laser gradient experiment

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Laser & diffraction grating.

August 29, 2017 English Posts , Light 76,814 Views

With the new Laser He-Ne (described in the Laser He-Ne post), you can easily test the physical properties of the diffraction grating . We propose, in particular, to measure the pitch of the grating through the measurement of the diffraction produced on the He-Ne laser beam.

The Diffraction Grating

When a collimated beam of light passes through an aperture, or if it encounters an obstacle, it spreads out and the resulting pattern contains bright and dark regions. This effect is called diffraction , and it is characteristic of all wave phenomena. It can be understood by considering the interference between different parts of the wavefront, which was altered in passing through the aperture. The angle of of diffraction is of order  λ / d with λ  the wavelength and d the dimension of the aperture. Thus, for visible light, apertures in the range 10-100 μm produce easily resolved diffraction patterns. The diffraction phenomena has been treated in the post  Light as a Wave : Slit Diffraction .

If instead of a single slit, two slits are illuminated by a plane wavefront, a series of interference fringes parallel to the slits will appear on a far screen, as shown in the image below.

This is the classical experiment of Thomas Young (1800). If the spacing between the slits is d and the width of the slits  b  is greater than the wavelength, the Fraunhofer diffraction equation gives the intensity of the diffracted light as:

Where the sinc function is defined as sinc( x ) = sin( x )/( x ) for  x  ≠ 0, and sinc(0) = 1. The sinc function includes the effects of diffraction due to the width of the slits.

The intensity of the principal maxima can be calculated and it decreases as the diffraction order is increased, as shown in the image below.

Experimental Setup and Gratings Measurements

The experimental setup is very simple and consists in pointing the beam laser emitted from the He-Ne source on the diffraction grating. The beam undergoes diffraction and produces on the screen behind the grating the diffraction pattern with the first and second order maxima. Measuring the distance between the grating and the screen and measuring the position of the maxima is immediate to obtain the angles  θ m  and from these we can calculate the grating pitch, using the equation previously described and knowing that λ  is 632.8 nm. In the image below you can see the laser, the diffraction grating and the screen on which you can see the luminous spots corresponding to the diffraction maxima.

From the measurements made with the Paton – Hawksley grating on the first and second order diffraction maxima we obtained the following data:

First Order – θ1  = 0.402 rad – d = λ / sin( θ1 )   = 1.62 μm which corresponds to a pitch of  617 l/mm Second Order – θ2  = 0.873 rad – d = 2λ / sin( θ2 )   = 1.65 μm which corresponds to a pitch of  605 l/mm

With the holographic grating for the first order we obtained the following data:

First Order – θ1  = 0.675 rad – d = λ / sin( θ1 )   = 0.99 μm which corresponds to a pitch of 1012 l/mm

As you can see, the results obtained fit quite well with nominal grating data .

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Tags diffraction grating Laser He-Ne

Detection of beta and alfa radiation with KC761B

Abstract: in this article, we continue the presentation of the new KC761B device. In previous posts, we described the device in general terms and its functionality as a gamma spectrometer. In this post, we describe its use as a beta and alpha radiation detector. To detect beta and alpha particles, the device uses a PIN-type semiconductor sensor positioned on the back of the device.

Description This is a simulation of a typical laser diffraction lab set up. Examine the set up in the 3D window, it shows a laser, a diffraction grating, and a screen. Use the checkbox to place the grating in front of the laser, and look at the pattern of dots that appear on the screen. Use the sliders to change the distance from the grating to the screen, the number of lines per millimeter in the diffraction grating, and the wavelength of the laser. Use the diagram of the screen in the top window to analyze the numerical positions of the dots on the screen using diffraction equations.