Interference in a Vertical Soap Film Bubble-blowing solution for kids consists o

Question

Interference in a Vertical Soap Film Bubble-blowing solution for kids consists of soapy water sold in a plastic jar. The jar usually includes a bubble-blowing wand such as the one shown at the right. Suppose that the wand is dipped in the soapy solution and then held vertically, as shown in the figure, and then one looks at a white source in reflection. Various wavelengths will be reflected strongly or weakly, depending on the thickness of the soap film. Since gravity pulls down on the soap film, it is thinnest at the top and thickest at the bottom. For this problem, assume that violet light has lambda = 400 nm and red light has lambda = 700 nm. The index of refraction of the soapy water is 1.33. Colored bands will be seen across the film, as shown in the figure. If a particular band appears red, it is because red light is reflected strongly, and similarly for the other colors. Over time, the film becomes very thin at the top, and then eventually it pops. Just before the film breaks it looks black at the top, where it is very thin. Why does it look black? (This question does not require extensive calculation.) Below the black stripe, where the film is a bit thicker, one will see colored stripes, such as red or violet. Which color appears first below the black stripe - red or violet? Why? Calculate the thickness of the film at the position of the first red stripe. Calculate the thickness of the film at the position of the first violet stripe.

Explanation / Answer

(a) since the soap film is soooo thin it doesnt have room for the wavelength to hit and refract so it just appears black since the light isnt bouncing off.It appears to reflect no light at all. (b) for finding the answer of this part, first we will do c and d parts

(c) So we solve for d = m*(lambda)/2n. Obviously, the minimum thickness is for m = 1 so
d = 700e-9/2.66 = 2.631 x 10^-7 m = 263 nm

(d) again, d = m*(lambda)/2n ,where m =1 d = 400e-9/2.66 = 1.503 x 10^-7 m = 150 nm

Now it is very clear that just below the top of the film, thickess of the film will be less coparision to the lower part of the film.

thinest at top and thickest at bottom.

so for part b....answer is one will see red colored stripes

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