Plot won't show up in Mathematica

[Victini11]

Homework Statement

My plot is not working, for some reason when I try to plot it it show nothing. I am not really used to using mathematica so maybe it's an syntax error but can really get what is wrong

Relevant Equations

Plot3D [(1.5/
1)*(2/1 +
n2)^2*((2*n1)/(n1 +
1.5))^2/(1 + ((n1 - 1.5)/(n1 + 1.5))^2 ((n1 + 1)/(1.5 +
1))^2 -
2*((n1 - 1.5)/(n1 + 1.5)*((n1 + 1)/(1.5 + 1)))*
Cos*(4*Pi*d*n1)), {n1, 1, 1.5}, {d, 100, 1000}]

 

[DrClaude]

Plot3D [(1.5/
1)*(2/1 +
n2)^2*((2*n1)/(n1 +
1.5))^2/(1 + ((n1 - 1.5)/(n1 + 1.5))^2 ((n1 + 1)/(1.5 +
1))^2 -
2*((n1 - 1.5)/(n1 + 1.5)*((n1 + 1)/(1.5 + 1)))*
Cos*(4*Pi*d*n1)), {n1, 1, 1.5}, {d, 100, 1000}]

The is a n2 in there and the cosine should be

Cos[4*Pi*d*n1]