A Simple Monochromator

A monochromator is device used to split polychromatic radiation into the monochromatic form or we can say it splits radiation into its component wavelengths and separate the desired portion of the spectrum to be studied. Monochromator is also known as a monochromatic illuminator. A simple monochromator is composed from two slits for entrance and exit, and a dispersion element like a prism or a diffraction grating. In both these elements, one takes advantage of the dependence of the refraction angle of the prism or the reflection angle of grating on the radiation wavelength. In the case of prism, the larger the photon energy the smaller is the refraction angle. The main purpose of the entrance slit is to define the geometric properties of the investigated irradiation. The diffraction can only be controlled by the collimated light, that is if all the rays of light are parallel. The goal of the prism is to disperse the light into a rainbow. At the exit slit, the colors of the light are spread out, in the visible this shows the colors of the rainbow. Because each color arrives at a separate point in the exit slit plane, there are a series of images of the entrance slit focused on the plane. Because the entrance slit is finite in width, parts of nearby images overlap. The light leaving the exit slit contains the entire image of the entrance slit of the selected color plus parts of the entrance slit images of nearby colors. The range of colors leaving the exit slit is a function of the width of the slits. The entrance and exit slit widths are adjusted together. It is obvious, therefore, that the monochromator refracting power increases with the decrease of the slit width and with the increase of the distance between the slit and the prism.

Posted in: