Question

White light is incident normally on a thin film which has n = 1.5 and a thickness of 5000 Å. For what wavelengths in the visible spectrum (4000 – 7000 Å) will the intensity of the reflected light be a maximum?

• A. 5000 Å, 6000 Å
• B. 5260 Å, 6000 Å
• C. 4285 Å, 6000 Å
• D. 4500 Å, 6500 Å

Answer 1

Answer: (C)

4285 Å, 6000 Å

Answer 2

Constructive interference occurs for

OPD = 2nt = kl +$\frac{\lambda}{2}$,

where t is the thickness of the film, k an integer. Hence for maximum reflection the wavelengths are

l =$\frac{4nt}{2k + 1}$=$\left\{ \begin{aligned} & 6000Åfork = 2 \\ & 4285Åfork = 3. \end{aligned} \right.\$