MeasuringPlancksConstantwithLEDs-MRSEC.pptVIP

Measuring Planck’s Constant with LEDs ? 2008 Stephen Ducharme In 1900, German physicist Max Planck (1858-1947) was trying to model the broad smooth spectrum of electromagnetic radiation (i.e., light) emitted by a warm body. This “black body radiation” is what you see coming from the sun, the filament of an incandescent light bulb, or a hot electric stove element. Its ‘spectrum’, the range of frequencies making up the radiation, is readily displayed by a prism or a diffraction grating. In explaining the shape of the black body spectrum, Planck assumed that the electromagnetic radiation came not in continuous waves of energy, but in discrete clumps of energy which we now call photons. Planck postulated the ‘photons’, at each frequency have a discrete energy E = hf, where E is the energy of the photon in Joules, f is the frequency in Hertz, and h is Planck’s constant. Schematic and picture of LED circuit STAR spectrophotometer. Slit Scale Adjustment Viewing Port Adjusting your STAR spectrophotometer Look through the viewing port with the slit pointing at a fluorescent ceiling light. See If the green spectrum line is at 546 nm on the scale. If needed, use scale adjustment to line up correctly. green spectrum line at 546 nm from the presence of Mercury vapor in the fluorescent bulb. Measuring The LEDs wavelength Adjust the current by turning the potentiometer so that the LEDs glow brightly. look at each LED with the Spectrophotometer and measure the emission wavelength λ (in nanometers) of each. Each person must record all data values on their own data sheet. Please ask for help if you are having problems or are not sure you are doing things properly. For more information on LEDs go to the web site: If the reading is not a definite line, read the middle and record this on your data chart. 620 nm LED Color Wavelength λ Voltage V Energy E = eV Frequency F = c/λ Planck’s Constant (h) Red 620 nm Orange Yellow Green Blue UV Voltage me