The angle in radians is used when calculating the distance between the virtual sources of Fresnel's biprism due to the small angle approximation, where sin(α) closely approximates α in radians. The formula for separation is d = 2(μ – 1) sin(α) y1, which simplifies to d = 2(μ – 1) α y1 for small angles. An example calculation shows that 2 degrees converts to approximately 0.0349 radians, aligning with the small angle approximation. A correction was noted in the calculations, emphasizing the need to use π when converting degrees to radians. Accurate calculations are crucial for precise results in optical experiments involving Fresnel's biprism.