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Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Lecture 23
Chapter 22
Wave Optics:
Interference of Light
Course website:http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII
Lecture Capture: http://echo360.uml.edu/danylov201415/physics2spring.html
04.17.2015Physics II
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Models of Light
Today, we are going to talk about light.Newton believed that light consists of particles –corpuscles. But Huygens thought that light is wave. The controversy between Newton and Huygens about the nature of light was settled in 1801 when Young demonstrated convincingly that light shows all the characteristic of waves.
The wave model: Under many circumstances, light exhibits the same behavior as sound or water waves. The study of light as a wave is called wave optics.
The ray model: The properties of prisms, mirrors, and lenses are best understood in terms of light rays. The ray model is the basis of ray optics.
The photon model: In the quantum world, light behaves like neither a wave nor a particle. Instead, light consists of photons that have both wave-like and particle-like properties. This is the quantum theory of light.
But today we will only focus on the wave character.
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Diffraction of Water Waves
A water wave, after passing through an opening, spreads out to fill the space behind the opening.
This well-known spreading of waves is called diffraction.
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Young’s Double-Slit Experiment
These experiment is similar.The nature of waves is different.
This experiment with two sources of light is equivalent to the waves emitted by two loudspeakers, so those results can be used here.
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Analyzing Double-Slit InterferenceLight wave(laser: λ, f )
dscreen
P
θ
A
B
θ
θ
zooms inSince L>>d, the paths from the two slits at angle θ are ALMOST parallel
L
↑↑
θ
Inherent phase difference:
= 2
C
Δ sinSince angles are small sin
It’s easier to measure distances than angles: θ → y
SO
∆ :
≃
, 0,1,2, …
Conditions for constructive interference(positions of bright fringes)
The wave from the lower slit travels an extra distance:
_
_
_
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Analyzing Double-Slit Interference II
12
,
0,1,2, …
Conditions for destructive interference
(positions of dark fringes)
dark fringesbright fringes
,
0,1,2, …
Conditions for constructive interference
(positions of bright fringes)
Δ
Similar for dark fringes
Let’s find distance between fringes: 1
There is no dependence on m,so they are equally spaced
A laboratory experiment produces a double-slit interference pattern on a screen. If the slits are moved closer together, the bright fringes will be
A) Closer together B) In the same positionsC) Farther apartD) There will be no fringes because the conditions for interference won’t be satisfied.
ConcepTest 1 Double-Slit Interference I
y Ld
and d is smaller, so . Is larger
distance between fringes
y
A laboratory experiment produces a double-slit interference pattern on a screen. If the screen is moved farther away from the slits, the fringes will be
A) Closer together B) In the same positionsC) Farther apartD) Fuzzy and out of focus.
ConcepTest 2 Double-Slit Interference II
y Ld
and d is smaller, so . Is larger
distance between fringes
y
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Intensity of the Double-Slit Interference Remember (Ch.34) that the intensity of a wave is
proportional to square of a wave amplitude
We got in the previous class
Δ sin
2
cos cos
the previous slide:
The intensity of the double-slit interference pattern at position y is:
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Example with a demonstration ?
Light wave(laser: λ, f )
A laboratory experiment produces adouble-slit interference pattern on a screen. If the left slit is blocked, the screen will look like
ConcepTest 3 Double-slit interference III
blocked
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Diffraction GratingSuppose we were to replace the double slit with an opaque screen
that has N closely spaced slits.
A large number of equally spaced parallel slits is called a diffraction grating.
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
The Diffraction Grating
When illuminated from one side, each of these slits becomes the source of a light wave that diffracts, or spreads out, behind the slit. A practical grating will have hundreds or even thousands of slits.
The figure shows a diffraction grating in which N slits are equally spaced a distance d apart.
This is a top view of the grating, as we look down on the experiment, and the slits extend above and below the page.
Physics and math are the same as for a double-slit experiment
Bright fringes will occur at angles m, such that:
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
The Diffraction Gratingscreen
θ1θ2
The y-positions of these fringes will occur at:
It’s easier to measure distances on the screen than angles: θ → y
The integer m is called the order of the diffraction.
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Bright spot intensityWe had two waves in the double-slit experiment
Now with N slits, the wave amplitude at the points of constructive interference is Na.
Because intensity depends on the square of the amplitude, the intensities of the bright fringes are:
∽
Not only do the fringes get brighter as N increases, they also get narrower.
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Measuring wavelength emitted by a diode laser (with a demonstration)
Light from a diode laser passes through a diffraction grating having 300 slits per millimeter. The interference pattern is viewed on a wall 2.5 m behind the grating.
Calculate the wavelength of the laser.
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Grating Spectrometer
Diffraction gratings are used for measuring the wavelengths of light(Grating Spectrometer)
Light wave(λ1, λ2 )
If the incident light consists of two slightly different wavelengths, each wavelength will be diffracted at a slightly different angle.
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
What you should readChapter 22 (Knight)
Sections 22.1 22.2 22.3
Department of Physics and Applied Physics95.144, Spring 2015, Lecture 23
Thank youSee you on Tuesday