| Date | May 2015 | Marks available | 3 | Reference code | 15M.3.HL.TZ1.13 |
| Level | Higher level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Outline | Question number | 13 | Adapted from | N/A |
Question
This question is about thin-film interference.
A thin film of oil lies on a puddle of water. White light from above shines on the film at normal incidence.
Outline the process by which coloured fringes are formed.
The following data are available:
Refractive index of oil = 1.4
Refractive index of water = 1.3
Thickness of the oil film = 250 nm
Calculate the maximum wavelength of the incident light for which destructive interference occurs.
Markscheme
light reflects from the top surface of the oil and the top surface of the water;
mention of interference/superposition;
path difference exists between both reflected rays;
different wavelengths interfere constructively for different positions/angles (hence colours appear/shift);
\(\lambda \left( { = t\frac{{2n}}{m}} \right) = 250 \times {10^{ - 9}}\frac{{2 \times 1.4}}{1}\);
\(\lambda = 700\left( {{\rm{nm}}} \right)\);
