(* Quantity: new in Mathematica 9 *)
cm = Quantity[ 0.01, "Meters"] ;
m = Quantity[ 1, "Meters"] ;
mm = Quantity[ 1/1000, "Meters"] ;
μm = Quantity[ 10^(-6), "Meters"] ;

numerical evaluation of 2010 exam q5a.

z0[ w0_, lambda_] = Pi w0^2/lambda ;
w[ w0_,lambda_, a_] = Sqrt[lambda(a^2 + z0[w0, lambda]^2)/Pi/z0[w0, lambda]] ;
w[ (Subscript[w, 0]),λ, a]  // TraditionalForm


z0[ 0.5  mm,0.8 μm ] 
w[ 0.5  mm,0.8 μm, 0.3 m ]  / mm

q5b

Clear[w0, n, l, d, theta]
divergence[w0_, lambda_, n_, l_, d_] = w0 Sqrt[ 2 n /((Pi^2 w0^4/lambda^2)  + (2 d + l/(2 n))^2)] ;
divergence[(Subscript[w, 0]), (Subscript[λ, 0]), n, L, D] // TraditionalForm 
theta= divergence[0.5  mm,0.8 μm, 1.5 , 5 cm, 10 cm] // N

(Pi theta /180) 60 60 ;
1 cm / (2 theta)
(*2 theta 5.8 m *100*)