© 2010 pearson education, inc. lecture outline chapter 24 college physics, 7 th edition wilson /...

© 2010 Pearson Education, Inc.

Lecture Outline

Chapter 24

College Physics, 7th Edition

Wilson / Buffa / Lou

Chapter 24Physical Optics: The Wave

Nature of Light


24.1 Young’s Double-Slit Experiment

• Light can behave as particles or as waves…we call this….

• In 1801, Thomas Young used interference to demonstrate the wave nature of light.

• Called the Young’s Double Slit Experiment

• 2 types of interference…they are...they do…

• Coherent vs. incoherent sources


Passing a beam of light through a pair of slits produces two beams that are in phase, and can interfere with each other.

The diagram to the left represents Young’s double slit experiment.



Whether the interference is constructive or destructive depends on the path length difference from the slits to the screen.



To calculate the path length the waves have traveled:



The condition for the location of an interference maximum is then (for constructive interference)

Here, n is called the order of the maximum.

The lateral distance between maxima and central maximum for small angles is,



• In a lab experiment, monochromatic light passes through 2 narrow slits that are 0.05 mm apart. The interference pattern is observed on a white wall 1.0 m from the slits, and the second order maximum is at an angle of 1.5 degrees. – a) If the slit separation decreases, the second order maximum

will be seen at an angle of greater than 1.5, at 1.5, or less than 1.5

– b) What is the wavelength of light and what is the distance between second order and third order maxima?

– c) If d = 0.040 mm, what is the angle for the second order?

24.2 Thin-Film Interference

If light reflects from a medium of higher index of refraction, there is a 180° phase shift. If it reflects from a medium of lower index of refraction, there is no phase shift.

Look at the pictures….you’ve seen this before!



In determining whether the interference will be constructive or destructive, we must look at the path length and the angle of incidence.

If the waves are out of phase - destructive interference

If the waves are in phase – constructive interference

Indices of Refraction determine in/out of phase


24.2 Thin-Film InterferenceThe colors of an oil spill come from interference of waves reflecting from the surface and from the oil–water interface.



Thin-film interference is also useful for determining the flatness of a reflecting surface.

If the surface is not flat, the bands will not be straight and even.

These are called Optical Flats.



A similar technique can be used to check lenses. If the lens is not shaped properly, the bands will be distorted.

Newton’s Rings


24.3 Diffraction• We like to think that light moves in straight

line paths (rays). If this were true, we wouldn’t be able to see interference.

• But WE DO!!! So…this means light must deviate from straight line paths!

• As waves pass through slits, they spread out. This spreading of light is called diffraction.

24.3 Diffraction

• Figure 24.12 in Book– When the width of the opening is much larger

than the wavelength of the wave there will be little diffraction.

– When the wavelength and width are around the same size (as the width of the slit becomes smaller), there will be noticeable diffraction.

24.3 DiffractionDiffraction patterns are created when waves encounter obstacles or openings, or are very close to the edges of objects.


24.3 DiffractionAgain, path length differences from different parts of the object or opening lead to interference.

The condition for diffraction minima is:

The diffraction pattern will be wider for longer wavelengths and narrower slits.

The central maximum is twice as wide as the side maxima; its width is:


24.3 Diffraction

• When you drive through a city or mountainous areas, the quality of your radio reception varies sharply from place to place, with stations seeming to fade out and reappear. Could diffraction be a cause of this phenomenon? Which of the following frequency bands would you expect to be least affected: – Weather (162 MHz)

– FM (88 – 108 MHz)

– AM (525 – 1610 kHz)

24.3 Diffraction

• Monochromatic light passes through a slit whose width is 0.05 mm.

• a) The resulting diffraction pattern is generally wider for longer wavelengths, wider for shorter wavelengths, or the same width for all wavelengths?

• b) At what angle will the third order minimum be seen and what is the width of the central maximum on a screen located 1.0 m from the slit, for a wavelength of 400 nm and 550 nm.

24.3 Diffraction

A diffraction grating is a series of small slits with uniform spacing. The maxima become quite narrow as the number of slits increase. The minima become wider.


24.3 DiffractionDiffraction gratings combine multiple-slit interference with single-slit diffraction, yielding the observed pattern.


24.3 Diffraction

• Two parameters define a diffraction grating:– The slit separation (d) [This is also called the

grating constant]– The slit width

• Let’s look back at slide #20

24.3 Diffraction

The maxima of a diffraction grating are the same as for a double slit:

If the light falling on a diffraction grating is not monochromatic, a spectrum will be seen, as different wavelengths will have peaks at different places. A familiar example of a (reflection) diffraction grating is a CD.

The grating constant be found with…


24.3 Diffraction

• A particular diffraction grating produces an n = 2 spectral order at an angle of 32 degrees for light with a wavelength of 500 nm. – a) How many lines per centimeter does the

grating have?– b) At what angle can the n = 3 spectral order be

seen?– c) What is the highest order maximum that can be

observed?

24.4 Polarization

Polarization refers to the orientation of the electric and magnetic fields in an electromagnetic wave.

This only happens in transverse waves.


24.4 Polarization

In unpolarized light, the electric fields have random directions.

In partially polarized light, the electric fields are clustered around a preferred orientation.

In fully polarized light, the electric fields are all in the same direction.


© 2010 pearson education, inc. lecture outline chapter 24 college physics, 7 th edition wilson /...

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