatic
radiation of frequency 5.40 x 1014 Hz and that has
an intensity in that direction of 1/683 watt per
steradian (sr). sr =angle intercepting a unit surface area on a unit sphere
Luminous Flux, F
is that part of the total energy radiated per unit of
time from a luminous source that is capable of
producing the sensation of light (notice, it’s a rate)
measured in lumens (lm)
The lumen is the luminous flux on a unit surface all
points of which are at a unit distance from a point
source of one candela. F = 4  I
Illuminance, E
is the density of a luminous flux on a surface
► measured in lux (lx)
The lux is the lumens/meter2.
E = F / A = I / r2
(assumes surface perpendicular to flux)
Illuminance on a surface area varies inversely with
the square of the distance from the luminous
source and directly with the cosine of the angle
between the luminous flux and the normal to the
E = I cos  / r2
Daily Challenge, 12/3
Photometry Mini-Lab
Use a bunsen “grease spot” photometer to
make photometry measurements as
instructed. Quantitatively compare these
“grease spot” measurements to the light
meter readings.
Daily Challenge, 12/4
What are some practical
applications of LASERS?

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