干涉和衍射

Interference occurs when two waves of the same kind overlap to produce a resultant wave with greater, lower, or equal amplitude. Diffraction is the bending of a wave around obstacles or through apertures, where different parts of the wave interfere to create spatial alternation of large and small amplitudes. Both phenomena demonstrate the wave nature of light.

When light passes through a narrow slit, different parts of the wavefront diffract and overlap. Constructive interference occurs where wave amplitudes add together, creating bright fringes, while destructive interference occurs where amplitudes cancel, producing dark regions. The center fringe is always bright, with intensity decreasing toward peripheral fringes along the slit's axis.

Young's double-slit interference pattern results from diffracted light from both slits interfering with each other. The interference fringes are much narrower than single-slit bright regions because the inter-slit separation controls fringe width, while slit width controls diffraction fringe width. The reciprocal of inter-slit separation determines the spacing of interference fringes.

Laser diffraction spectroscopy uses diffraction patterns created when a laser beam passes through objects ranging from nanometers to millimeters in size. A sensor detects the angling of diffracted laser light, and a computer analyzes the light energy and its spatial distribution to quickly determine the particle's geometrical dimensions without direct contact.

Interferometry uses superposition and interference of waves with the same frequency but different path lengths to make precise measurements of distances, displacements, refractive index changes, and surface irregularities. The resulting interference pattern reveals unknown parameters. LIGO detectors use this technique to detect gravitational waves by measuring tiny path length differences.

Slits are created by cutting straight lines in aluminum foil using razor blades. For single-slit experiments, one slit approximately 1 centimeter long is cut in the foil's center. For double-slit experiments, two closely spaced parallel slits are cut using a stacked razor blade technique. The foil is then mounted on cardboard with a centered hole and positioned perpendicular to a laser beam.

The observation of diffraction and interference patterns of light played essential roles in establishing that light is an electromagnetic wave. These characteristic wave phenomena demonstrated that light behaves like other waves, such as sound waves and water waves, fundamentally changing our understanding of light's nature and enabling modern optical and photonic technologies.