You can think of the hair as being cylindrical. In your imagination, scale the hair-sized cylinder up to, say, a half inch diameter. Illuminate the cylinder with a beam as wide as the cylinder, at the 20 degree angle of your drawing. Now consider all the directions the light will be reflected into at different points of incidence on the cylinder. You will see that the reflected rays will be confined to a conical range of angles corresponding to the curve you've drawn.