Question

1. Why do the colours of a soap bubble floating in the air or the soap film on a child's plastic hoop change continually? 2. Explain why the headlights of a car are not able to form an interference pattern.

Answer 1

(1)Soap bubble is consists of a thin layer of water film trapped between layer of soap molecules.When light rays falls on it then a part of light is reflected by the outer layer of film while some part is transmitted inside the film and reflected from the inner surface of film.These reflected rays from outer and inner surface of film suprempose with each other and produces interference pattern.

If rays meet with each other in same phase(i.e. phase difference is multiple of $2 \pi$ radian) then interference is constructive while if these two rays meet in opposite phase (i.e. phase difference are $\pi,3 \pi,5 \pi$ etc) then interference is destructive.

Since white light is made up of different colors having different wavelengths.Since white light falls on the bubble from different different angles so path difference between the rays reflected from outer and inner surface of film will be also different.So for some angle,the condition of constructive interference satisfied for some type of wavelength of light.While for other angles the same condition satisfied for other wavelength of light.

Thats why we see the colourful soap bubble.As the bubble floating in the air then angle of light falling on it changes as a result condition for constructive and destructive interference also changes so we see that colour of bubble changing.Also with time thickness of soap bubble decreases so path difference between rays also changes and we see that colour of bubble changes..

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(2)To produce interference pattern we require monochromatic source of light that emits only single frequency light.Headlight of a car emits polychromatic light which contains a large number of wavelengths.Also we require two coherent sources to produce interference which is not possible with headlightsof car.The light rays by the two headlight changes their phase at any point so quickly that after their superposition it is impossible to see a stationary pattern of interference.Because at every point condition of constructive and destructive interference occurs so quickly that we could not judge them.