Optics
Young's double-slit interference
Test the small-angle relation i = λD/a and identify the bright and dark positions on a screen.
Equipment
- Monochromatic laser with adjustable wavelength
- Double slit with adjustable separation a
- Screen on a measured rail, with a ruler for fringe positions
Procedure
Align the apparatus
In a real setup, place the slit plate perpendicular to the beam, set the screen distance D, and align the central bright fringe with the ruler's zero. Record D and a.
Measure several fringes
Measure the distance from the central maximum to the m-th bright fringe on both sides. Average the two readings and divide by m to reduce reading error.
Vary one parameter at a time
Change λ while keeping a and D fixed, then repeat by changing a and D separately. Compare each measured interfringe with the predicted i = λD/a; note that dark fringes lie halfway between adjacent bright fringes.
Simulation
Experiment history
In 1801, Thomas Young presented evidence that light can behave as a wave, using interference from two narrow openings. The regularly spaced bright and dark bands were difficult to explain with a simple stream of particles and became a landmark demonstration of superposition.
Young's original apparatus was not the modern laser-and-slit plate. Later refinements made the geometry easier to control and the fringes easier to measure. The relation i ≈ λD/a follows when the screen is far from the slits and the angles are small; it provides a practical way to infer a wavelength from measured fringe spacing.