csc(theta) = sqr2 = 1/sin(theta)
sin(theta) = 1/sqr2 = sqr2/2 = about 1.414/2 = 0.707
the angle whose sine is sqr2/2 is 45 degrees or pi/4 radians
a right triangle is formed with sides 1, 1 and sqr2. It's a common angle in trigonometry that people generally recognize. Or use a calculator with an inverse trig function. theta = sin^-1(.707) = 45 degrees. If you used 0.707, you'd get 44.99 degrees. You'd need to put in sqr2/2 to get the exact solution of 45 degrees.
the limits on the angle place it in quadrant I, otherwise, theta could have been 135 degrees & 3pi/4
as well as 45 and pi/4. With each angle also plus n360 or 2npi where n=any integer.