theta1=[1:0.5:90]';    %input half angle in degrees
theta1=theta1/180*pi;

n1=1.000;%2761;    %refractive index of air
n2=1.513;%;9;    %refractive index of optic (N-BK7)
n3=1.335;%936625;    %refractive index of seawater: 671 nm, 1500 m, 10 C, 30 ppt

theta2=asin(sin(theta1)*n1/n2);
theta3=asin(sin(theta2)*n2/n3);

figure(1)
plot(theta1/pi*180*2,theta3/pi*180*2)
grid on
xlabel('Input half angle (deg)')
ylabel('Output half angle (deg)')

init=theta1/pi*180;
final=theta3/pi*180;

interp1(init,final,22.5)*2
interp1(final,init,22.5)*2

0.5*tan(45/2/180*pi)*2