Physics Light Waves & Physical Optics
Physical Optics Physical optics or wave optics, involves the effects of light waves that are not related to the geometric ray optics covered previously. We will use the wave front model of light primarily Diffraction Interference Polarization
Huygens Principle 1. Every oscillator in a wave may be considered the source of secondary spherical wavelets that propagate outward. 2. The wave front created by these oscillators is due to the combined interference of the wavelets.
Huygens Principle
Huygens Principle Huygens Principle helps us better understand the principles of reflection and refraction.
Huygens Principle It also helps us understand one of the main topics of Ch 29, Diffraction.
Diffraction Diffraction is the bending of light as it passes the edge of an object, creating a fuzzy edge. It also occurs when a wave passes through an aperture.
Diffraction The smaller the opening, the greater the diffraction.
Diffraction and Huygens Principle
Diffraction and Huygens Principle
Diffraction If an object is smaller than the wavelength of light illuminating it, no structure can be seen the entire image is lost due to diffraction.
Diffraction Opening is large compared with the wavelength of light: The result is a sharp shadow with a bit of fuzziness at its edges.
Diffraction Opening is thin, razor like: Diffraction is much more apparent. Light spreads out like a fan without sharp edges a very fuzzy shadow.
Diffraction An array of very thin slits is called a diffraction grating. It can be used to split light into its component colors using diffraction.
Diffraction
Interference of Light When 2 waves interact, the amplitude of the resulting wave is the sum of the amplitudes of the two individual waves. Principle of Superposition
Interference of Light
Interference of Light
Thomas Young s Experiments In 1801, British physicist Thomas Young performed his now famous interference experiments with light.
Thomas Young s Experiments Bright spots of light were formed when waves from both slits arrived in phase.
Young s Double-Slit Experiment
Young s Double-Slit Experiment x m mλl d d sin θ = mλ
Young s Double-Slit Experiment
Problems Monochromatic light passes through two narrow slits and is projected onto a screen, creating a double-slit interference pattern. The first bright maximum occurs at a distance of 0.004 meters from the central maximum. The slit spacing is 3.0 x 10-6 meters, and the distance to the screen is 2.0 meters. Determine the wavelength of light used in this experiment.
Problems Light incident on two narrow slits is used to project an interference pattern onto a screen. How will the interference pattern change if the distance between the slits is doubled?
Problems Light with wavelength, λ, is incident on two narrow slits spaced d meters apart. A distinct pattern of alternating bright and dark regions is visible on a screen located L meters behind the slits. The second bright region (maximum) is observed to be x meters from the central bright maximum. The paths of the rays of light, extending from each slit to the second bright maximum, are shown converging at the second maximum. What is the path difference?
Problems Which of the following is true regarding the experiment in the previous problem? a) The experiment provides evidence that light has a wave characteristic. b) Light passing through each slit diffracts into circular wave fronts. c) The light from each slit interferes to create the resulting bright maximums and dark minimums seen on the screen. d) The experiment must be conducted with monochromatic light. e) All of these are true.
Single Slit vs. Double Slit Interference
Single Slit vs. Double Slit Interference
Problems Monochromatic light is incident upon a single slit. The light passes through the slit and is projected on a screen behind the slit. Which of these describes the pattern seen on the screen? a) There is single maximum consistent with a single slit. b) Light spreads out completely and fills the entire screen evenly. c) There is a large central maximum with faint secondary maxima. d) The pattern is identical to the results obtained using a double slit. e) The light converges into a single bright line.
Thin Film Interference The beautiful colors you can see in soap bubbles are due to interference of reflected light from the outside and inside surfaces of the soap film.
Thin Film Interference We see similar colors from a thin film of gasoline floating on water on wet pavement.
Thin Film Interference Some of the sunlight incident on a layer of gasoline is reflected at the surface, while some of the sunlight penetrates the gasoline layer only to be reflected back by the water. When these reflected rays interfere, some frequencies are cancelled out through destructive interference.
Thin Film Interference
Thin Film Interference Different colors correspond to different thicknesses of the thin-film.
Thin Film Interference The colors you see with thin film interference are primarily Cyan, Yellow, Magenta due to the subtraction of primary Red, Green, Blue in sunlight.
Light Polarization Waves are either longitudinal or transverse.
Light Polarization The girl is creating a transverse wave by shaking a rope. The wave moves along the rope in one plane. The upper image shows a vertically polarized wave. The lower image shows a horizontally polarized wave.
Light Polarization Because the oscillating electric field occurs in a set plane, light waves are polarized. The plane in which the electric field oscillates is known as the plane of polarization.
Light Polarization Lamps, candles and the sun produce unpolarized light.
Light Polarization
Light Polarization If unpolarized light passes through a polarizing filter, it will become polarized.
Light Polarization
Light Polarization Light reflecting off surfaces is partially polarized. The direction of this polarization matches the surface from which the light is reflected.
Light Polarization
Light Polarization
Light Polarization
Light Polarization
Light Polarization Two final important points: Polarization occurs ONLY for transverse waves. Light polarization is then an important way of determining whether a wave is transverse or longitudinal. Light is transverse!
Light Polarization What other fun things can you do with polarization?