We begin by noting that the angle of incidence is the one that's taken with respect to the normal to the surface in question. In this case the angle of incidence is 30. The material is Flint Glass according to the original question. The refractive indez of air n1=1, the refractive index of red in flint glass is nred=1.57, finally for violet in the glass medium is nviolet=1.60. Snell's Law dictates:
[tex]n_1sin(\theta_1)=n_2sin(\theta_2)[/tex]
Where [tex]\theta_2[/tex] differs for each wavelenght, that means violet and red will have different refractive indices in the glass.
In the second figure provided details are given on which are the angles in question, [tex]\Delta x [/tex] is the distance between both rays.
[tex]\theta_{2red}=Asin(\frac{sin(30)}{1.57})\approx 18.5705[/tex]
[tex]\theta_{2violet}=Asin(\frac{sin(30)}{1.60})\approx 18.21[/tex]
At what distance d from the incidence normal will the beams land at the bottom?
For violet we have:
[tex]d_{violet}=h.tan(\theta_{2violet})\approx 0.0132m[/tex]
For red we have:
[tex]d_{red}=h.tan(\theta_{2red})\approx 0.0134m[/tex]
We finally have:
[tex]\Delta x=d_{red}-d_{violet}\approx2.8\times10^{-4}m[/tex]
