For those interested in a mathematical take on a natural occurrence.
This was originally a paper written for a math modeling class.
Everyone knows the pot of gold at the end of the rainbow is a myth, don’t they? Though this may be accepted as fact, it is quite impossible to prove. First, a rainbow does not occupy a point or region in space; instead it ‘floats’ along a path that is based on the position of the observer relative to the Sun. This means that, though two people may both see a rainbow at the same time in approximately the same place, there will be a slight difference in the observed position of the rainbow. Second, the ‘end’ of the rainbow is merely the point(s) at which it disappears behind the horizon. In reality, the refraction of light that is observed as a rainbow is a circle, the visible arc of which varies with the elevation of the Sun.
A rainbow is the manifestation of the refraction of sunlight by water droplets in the atmosphere. The angle of the projected prism is approximately 42 degrees to the angle of sunlight, with the angle decreasing toward the red end of the spectrum. This variation in the angle of refraction for the different parts of the spectrum is what splits the white light into its component colors and causes the ‘ROYGBIV’ arrangement of the rainbow.
Despite the tenuous nature of the rainbow, it still may be tempting to quantify or measure it in some way. One way would be to measure its height or width based on incline or azimuth from the observer’s point of view. On deeper reflection, this proves impossible. Not only is there no way to measure the distance to the plane of the rainbow in order to calculate from the other two measurements, it turns out that these angular measurements are purely a factor of the elevation of the Sun, as long as the elevation of the observer relative to the landscape does not change. In essence, the rainbow can be thought of as a cone projection in reverse; the sun strikes water vapor and is refracted at 42 degrees. If the observer is thought of as the point of this cone, the base is the circular rainbow projecting back the sunlight from behind the observer. The ‘bow’ shape is merely the part of the base of the cone that is above the horizon; the higher the sun in the sky, the less the arc length visible. As the distance from the observer to the plane of the rainbow increases, the diameter of the rainbow grows, but the angles remain the same, it is similar to taking slices perpendicular to the cone at different heights. If one were to look deeper and try to use the thickness of the rainbow to determine distance, the same property arises. Think now of this cone as two concentric cones, one of 40 degrees from the main axis to the sides, the other at 42. As planar slices are taken, the region between the surfaces of the cones is the span from red to violet; again this width or thickness increases with distance, but the angular measurement from the observer’s point of view is unchanged. Effectively, there is no means by which to use the observation of a rainbow to measure either its size or its distance. On the other hand, since the angles involved are known if either size or distance is given, the other can be calculated. If one, for the sake of superstition, argued that the pot of gold existed at the ‘apparent’ base of the rainbow, this point could be found from angular measurements, or from the known location of the Sun, and the distance to the horizon.
Say that the horizon is 2.5 miles away and the sun is at 20 degrees above the horizon behind the observer. Since the radius of the rainbow is the opposite side of a right triangle with angle 42 degrees and adjacent side 2.5 miles, determining the total size of the rainbow is simple trigonometry: Tan (42 degrees)*2.5miles=2.25 miles. Since only 42-20=22 degrees of elevation exist between horizon and top arc of the rainbow, we have a visible height that is 2.25 miles-tan (20)*2.5=1.34 miles, which relates to a ‘y-intercept’ of .91 miles. Now, with a circle of radius 2.25 and a y-intercept, the x coordinates are needed. Since a circle is y^2+x^2=r^2, we have .91^2+x^2=5.0625, x=2.058 miles. So our distance along the z-axis is 2.5 miles, in the x-axis 2.058 miles and our straight line distance to the pot of gold is 3.24 miles at a heading of +/-42 degrees. As long as the observer leaves his eyes behind, he can see himself reach the end of the rainbow.