Solving a problem in physics section 42 point 61 optics

61. Determine the smallest thickness of the crystal plate a quarter wavelength for λ = 530 nm, if the difference in the refractive indices of the ordinary and extraordinary rays for a given wavelength ne- n0 = 0,01. A quarter wave plate called crystal plate cut parallel to the optical axis, through which passes in a direction perpendicular to the optical axis, the ordinary and extraordinary beams without changing their direction, become the path difference equal to λ / 4.