Wave Nature of Light

To explain why oil looks white at the edges or why soap bubbles go black just before they break.

When light hits an interface (i.e. air to water) some of the light is transmitted or absorbed (pavement) and a portion of the ray reflects back through your pupil. The portion that reflects back is always inverted at the first interface because the index of refraction, n, is greater (n water and oil >n air).

For a soap bubble the portion of the light that is reflected back from the soap to air interface does not invert because it is going from a higher to a lower n. If the film is very thin, say less than 1/20 the wavelength of light, then the bottom ray hasn't traveled a significant amount. So the inverted ray is completely out of phase with the non inverted lower ray, crests meet troughs and destructive interference of all light wavelengths occurs. (oil n=1.4 on water n=1.33 will have the same result)

Similarly, for oil on glass, the portion of the light that is reflected back from the oil to glass interface also inverts because it is also going from a lower to a higher n. If the film is very thin, then the bottom ray hasn't traveled a significant amount. So the inverted rays are completely in phase, crests meet crests and constructive interference of all light wavelengths occurs.

/50 Wave nature of Light ISU test Outline 12U (non AP) Name ________________________

449 5 458 3 (wavelength = wavelength in air/n, need this for 10.1 thin films)

Read 9.3 Ripple tanks 468 1-3 469 4 Plus ripple tank Handout /12

Read 9.4 Double slit interference 476 3 5-10 482 1-3 484 1-9 497 65 68-76 /30

Read 10.2 Single slit 516 1-3 519 1-8 /4

Read 10.3 Diffraction gratings 523 1-3 525 4-7 /4

Bonus 10.2 Thin films 507 1-4 511 1-4 (answer to 1 is 550nm) /8 (transferable to other test marks if total over 50.

I recommend you do the MP assignment and the Physics Quizzes for extra practice