A ray of monochromatic light is incident on a thin film of soap water that is suspended in air. The diagram shows the reflection of this ray from the top and bottom surfaces of the film.

On the diagram, label, with the letter P, the point at which a phase difference of \(\pi \) occurs.

White light is incident normally on the soap film. The thickness d of the soap film is 225 nm and its refractive index is 1.34.

(i) Show that the longest wavelength of light λ in air for which the reflected rays destructively interfere is 603 nm.

(ii) Explain why the soap film will appear coloured.

P as shown;

(i) for destructive interference, use of 2μd=nλ to give λ=2μd (ie n=1);
λ=2×1.34×225;
(603nm)
Answer given, look for correct working.

(ii) the reflected light is white minus the wavelengths that suffer destructive interference;
some colours are determined by the missing wavelengths;
some colours are enhanced due to constructive interference; 

There is considerable uncertainty amongst candidates about the surface at which a phase change occurs on reflection.

(b)(i) was an easy two marks for most. However (b)(ii) was poorly answered. More often than not only refraction, diffraction or dispersion were mentioned - even though the G5 question was clearly indicated as being about thin film interference. It is very good practice for candidates to read the stem of the question carefully before starting, and to highlight key phrases or data.