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

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Presentation on theme: "Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Interference and Diffraction Introduction to Physical Optics."— Presentation transcript:

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

2 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science What is Light? To understand physical optics, let’s review how we think about and measure light, which is part of electromagnetic radiation.

3 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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. Gamma Rays Ultraviolet Rays X Rays Light Infrared (IR) Microwave Radio wave m m 10 3 m10 -2 m

4 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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

5 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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.

6 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Wavelength Wavelength is the distance between two identical points on a wave. (  lambda)

7 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Frequency Frequency is the number of cycles per unit of time. (  nu) lIt is inversely proportional to the wavelength. time unit of time

8 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Wavelength and Frequency Relation lWavelength is proportional to the velocity, v. lWavelength is inversely proportional to the frequency. leg. AM radio wave has a long wavelength (~200 m), therefore it has a low frequency (~KHz range). lIn the case of EM radiation in a vacuum, the equation becomes  v   c  Where c is the speed of light (3 x 10 8 m/s)

9 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Photons lPhotons are little “packets” of energy. lEach photon’s energy is proportional to its frequency. A photon’s energy is represented by “h ” E = h Energy = (Planck’s constant) x (frequency of photon)

10 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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.

11 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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 IntensityLower Intensity

12 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Phase: –The phase of light refers to the timing and position of two or more waves. Wave Phase

13 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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.

14 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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.

15 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Interference Two waves of the same type and frequency can interfere when they meet at the same place.

16 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Superposition occurs when waves combine to form a new wave. –Constructive Interference Waves in phase always superpose to add amplitudes. Interference =

17 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Superposition occurs when waves combine to form a new wave. –Destructive Interference Waves out of phase superpose to subtract amplitudes. Interference =

18 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Coherent waves are continuously in phase with each other. –Example: Laser Light Interference

19 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science The phases of incoherent waves vary randomly. Interference –Example: Light bulb

20 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Interference Laser Beam-splitter 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.

21 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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.

22 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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. s1s1 s2s2

23 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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.

24 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Interference Fringes –When two or more beams of coherent light interfere, patterns appear in the form of fringes (dark and bright bands of light). Constructive interference: waves from two slits combine in phase

25 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Interference Fringes –The bright spots are caused by constructive interference, and the dark spots by destructive interference Destructive interference: waves from two slits combine out of phase

26 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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.

27 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science Diffraction A circular hole or just a tiny circular shaped particle could create the diffraction pattern below: –Alternating fringes of concentric circular rings

28 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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 HighLow Resolving Power:

29 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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.

30 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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

31 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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!).

32 Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science 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.


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