Computes wavelength of incident light

gridcalc.c

/*
 * Computes wavelength of incident light
 * Formulas used:
 * Arctangent - atan(c / a) - thanks Cyparagon
 * Diffraction grating formula - nλ = d sin(ϴ)
 * Pythagorean theorem - a² + b² = c²
 * The Lightning Stalker 2013
 */

#include <math.h>
#include <stdio.h>
#include <stdlib.h>

int main (int argc, char **argv)
{
    float a, c, d, wl;

    if (argc == 4)
    {
        a = atof(argv[1]);
        c = atof(argv[2]);
        d = atof(argv[3]);
        wl = d * sin(atan(c / a));
        printf("Wavelength is %1.1fnm\n", wl);
    }
    else
    {
        puts("gridcalc is a diffraction grating wavelength calculator.\n");
        puts("Usage: gridcalc a c d");
        puts("a = Distance from grating to target in mm");
        puts("c = Distance from center to first order spectrum line of interest in mm");
        puts("d = Distance between diffraction grid elements in nm");
        puts("Example: gridcalc 100 88 1000");
        puts("Output should be 660.6nm");
        return(1);
    }

    return(0);
}