Please answer this prism question ( geometrical optics)

Dear Manish

 

assuming the angle is small( which is the case in present scenario) we can writeif the convergence point is situated at x in focul plane(x/f)=angle the rays make with principal axisx=f*dx=(R/2)*dx=(20/2)*0.9*pi/180x=0.15707 cmx=1.57 mm (rounded off)the parallel beam will be deviated by the small angle prism by d=(u-1)A = (1.5-1)*1.8=0.9 degrees= 0.9*pi/180now this beam will also be parallel and will be incident on the concave mirror at an small angle to the principal axis and after reflection will be focused at a point in the focul plane FF`