Spectrum Analyzer

Challenge: To design a spectroscope to analyze the spectrum emitted by an LED light source.

Parameters: Your device will be designed around a diffraction grating and photoresistor provided by Mr. J. You will need to design it so that it can measure the angle to the first maximum in the interference pattern produced by the diffraction grating, so that you can calculate the wavelength of the light. The photoresistor will allow you to measure the relative intensity of the light at each wavelength. You need to build the circuit that powers the LEDs and another one that measures the current in the photoresistor.

Scoring: Your score will be determined by your ability to reproduce the spectrum graph of a "mystery" LED. The graph parameters will be provided by Mr. J.

Physics: Parallel light hitting a diffraction grating perpendicularly will produce an interference pattern unique to that light source. The photoresistor will decrease resistance in proportion to the intensity of the light striking it. The angle at which you place the photoresistor is related to the wavelength of the light at that part of the interference pattern. This will allow you to produce a relative intensity vs. wavelength spectrum graph.

Extensions: The angles of the interference pattern are dependent on the microscopic distance between the thousands of tiny ridges in the diffraction grating. If you changed to a different diffraction grating, you would have to recalibrate your device! You can use some simple geometry and an understanding of wave interference to determine the wavelength of unknown light using a known diffraction grating, and vice versa.

Help/Hints: Your device needs a convex lens to ensure diverging light is not hitting the diffraction grating. Ensure that the photoresistor sensor is firmly supported so that there is no variation during use. Also, your angle markings and system of angle measurement must be reliable and repeatable.

Online Text: 13 (Light as a Wave), 15 (Electromagnetic Spectrum), 16 (Lenses), 17 (Interference), 21 (Quantum Theory of Light)