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Light as a Wave : Slit Diffraction


Diffraction is a physical phenomenon associated with the propagation of the waves, the effects of which are relevant when a wave encounters an obstacle or a slit whose dimensions are comparable or lower than its wavelength.

According to the principle of Huygens – Fresnel, after the light has passed through a slit, each point of the slit acts as if it were itself a source of circular waves and these waves interact with each other by means of the interference phenomenon. Diffraction is actually the interference between the different parts of the wave that occurs after the wave itself has encountered an obstacle .

In the image below it is presented the mathematical calculation of the diffraction pattern on the basis of the Huygens- Fresnel principle :


Experimental Setup

For the detection of the diffraction pattern a normal webcam has been used, from the webcam the IR filter and the focusing lens have been removed. On the CMOS sensor of the webcam it was projected the diffraction pattern produced by a red laser of 650nm which passes through a slit of manually adjustable width. The red laser is attenuated by a pair of polarizing filters. The images from the webcam have been acquired with the software “Theremino Spectrometer”.

Laser, Webcam, Filters, Adjustable Slit

Diffraction Patterns acquired with Webcam

Wide Slit


Diffraction with”wide” slit

Medium Slit


Diffraction with “medium” slit

Narrow Slit


Diffraction with “narrow” slit

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