Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Interference and...

$: Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Interference and Diffraction Introduction to Physical Optics.$
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

Interference and Diffraction

Introduction to Physical Optics


What is Light?

• To understand physical optics, let’s review how we think about and measure light, which is part of electromagnetic radiation.


Electromagnetic Radiation

• EM radiation is made up of an electric field and a magnetic field.

• Particle-wave duality of EM radiation.– Light as a particle– Light as a wave (physical optics)

• Includes x-rays as well as light, IR (heat) and radio waves.

Gamm

a Ray

s

Ultrav

iolet

Ray

s

X Ray

s

Light

Infra

red (I

R)

Microwav

e

Radio

wav

e

10-15 m 10-6 m 103 m10-2 m


Optics• Optics contains two areas of study:

– Geometrical Optics– Physical Optics

• Recall: Geometrical optics, or ray optics, is the study of light that travels as a “ray,” in straight lines.– Light rays passing through lenses and

bouncing off mirrors


What is Physical Optics?• Physical optics, or wave optics, is the

study of how light interacts with objects similar in size to its wavelength.– Light energy travels as a wave (not a

ray).– Wave optics concerns the characteristics

of light such as wavelength, intensity, phase, and orientation.


Wavelength

Wavelength is the distance between two identical points on a wave. (lambda)


Frequency

Frequency is the number of cycles per unit of time. (nu)

It is inversely proportional to the wavelength.

time

unit of time


Wavelength andFrequency Relation

Wavelength is proportional to the velocity, v. Wavelength is inversely proportional to the frequency. eg. AM radio wave has a long wavelength (~200 m), therefore

it has a low frequency (~KHz range). In the case of EM radiation in a vacuum, the equation becomes

v

cWhere c is the speed of light (3 x 108m/s)


Photons Photons are little “packets” of energy. Each photon’s energy is proportional to

its frequency. A photon’s energy is represented by “h”

E = hEnergy = (Planck’s constant) x (frequency of photon)


Light Wave• Transverse Wave

– Travels perpendicular to change of amplitude.

E

B

Direction of Travel

– The case of light:• Light waves are called electromagnetic waves because

they contain two types of energy that change amplitudes.

• Both electrical and magnetic energy vary perpendicular to each other.

• Light is a transverse wave because the direction of travel is perpendicular to the amplitude change of BOTH electrical and magnetic fields.


Light Intensity• Intensity of a monochromatic light

relates to the brightness of that light. – The intensity of an electromagnetic

wave is proportional to the amplitude squared.

Higher Intensity Lower Intensity


• Phase:– The phase of light refers to the timing and

position of two or more waves.

Wave Phase


• In Phase:– Two waves that are “in

phase” move together with the same motions.

• They are at the same cyclic position at the same time.

Waves ‘In Phase’

– Example• The turn signal on the car in front of you blinks at

the same time as your signal.


• Out of Phase:– Two waves that are “out of

phase” do NOT move together with the same motions.

• At the same time they are at different cyclic positions.

Waves ‘Out of Phase’

– Example• The turn signal inside your car alternates with

the signal of the car in front of you.


Interference• Two waves of the same type and

frequency can interfere when they meet at the same place.


• Superposition occurs when waves combine to form a new wave.

– Constructive Interference• Waves in phase always superpose to add amplitudes.

Interference

=


• Superposition occurs when waves combine to form a new wave.

– Destructive Interference• Waves out of phase superpose to subtract amplitudes.

Interference

=


• Coherent waves are continuously in phase with each other.

– Example:Laser Light

Interference


• The phases of incoherent waves vary randomly.

Interference

– Example:Light bulb


Interference

LaserBeam-splitter

Mirror

Mirror

Viewing Screen

• To observe interference:• Use light that that has the same frequency,

and is coherent; e.g. LASER light.• Split a light beam into two paths.

– amplitude splitting

• Allow the two beams to meet (recombine) at the same location on a viewing screen or detector.


Interference• To observe interference:

• When the two beams recombine at the viewing plane they produce interference patterns of dark and bright fringes because the distances traveled by the beams determine their phases relative to each other.


Thin Films• Thin films, such as

gasoline, oil, or soap bubbles, also cause interference by amplitude splitting.– One light source (sun), is

used to create two virtual light sources, splitting the amplitude of the original.

• The first source is the reflection off the first surface of the film.

• The second source is the reflection off the second surface of the film.

s1 s2


Thin Films• Thickness

– The distance between the two surfaces is half the distance between the virtual sources -- this path difference determines the wavelength(s) of the reflected light.

• Oil or gasoline on wet pavement is seen as different colors as the thickness of the film changes.


Interference• Fringes

– When two or more beams of coherent light interfere, patterns appear in the form of fringes (dark and bright bands of light).

Constructiveinterference:

waves from two slits combine in

phase


Interference• Fringes

– The bright spots are caused by constructive interference, and the dark spots by destructive interference

Destructiveinterference:

waves from two slits combine out of phase


Diffraction• Diffraction will cause interference

fringes to form when a single beam interacts with an object nearly the same size as the wavelength of the light.


Diffraction• A circular hole or just a tiny circular

shaped particle could create the diffraction pattern below:– Alternating fringes of concentric circular

rings


Diffraction• Many tiny dust particles can

decrease the resolving power of a lens by overlapping many diffraction patterns.

• Loss of resolving power = difficult to distinguish fine details

High Low

Resolving Power:


Diffraction Gratings• What is a diffraction grating?

– A transmission grating is an opaque plate that has numerous equally spaced parallel slits that serve to break up light into its component wavelengths.

– A reflection grating is to similarly space numerous reflective surfaces.


Diffraction Gratings• The slits cause light to diffract

– Shorter wavelengths (e.g. blue) interfere constructively at a smaller angle than longer wavelengths

– As a result, a grating spreads incident white light out into a spectrum of colors.

Grating


Interference Applications• Diffraction Gratings

– Iridescent colors: When the diffracted color changes with the angle an object is looked at.• Many birds, insects, and fish have iridescent

colorings via diffraction (feathers make excellent gratings!).


Summary• Physical optics is

the study of light traveling as a wave.

• Coherent light of the same frequency can interfere both constructively and destructively, sometimes forming fringe patterns.

• Diffraction of light due to tiny objects causes diffraction patterns to form.

• Diffraction gratings are used to diffract light into its component wavelengths.

Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Interference and...

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