Interference Introduction to Optics Coherent source Two Slit Interference Thin film interference...

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Transcript of Interference Introduction to Optics Coherent source Two Slit Interference Thin film interference...

  • InterferenceIntroduction to OpticsCoherent sourceTwo Slit InterferenceThin film interferenceInterference from a Grating

  • Reading QuestionWhat was the first experiment to show that light is a wave? 1. Youngs double slit experiment2. Galileos observation of Jupiters moons3. The Michelson-Morley interferometer4. The Pound-Rebka experiment5. Millikans oil drop experiment

  • Reading QuestionWhat was the first experiment to show that light is a wave? 1. Youngs double slit experiment2. Galileos observation of Jupiters moons3. The Michelson-Morley interferometer4. The Pound-Rebka experiment5. Millikans oil drop experiment

  • Reading QuestionWhat is a diffraction grating?1. A device used to grate cheese and other materials2. A musical instrument used to direct sound3. A plaque with a tiny circular aperture4. An opaque objects with many closely spaced slits5. Diffraction gratings are not covered in Chapter 22.

  • Reading QuestionWhat is a diffraction grating?1. A device used to grate cheese and other materials2. A musical instrument used to direct sound3. A plaque with a tiny circular aperture4. An opaque objects with many closely spaced slits5. Diffraction gratings are not covered in Chapter 22.

  • Reading QuestionWhen laser light shines on a screen after passing through two closely spaced slits, you see1. a diffraction pattern.2. interference fringes.3. two dim, closely spaced points of light.4. constructive interference.

  • Reading QuestionWhen laser light shines on a screen after passing through two closely spaced slits, you see1. a diffraction pattern.2. interference fringes.3. two dim, closely spaced points of light.4. constructive interference.

  • InterferenceWavesMechanicalMatterElectromagnetic

  • InterferenceOpticsGeometric OpticsPhysical Optics

  • InterferenceCoherence and MonochromaticNo coherence between two light bulbsCoherence timeCoherence lengthSome later time or distancecoherence - two or more waves that maintain a constant phase relation.monochromatic - a wave that is composed of a single frequency. Heisenberg uncertainty relation.

  • Double Slit InterferenceWe will work through this tutorial together. We have setup a laser and a slide (2 inches X 2 inches) with different double slit patterns labeled A, B, and C. First we will look at double slit A. On the wall you will see a double slit interference pattern. Draw the interference pattern below and mark the center bright spot. Label the center bright spot 0. Label the next bright spot on the right +1. Continue to label the spots +2, +3, etc. Do the same for the spots on the left but label them -1, -2, -3, etc.Interference

  • Double Slit InterferenceBelow we have reproduced Figure 37.5 on page 975. On the figure label the points that corresponds to the orders -2, -1, 0, +1, and +2. If the lines represent a crest then where two crest cross is a bright spot.

    +1

    0

    -1Interference

  • Interference

  • Interference

  • Double Slit InterferenceWrite the equation that allows you to calculate the position of the bright fringes as a function of angle.

    Write the equation that allows you to calculate the position of the bright fringes as a function of displacement ym from the center line.

    The labels we used to label the bright spots is called the order m.Interference

  • Double-Slit Interference (Youngs Experiment)Interferenceym

  • Double Slit InterferenceCan you explain what the two patterns look like for a red and green laser?

    +1

    0

    -1Interference

  • InterferenceIdealReal

  • Double-Slit Interference (Youngs Experiment)Measure the distance from the slits to the screen and calculate the wavelength for the laser. The separation between the slits is 0.6 mm.

    d = 0.6 mmL = 8 mym+1 - ym = 910 mm

    Interference

  • Student Workbook

  • Student Workbook

  • Student Workbook

  • Student Workbook

  • Class QuestionSuppose the viewing screen in the figure is moved closer to the double slit. What happens to the interference fringes? 1. They get brighter but otherwise do not change.2. They get brighter and closer together.3. They get brighter and farther apart.4. They get out of focus.5. They fade out and disappear.

  • Class QuestionSuppose the viewing screen in the figure is moved closer to the double slit. What happens to the interference fringes? 1. They get brighter but otherwise do not change.2. They get brighter and closer together.3. They get brighter and farther apart.4. They get out of focus.5. They fade out and disappear.

  • Class QuestionLight of wavelength l1 illuminates a double slit, and interference fringes are observed on a screen behind the slits. When the wavelength is changed to l2, the fringes get closer together. How large is l2 relative to l1?1. l2 is larger than l1.2. l2 is smaller than l1.3. Cannot be determined from this information.

  • Class QuestionLight of wavelength l1 illuminates a double slit, and interference fringes are observed on a screen behind the slits. When the wavelength is changed to l2, the fringes get closer together. How large is l2 relative to l1?1. l2 is larger than l1.2. l2 is smaller than l1.3. Cannot be determined from this information.

  • InterferenceThin Film Interference

    The most common example of interference is interference in thin films. One example of this is an oil slick. This is a thin film of oil on top of water. You find this a lot of times on a road after a rain storm. You will see bands of different colors on the surface of a puddle. The different colors come from the varying thickness of the oil film. Can you explain this?

  • We can also use interference to measure the thickness of small objects as shown in the following example. For interference from a thin film we need to consider two things:

    1. The phase shift when the light (wave) is reflected at both surfaces.

    2. The phase shift do to the difference in length of the two paths.Interference

  • InterferenceFirst lets consider the phase shift do to the reflection from an interface between two different materials with different indexes of refraction. What happens when light is reflected from a interface with the second medium having a larger index? We can use the analogy of a wave on a string being reflected from a fixed end. Do you recall what happen to the reflected pulse? Draw the reflected pulse on the top diagram below.

  • InterferenceDoes the light wave come back in phase or out of phase? Discuss this in your group.

    string fixed end

    light medium with smaller index180 degree or l/2 phase shift on reflection

  • InterferenceDo the same for light reflected from a medium with a smaller index of refraction. Draw the reflected light wave on the lower diagram. This is like a wave on a string being reflected from a free end.

    string free end

    light medium with larger index0 degree phase shift on reflection

  • InterferenceDoes the light wave come back in phase or out of phase? Discuss this in your group.

    l/2 phase shiftno phase shift

  • InterferenceLets do an example. Light is reflected from an oil slick:a thin film of oil on top of water.airoilwaterincident beamreflected beam

  • InterferenceMulti-slit InterferenceNext we would like to see what happens if we increase the number of slits from 2 to 3, 4, 5, etc. We will setup a slide (1 inch X 5 inches) with different slit patterns. The numbers across the top indicate the number of slits. Thus 4 indicates four slits. We will use the same set-up as before and observe the slit pattern for the double and triple slits. Do you see any difference between the two patterns? Discuss this and write your answer.

  • InterferenceDraw the triple slit interference pattern below.

    -3 -2 -1 0 1 2 3

    Draw the interference pattern for 4 slits.

    -3 -2 -1 0 1 2 3

  • InterferenceObserve the pattern for 4 and 5 slits. Do you see any pattern developing between the number of slits and the interference pattern? Explain.

  • InterferenceDiffraction GratingWhat is a diffraction grating?

    What are the two types?

    A diffraction grating is a slide with a large number of slits. Usually expressed in the number of slits per mm.transmission and reflection gratings

  • InterferenceWe have set up a transmission diffraction grating with a laser and a white light source. Observe the spectra from a white light source and laser. Can you see more than first order from the grating and laser? Can you see more than one spectra from the white light source? Discuss what you each see in your group. Draw the light pattern from the laser and white light source below.whitelightlaser

  • You should see at least t