Question

Blue light of wavelength $480nm$ is most strongly reflected off a thin film of oil on a glass slab when viewed near normal incidence. Assuming that the index of refraction of the oil is $1.2$ and that of the glass is $1.6$, what is the minimum thickness of the oil film (other than zero)?

• A. $100nm$
• B. $200nm$
• C. $300nm$
• D. None

Answer 1

Answer: (A), (B)

$200nm$
$100nm$

The two reflections that cause interference are from the air oil interface and the oil glass interface. In both cases the electric field reflected is shifted ${180}^o$, so to have a strong reflection they are already in phase. A thickness of zero would give a strong reflection; the next possibility is a thickness such that the extra path covered by the second reflection is one wavelength longer, so:

$2\times thickness={\lambda }_{oil}$

or

$thickness=\frac{{\lambda }_{oil}}{2}=\frac{{\lambda }_{air}}{2n_{oil}}=\frac{480}{2.4}=200nm$