Transcript Light and Optics - bba-npreiser

More Light and Optics
Light – Wave or particle?
• For many years scientists argued over the
nature of light, "Is light a wave or a stream of
particles?"
• In some experiments light exhibits wave like
properties, the Doppler effect, interference,
refraction, diffraction
• and in other experiments, like the photo
electric effect, it exhibits particle like
properties
• The fact is that light exhibits behaviors which
are characteristic of both waves and
particles.
Models of Light - Waves
• Electromagnetic waves (light) originate from vibrating or
accelerating electric charges
• Electromagnetic waves are made up of an electric field
and a magnetic field oscillating at right angles relative to
one another
• An electromagnetic wave
(light) is a transverse wave
Unlike other waves, light waves can travel
through a vacuum
Models of Light - Particles
• Particle of light are called photons
• Photons have zero rest mass and travel at the speed of
light through a vacuum.
Speed of Light - c
• In the early 17th century, many scientists believed that
there was no such thing as the "speed of light"; they
thought light could travel any distance in no time at all.
• In the 1670's Roemer was able to calculate a value for
the speed of light by carefully studying the orbit of one
of Jupiter’s moons, Io. He noticed that the time between
the eclipses of the moons of Jupiter was less as the
distance away from Earth is decreasing than when it is
increasing.
• In 1926 scientist Albert Michelson used the reflection
from a rotating mirror on a distant mountain and
measured the speed of light at 299,796 km/second
• The current accepted value is 300,000,000 meters per
second (3 x 108 m/s) or 186,000 miles per second. Light waves
obey the wave
equation, c = lf
Things the produce electromagnetic waves
• Radio waves
– electrons moving up and down an antenna
• Visible Light
– electrons changing energy states in an atom
Wave/Particle
Phenomenon
Can be explained in terms
of waves.
Can be explained in terms
of particles
Reflection
yes
yes
Refraction
yes
yes
Interference
yes
no
Diffraction
yes
no
Polarization
yes
no
Photoelectric effect
no
yes
Light and Energy
• For waves, the amplitude or intensity is usually
related to the energy of the wave
• For light, this is not true. The energy of light waves
was found to be directly related it is frequency.
• An experiment demonstrating the photoelectric
effect demonstrated the particle nature of light and
that E = hf, where E is energy, h is Planks constant,
and f is frequency.
http://phet.colorado.edu/new/simul
ations/sims.php?sim=Photoelectric
_Effect
The Photoelectric Effect
• Laws of photoelectric emission
• For a given metal and frequency of incident radiation, the rate at which
photoelectrons are ejected is directly proportional to the intensity of the
incident light.
• For a given metal, there exists a certain minimum frequency of incident
radiation below which no photoelectrons can be emitted. This frequency
is called the threshold frequency.
• Above the threshold frequency, the maximum kinetic energy of the
emitted photoelectron is independent of the intensity of the incident light
but depends on the frequency of the incident light.
• The time lag between the incidence of radiation and the emission of a
photoelectron is very small, less than 10-9 second
The equation is
, where h is Planck's constant, f is
the frequency of the incident photon,Φ is the work function (sometimes
denoted W instead), the minimum energy required to remove a
delocalised electron from the surface of any given metal.
The Electromagnetic Spectrum
The electromagnetic spectrum is the range of electromagnetic waves
extending from radio waves to gamma rays
Increasing frequency
ROYGBIV
The Visible Spectrum
• We can only see a small part of the
electromagnetic spectrum
• The visible spectrum is a range of light waves
extending in wavelength from about 400 to
700 nanometers.
Increasing wavelength
Increasing frequency
Increasing energy
Things that can separate white light
•
•
•
•
Prism
Raindrops
CD’s
Diffraction Grating
Young’s Double-Slit Experiment
• The wave theory of light came to prominence with Thomas Young’s
double-slit experiment, performed in 1801.
• The double-slit experiment proves that light has wave properties because
it relies on the principles of constructive interference and destructive
interference, which are unique to waves.
