[ahhppull]

 Calculate the dispersion at Φ = 19° of the first order diffraction for a grating with 1.60 × 10^3 grooves per centimeter. Give the answer in °/μm.

I am getting this question wrong.

Using the calculator in radians mode
 ΔΦ /Δλ = n/dcosΦ = 1/[(1.6x10^3)cos(19*pi/180)] = 0.000661

0.000661*100 = 0.0661 rad
0.0661*180/pi = 3.78 deg