For a N-BK7 singlet below (a) Use the equations (assume it is a thin lens) to explain why the spherical aberration is reduced to V/4 when the singlet is split to two singlets with equal power (b) This lens has coma as identified from the off-axis rays. How can you reduce coma to zero? (c) It is obvious that this lens has field curvature, how can you reduce the field curvature? (d) How would you choose another glass for a cemented doublet to reduce the axial chromatic aberration