|To determine the refractive index of the
glass ball (D=70 mm), the effective focal length f from
the center and the back focal length fb were measured (f =
The supplier of the sphere specifies the material as borosilicate glass. Searching on the internet:
The rainbow angle δ of the photo can be calculated, reducing the measured distance L and diameter D of the rainbow by means of the applet:
δ = arctan [(D/2-y)/(L-x)]
Result (5 experiments):
The refractive index n is used to compute the point (x|y) which is necessary to reduce the measured distances L and D.
Pernter, in his book, mentions a rainbow angle of 22° 51' = 22.85° for glass.
The applet may be used to calculate the refractive index of the glass from the rainbow angle measured by my experiments:
The result is nglass = 1.510 ± 0.003, instead of 1.52 by the focal length.
Understanding Ball Lenses (Edmund Optics)
|Pernter, J. M., Exner, F. M.:
Meteorologische Optik; Salzwasser Verlag, Paderborn, 2012;
Nachdruck der 2. Auflage 1922.
Pernter, J. M., Exner, F. M.: Meteorologische Optik; Wilhelm Braumüller, Wien und Leipzig, 2. Aufl. 1922.
2016 J. Giesen
updated: 2016, Apr 28