SPRAY is used to investigate the shift of the focal plane of a collecting lens with wavelength.
A circular light source (left) creates a parallel beam which is focussed by a collecting lens (right):
Sending some test rays the function of the lens is demonstrated:
In the focal plane of the lens an array detector (length: 3 mm, 30 pixels) is placed which records position-dependent spectra along the symmetry axis of the system:
If the optical constants of BK7 glass are used for the lens the following intensity distribution is observed:
Clearly blue light (400 nm) is focussed closer to the lens than red light (700 nm) which is due to the dispersion of the glass. The refractive index of BK7 has the following spectral dependence:
