Lab: Interference, Diffraction, and Planck's Constant

A wavelength of visible light is around 500 nanometers, far too small to see directly. But when light passes through two narrow slits, it diffracts and the two waves overlap on a distant screen, interfering constructively in some places and destructively in others. That overlap pattern is huge compared to the wavelength itself, stretching into millimeters you can measure with a ruler. The bright fringes sit at spacing dy = lambda·L/d, where L is the slit-to-screen distance and d is the slit separation. Because dy is easy to measure and L and d are set by the equipment, you solve for lambda: lambda = dy·d/L. A tiny laser wavelength becomes a readable number on a wall.

The second half of this lab trades photons for LEDs. An LED lights up only once each electron crossing it carries enough energy to produce a photon of that color: qV = hf, where V is the turn-on voltage and f is the light's frequency. Measure turn-on voltage for LEDs of several colors, plot V against f, and the slope is h/q. Multiply by the electron charge q and you recover Planck's constant, the same number buried in every photon energy calculation. Both halves of this lab turn an abstract constant, a wavelength or Planck's constant itself, into a slope or spacing you read off a graph or a ruler.

This lab sits among the course's graph-linearizing exercises: instead of calculating one answer from one measurement, you take several data points, plot a line, and let the slope or intercept carry the physics. Swapping the double slit for a diffraction grating sharpens the fringes into narrow, bright, widely spaced maxima because many more slits reinforce the same constructive-interference condition and cancel out everywhere else.