A parallel beam of light is incident on upper part of a prism of angle 1.8(degree) and R.I. = 3/2.

the light comming out of the prism falls on the concave mirror of radius of curvature 20 cm. the distance of the point ( where the rays are focused after reflection from the mirror) form the principle axis is:

1. 9 cm

2. 1.57 mm

3. 3014 mm

please ans. with full explanation

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/180
now 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`

assuming the angle is small( which is the case in present scenario) we can write
if the convergence point is situated at x in focul plane
(x/f)=angle the rays make with principal axis
x=f*d
x=(R/2)*d
x=(20/2)*0.9*pi/180
x=0.15707 cm
x=1.57 mm (rounded off)
hence the second option is the naswer