Lesson 26 Diffraction and Interference Eleanor Roosevelt High School Chin-Sung Lin

Diffraction

Christiaan Huygens Christiaan Huygens, a Dutch physicist, astronomer, and mathematician Telescopic studies elucidating the nature of the rings of Saturn and the discovery of its moon Titan The invention of the pendulum clock Studies of both optics and the centrifugal force

Huygens’ Principle Light waves spreading out from a point source may be regarded as the overlapping of tiny secondary wavelets Every point on any wavefront may be regarded as a new point source of secondary waves Wavefronts are made up of tinier wavefronts

Huygens’ Principle As wavefronts spread, they appear less curved. Wave fronts seem to form a plane very far from the original source

Huygens’ Principle Huygens’ principle applied to reflection

Huygens’ Principle Huygens’ principle applied to refraction

Huygens’ Principle Straight wave passing through wide opening

Huygens’ Principle Straight wave passing through narrow opening

Huygens’ Principle When the opening becomes smaller, Huygens’ idea that every part of a wave front can be regarded as a source of new wavelets becomes apparent

Huygens’ Principle Huygens’ principle applied to diffraction

Diffraction The bending of a wave by means other than reflection or refraction as it passes around the edge of an obstacle

Diffraction When light passes through an opening that is large compared to the wavelength of light, it casts a rather sharp shadow When it passes through a small opening it casts a fuzzy shadow

Diffraction Single-color light passing through a razor blade creates diffraction fringes In white light, the fringes merge together to create a fuzzy blur at the edge of the shadow

Diffraction The amount of diffraction depends on the size of the wavelength compared to the size of the obstruction that casts the shadow The longer the wave compared to obstruction, the greater the diffraction is Long waves are better at filling in shadows

Diffraction Long waves diffract or bend readily around buildings and reach more places than shorter waves do FM radio waves have shorter wavelengths than AM waves do, so they don’t diffract as much around buildings Many places have poor FM reception but clear AM stations

Diffraction Diffraction limits the function of optical microscope – Size of the object = the wavelength of the light: blurred – Size of the object < the wavelength of light: no structure will be seen

Diffraction Electron microscopes are used to illuminate tiny things where the is much less than that of an optical microscope

Interference

When two or more waves happen at the same time, they cross each other and produce an interference pattern Wave effects may be increased, decreased or neutralized within this pattern

Young’s Interference Experiment British physicist, Thomas directed monochromatic light through two closely spaced slits, fringes of brightness and darkness were produced on a screen behind to demonstrated wave nature of light that Huygens had previously suggested

Interference The bright fringes of light resulted from light waves from both slits arriving crest to crest Dark areas resulted from light waves arriving trough to crest

Interference The path difference is the reason of the interference

Interference Color changes, the interference pattern changes

Interference Diffracting grating is produced by a multitude of closely spaced parallel slits Many spectrometers use diffracting gratings rather than prisms to disperse light into colors

Interference Whereas a prism separates colors of light by refraction, a diffracting grating separates colors by interference Diffracting grating can be observed in peacock’s feathers, CD discs etc.

Interference from Thin Films Interference fringes can be produced by the reflection of light from two surfaces that are closely together When monochromatic light is shone onto two plates of glass, one atop the other, dark and bright bands can be observed

Interference from Thin Films Iridescence: interference of light waves of mixed frequencies reflected from the top and bottom of thin films, produces a spectrum of colors

Interference from Thin Films Testing of precision lenses can be a practical use of interference fringes When a lens that is to be tested is placed on a perfectly flat piece of glass light and dark concentric, and regularly spaced fringes are seen

Interference from Thin Films Testing of precision lenses can be a practical use of interference fringes When a lens that is to be tested is placed on a perfectly flat piece of glass light and dark concentric, and regularly spaced fringes are seen

Laser Light Laser (Light Amplification by Stimulated Emission of Radiation)

Laser Light Light emitted by a common lamp is incoherent (many phases of vibration) Interference within a beam of incoherent light is rampant and a beam spreads out after a short distance, becoming wider and less intense

Laser Light A filtered monochromatic beam is still incoherent since the waves are out of phase and interfere with one another. The slightest differences in their directions result with increased distance

Laser Light Laser produces coherent light that has the same frequency, phase and direction. There is no interference of waves within the beam. Only a beam of coherent light will not spread and diffuse

Laser Light A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons

Laser Light Laser is on

Laser Light Within a laser, light wave emitted from one atom stimulates the emission of light from a neighboring atom so that the crests of each wave coincide

Laser Light These waves stimulate the emission of others in cascade fashion and a beam of coherent light is produced

Hologram Hologram is a 3-D version of a photograph that contains the entire message in every portion of its surface. Light diffracted from these fringes produces an image that is extremely realistic

Hologram

Holograms are produced by interference between two laser light beams on photographic film The object scatters the light in all directions including onto the film the film is also illuminated by means of another diverged beam from the same laser in order to produce interference Interference between the reference beam and light reflected from the different points on the object produces a pattern of microscopic fringes on the film Light from nearer parts of the object travels shorter paths than light from farther parts of the object. The different distances traveled will produce slightly different interference patterns with reference beam. In this way, information about the depth of an object is recorded

Hologram When light falls on a hologram, it is diffracted by the fringed pattern to produce wave fronts identical in form to the original wave fronts reflected by the object. The diffracted wave fronts produce the same effect as the original reflected wave fronts

Hologram If holograms are made using short-wavelength light and viewed with light of a longer wavelength, the resulting image is magnified in the same proportion as the wavelengths Holograms made with X-rays would be magnified 1000x when viewed with visible light and appropriate viewing arrangements

The End