Fresnel's Biprism and Fresnel's Mirror can be used to create two Virtual coherent light sources and observe interference phenomenon. Monochromatic light in these experiments is produced by sodium light source or He-Ne laser. The wavelength (λ) can be determined by the formula = d/D
Where, is the fringe width,
D is the distance between virtual source and screen or eyepiece, and d is the distance between two virtual sources.