 Lloyd 's mirror generates interference fringes by combining direct light from a source ( blue lines ) and light from the source 's reflected image ( red lines ) from a mirror held at grazing incidence . The result is an asymmetrical pattern of fringes . Interestingly , the band of equal path length , nearest the mirror , is dark rather than bright . In 1834 , Humphrey Lloyd interpreted this effect as proof that the phase of a front @-@ surface reflected beam is inverted . 
