(a) When illuminated from above at

(a) When illuminated from above at A, a light ray reflected from the air-oil interface undergoes aphase shiftof φ=π. A ray reflected at the oil-water interface undergoes no phase shift. If theoil thickness at A is negligible compared to the wavelength of the light, then there is nosignificant shift in phase due to a path distance traveled by a ray in the oil. Thus the lightreflected from the two surfaces will destructively interfere for all visible wavelengths, and theoil will appear black when viewed from above.b) From the discussion in part (a), the ray reflected from the air-oil interface undergoes a phaseshiftof φ, =π. A ray that reflects from the oil-water interface has no phase change due to