Wave Optics

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Slide 12

Thin-Film Interference, Case 1

Waves reflected by a thin film undergo a phase change

The number of extra cycles traveled by the ray inside the film completely determines the nature of the interference

If the number of extra cycles, N, is an integer, there is constructive interference

If the number of extra cycles is a half-integer, there is destructive interference

Only if: nair < nfilm < n(substance below the film)

Slide 13

Thin-Film Interference, Case 2

Assume the soap bubble is surrounded by air

There is a phase change at the top of the bubble

There is no phase change at the bottom of the bubble

Since only one wave undergoes a phase change, the interference conditions are

Slide 14

Example 25 .3 Color and Thickness of a Soap Film

Consider a bubble formed from a thin, soapy film that looks blue when viewed at normal incidence. Estimate the thickness of the film. Assume its index of refraction is nfilm=1.35 and blue light has a wavelength λblue=400 nm. Also assume the film is so thin that thinner films are not able to give constructive interference.

Solution:

The interference here is destructive and we consider m=0:

Slide 15

Light through a Single Slit: Qualitative Behavior

Light passes through a slit or opening and then illuminates a screen

As the width of the slit becomes closer to the wavelength of the light, the intensity pattern on the screen and additional maxima become noticeable

Slide 16

Single-Slit Diffraction

Water wave example of single-slit diffraction

All types of waves undergo single-slit diffraction

Diffraction is the bending or spreading of a wave when it passes through an opening

It is useful to draw the wave fronts and rays for the incident and diffracting waves

Huygen’s Principle can be stated as all points on a wave front can be thought of as new sources of spherical waves

Slide 17

Double-Slit Interference: Young’s Experiment of Light from a Single Slit

Double-Slit Interference

Light passes through two very narrow slits

When the two slits are both very narrow, each slit acts as a simple point source of new waves

The outgoing waves from each slit are like simple spherical waves

The double slit experiment showed conclusively that light is a wave

Experiment was first carried out by Thomas Young around 1800

Slide 18