can some one help me with this problem Suppose you have a fairly large plastic r

Question

can some one help me with this problem

Suppose you have a fairly large plastic ring that you've dipped into some bubble solution, so that it supports a film of the soapy liquid. The index of refraction is about the same as that of water, 1.33. If you hold it vertically for a while, gravity competes with surface tension and the film will acquire a thickness gradient, being thin at the top and steadily becoming thicker as it goes down. Wien illuminated with white light from the front side (the side you're looking at it from), colored bauds appear, as you can see in this image at right, which you can also view Very close to the top of the ring, this film is very thin - its thickness is much less than the wavelength of visible light. Explain why that produces destructive interference in the reflected light, making it look black. Your explanation should state whether there is or is not a phase change for each relevant reflection. What is the approximate thickness of the film in the middle of the top bright band, shown by the upper arrow in the figure? The colors are not very much separated in that band, so use an average wavelength of visible light, 550 nm in air, in your calculation. (Hint: remember that the wavelength inside the film is what matters, and that is not equal to the wavelength in air.) What is the approximate thickness of the film at the third red band, shown by the lower arrow in the figure? Thin films are useful on optical components to either discourage or enhance reflections, depending on the application. (Below, assume that the light is at normal incidence, or at least close to it.)

Explanation / Answer

Constructive and destructive interference of light waves is also the reason why thin films, such as soap bubbles, show colorful patterns. This is known as thin-film interference, because it is the interference of light waves reflecting off the top surface of a film with the waves reflecting from the bottom surface. To obtain a nice colored pattern, the thickness of the film has to be on the order of the wavelength of light.

Consider the case of a thin film of oil floating on water. Thin-film interference can take place if these two light waves interfere constructively:

An important consideration in determining whether these waves interfere constructively or destructively is the fact that whenever light reflects off a surface of higher index of refraction, a 180

Related Questions

Navigate

• Browse (All)
• Browse C
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.