Constructive and Destructive Interference
• At any point P on the back screen, there is light
from two different sources: the two slits. The line
joining P to the point exactly between the two slits
intersects the perpendicular to the front screen at
an angle .
• The light from the right slit—the bottom slit in our
diagram—travels a distance of l = d sin more than
the light from the other slit before it reaches the
screen at the point P.
• As a result, the two beams of light arrive at P out
of phase by d sin. If d sin = (n + 1/2), where n is an
integer, then the two waves are half a wavelength
out of phase and will destructively interfere.
• On the other hand, if d sin = n, then the two waves
are in phase and constructively interfere, so the
most light hits the screen at these points.
Accordingly, these points are called the maxima of
the pattern.
Double-Slit Diffraction/Interference Pattern
Double-Slit Diffraction/Interference Pattern
For L>>d, Q in large and small
triangles are equal. L is
distance to screen.
If Dx = l, then l = d sin Q
Double-Slit Diffraction/Interference Pattern
l = d sin Q
y
L
tan Q = y/L,
where y is
the distance
between the
bright spots
and L is the
distance to
screen. For
small angles
tan Q = sin Q
so l = d y/L
y = lL/d
Single-Slit Diffraction Pattern
l = W sin Q
OR
l = a sin Q
a and W are slit width
This time the solution is for
the destructive points (the dark spots)
By the same argument as in double slits
y = lL/a y is measured from the center
to the first dark spot.
Diffraction Grating
• A diffraction grating is a screen with a bunch of
parallel slits, each spaced a distance d apart
• The condition for maximum intensity is the same as
that for a double slit. However, angular separation of
the maxima is generally much greater because the
slit spacing is so small for a diffraction grating.
Polarization
• As an electromagnetic wave
traveled towards you, then
you would observe the
vibrations of the slinky
occurring in more than one
plane of vibration
• A light wave which is vibrating
in more than one plane is
referred to as unpolarized
light.
• Light emitted by the sun, by a
lamp in the classroom, or by a
candle flame is unpolarized
light
The Structure of the Atom and Emission
• An atom is composed of electrons,
protons and neutrons.
• When an electron is raised to a
higher energy level, the atom is said
to be excited.
• When the electron returns to a lower
energy level, energy is released in the
form of light.
• Different transitions from high levels
to low levels result in different colors
of light.
The Kirchhoff-Bunsen Experiment
• These two scientists found that burning
chemicals over an open flame resulted in a
spectrum with bright lines.
• They found that each chemical element
produced its own characteristic pattern of
bright spectral lines.
Emission Spectra of Hydrogen
Hot gas produces a bright line emission spectrum.
RH is 1.09678 x 10-2 nm-1
Discrete Emission Spectrum
Slit
Film
Low Density
Glowing
Hydrogen Gas
Prism
Photographic Film
Calculating the energy in a transition
1 eV = 1.6 x 10-19 J
The Rydberg Equation:
RH is 1.09678 x 10-2 nm-1
Every element can be “fingerprinted” by it spectra.
Hydrogen
Helium
Oxygen
Carbon
Incandescence
• Hot, dense solids produce a continuous
spectrum.
Continuous Spectrum
• The brightness and color of light emitted by a
hot object changes with its temperature.
• Glowing object colors:
•
•
•
•
•
Reddish
 coolest glowing object
Orange-ish
Yellowish
White
Bluish
 hottest glowing object
Absorption Spectra
• Cool gas in front of a continuous source of
light produces an absorption line spectrum.
• Fraunhofer lines in our Sun's spectrum
showed that cool helium gas surrounds the
Sun
Absorption Spectrum
Absorption Spectra of Hydrogen
Discrete Emission Spectrum
Discrete Absorption Spectrum
Slit
Hydrogen Gas
Film
White Light
Source
Prism
Photographic Film
Sources
• Conceptual Physics by Paul Hewitt
• www.physicsclassroom.com
• http://observe.phy.sfasu.edu/courses/phy101
/lectures101/