theta_i = linspace(0,pi/4);
%f = function
f = cos(2.*theta_i);
%f_approx = tangentile line approximating f
f_approx = pi/2 - 2*theta_i;

%figure
g = figure();
plot(theta_i,f,'Color','black','LineWidth',2)
hold on
plot(theta_i,f_approx,'Color','black','LineStyle','--','LineWidth',2);


xlabel('Twist angle, \phi [rad]','FontSize', 18)
ylabel('Transmittance','FontSize', 18)
%xlabel('Frequency, THz','FontSize', 18)
yticks([0 0.5 1 1.5])
xticks([0 pi/8 pi/4]);
xticklabels({'0','\pi/8','\pi/4'});

ax = gca;
ax.FontSize = 15;
ax.YLabel.FontSize = 20;
ax.XLabel.FontSize = 20;
ax.LineWidth = 1.2;
ax.FontName = 'Arial';

box on



%ax = gca; exportgraphics(g,'SingleApproxFunc.png','Resolution',600)