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Czerny-Turner Grating Monochromator

This page provides the interactive animated visuals needed to fully understand how a monochromator works. The learner has the option of changing the grating tilt which changes the output wavelength of the light. The option of changing the angle of the facets (the blaze angle). For maximum efficiency, this angle should be such that the facets are vertical so that the FACET angle of reflection to the second curved mirror is equal to the FACET angle of incidence. First note that the light (or wave) source is at the top left of the animation and is essentially a point source. As such, the wavelets diverge cylindrically away from this point where the radius of these cylinders is the distance away from the source. The task of the top mirror is to convert these spherical wavefronts to planar ones. To do this, the mirror (of radius of curvature, `r_c`) must be located at a distance `r_c/2` away from the point source. The plane waves impinge on the grating which can be tilted to a large range of angles. The reflection (or diffraction) from the grating results in plane waves in many different directions which correspond to the different wavelengths in the point source. For this particular type of monochromator, the bottom concave mirror focuses the reflected planar wavefront that is symmetrical to the top planar wavefronts to a slit or hole that is also symmetrical to the location of the top point source. Basically, the narrower the holes or slits at the top and bottom, the narrower is the range of wavelengths that pass through. This particular monochromator (called Czerny-Turner after its inventors) has the advantage that the tilt angles of the curved mirrors can stay constant over a large range of grating tilt angles and that it is compact because of the folded design. It should be obvious that, in its wavelength reflection ability, the grating approximates a simple (slightly rough) mirror when its front surface is vertical. When its front surface is not vertical, then the difference in optical paths between adjacent facets is

` deltaL=d(sintheta_r-sintheta_i)`

where `d` is the facet spacing, `theta_r` is the angle of reflection with respect to the normal of the grating's front surface median plane and `theta_i` is the incident ray angle with respect to that same normal. Since both of these angles are much less than `pi/2`, we can simplify the path difference to

`deltaL=d(theta_r-theta_i)`

and that too can be simplified to read `deltaL=2d*theta_G` where `theta_G=theta_r-theta_i` is the counter-clockwise measured tilt angle of the grating front surface. The first diffraction order wavelength that is "seen" by the detector is then just this path difference. Second and higher order diffractions will be much weaker than the first because the blaze angle is usually chosen to be close to maximum efficiency for the first order diffraction.

Instead of drawing waves as sinusoids, I have drawn just the line through the phase of maximum intensity which I call the wavefront. I should point out that I have chosen to show just the colors of the waves that would be sensed through a very narrow aperture placed in front of the detector.