Mathematical and Physical Journal
for High Schools
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# Problem P. 4172. (May 2009)

P. 4172. With appropriate washing-up liquids the surface tension of water can be significantly decreased. If a frame is immersed into a mixture of water and washing-up liquid, and then it is carefully taken out, there will be a thin layer of soap film on the frame. Coloured stripes can be observed on the soap film (if it is in front of a black background.)

a) What is the direction of the stripes if the plane of the frame is vertical? Why do they have this direction?

b) What is the colour of the film if its width is only 10-8 m?

(4 pont)

Deadline expired on June 11, 2009.

### Statistics:

 17 students sent a solution. 4 points: Benyó Krisztián, Emroz Khan, Hartstein Máté, Kardos Örs, Kovács 235 Gábor, Kovács Benjámin, Krämer Zsolt, Lovas Lia Izabella, Sass Zoltán, Varju 105 Tamás. 3 points: Kovács Attila. 2 points: 4 students. Unfair, not evaluated: 2 solutions.

Problems in Physics of KöMaL, May 2009