chapter 36. longer wavelength, larger diffraction

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DIFFRACTION (PHYSICAL OPTICS) Chapter 36

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Page 1: Chapter 36. Longer wavelength, larger diffraction

DIFFRACTION (PHYSICAL OPTICS)Chapter 36

Page 2: Chapter 36. Longer wavelength, larger diffraction

SINGLE SLIT DIFFRACTION

Page 3: Chapter 36. Longer wavelength, larger diffraction

WAVELENGTH DEPENDENCE

Longer wavelength, larger diffraction

Page 4: Chapter 36. Longer wavelength, larger diffraction

NEAR-FIELD AND FAR-FIELDFresnel diffraction (near-field):Source, obstacle, screen are all close to each other.Fraunhofer diffraction (far-field):Source, obstacle, screen are far from each other. Light rays can be considered parallel to a good approximation. This is the case we will study.

Page 5: Chapter 36. Longer wavelength, larger diffraction

INTENSITY OF DIFFRACTION

Page 6: Chapter 36. Longer wavelength, larger diffraction

CHARACTERISTIC OF DIFFRACTION

Central peak twice as wide

Page 7: Chapter 36. Longer wavelength, larger diffraction

ANGULAR WIDTH

Page 8: Chapter 36. Longer wavelength, larger diffraction

MINIMA

Page 9: Chapter 36. Longer wavelength, larger diffraction

FIND THE DARK FRINGE

Page 10: Chapter 36. Longer wavelength, larger diffraction

FIND THE SLIT WIDTH

Page 11: Chapter 36. Longer wavelength, larger diffraction

DIFFRACTION THROUGH A CIRCULAR APERTURE

Page 12: Chapter 36. Longer wavelength, larger diffraction

RESOLVABILITY FOR DIFFERENT α

Cannot be resolved Can be resolved

α

Page 13: Chapter 36. Longer wavelength, larger diffraction

LARGE ANGULAR SEPARATION

α

Can resolve the two stars easily

Page 14: Chapter 36. Longer wavelength, larger diffraction

SMALL ANGULAR SEPARATION

α

Cannot resolve the two stars

Page 15: Chapter 36. Longer wavelength, larger diffraction

RAYLEIGH’S CRITERION

α

Just able to resolve the two stars when the maximum falls directly on the first minimum

Page 16: Chapter 36. Longer wavelength, larger diffraction

RESOLVABILITY (RESOLVING POWER)

Page 17: Chapter 36. Longer wavelength, larger diffraction

DIFFERENT APERTURE

Page 18: Chapter 36. Longer wavelength, larger diffraction

EXAMPLE

θR

Page 19: Chapter 36. Longer wavelength, larger diffraction

DIFFRACTION GRATING (MULTIPLE SLITS)

Page 20: Chapter 36. Longer wavelength, larger diffraction

DIFFRACTION GRATINGS

Peaks much narrower than a double slit. N is the total number of slits.

Page 21: Chapter 36. Longer wavelength, larger diffraction

DIFFERENT NUMBER OF SLITS

The width of the peaks decreases as N increases.

Page 22: Chapter 36. Longer wavelength, larger diffraction

DIFFERENT COLORS (WAVELENGTH) DIFFRACT DIFFERENTLY

Page 23: Chapter 36. Longer wavelength, larger diffraction

EXAMPLE: WHITE LIGHT ON A GRATINGFind the angular spread of the first order bright fringe when white light falls on a diffraction grating with 600 slits per millimeter. The wavelengths of the visible spectrum are approximately 400nm (violet) to 700nm (red).