physics
posted by rudi .
A very narrow beam of white light is incident at 40.40° onto the top surface of a rectangular block of flint glass 11.8 cm thick. The indices of refraction of the glass for red and violet light are 1.637 and 1.671, respectively.
Calculate the dispersion angle (i.e., the difference between the directions of red and violet light within the glass block)

This is a Snell's law problem, of course.
For the red light:
1.637 sin Ar = 1.000 sin 40.4
sin Ar = 0.39592
Ar = 23.32 degrees
For the blue light:
1.671 sin Ab = 1.000 sin 40.4
Solve for angle Ab (the blue ray refraction angle) and then compute the dispersion angle Ab  Ar