© 2012 Pearson Education, Inc.

{{Chapter 36Chapter 36

Diffraction (cont.)Diffraction (cont.)

© 2012 Pearson Education, Inc.

The diffraction grating The diffraction grating

• A A diffraction gratingdiffraction grating is an is an array of a large number of slits array of a large number of slits having the same width and having the same width and equal spacing. (See Figure equal spacing. (See Figure 36.16 at the right.)36.16 at the right.)

© 2012 Pearson Education, Inc.

Grating spectrographsGrating spectrographs

• A diffraction grating can be used to disperse light into a spectrum.A diffraction grating can be used to disperse light into a spectrum.• The greater the number of slits, the better the resolution.The greater the number of slits, the better the resolution.• Figure 36.18(a) below shows our sun in visible light, and in (b) Figure 36.18(a) below shows our sun in visible light, and in (b)

dispersed into a spectrum by a diffraction grating.dispersed into a spectrum by a diffraction grating.

© 2012 Pearson Education, Inc.

Consider two diffraction gratings. One grating has 100 lines/mm, and the other one has 200 lines/mm. Both gratings are illuminated with a beam of the same monochromatic light. Which grating produces the greater dispersion?

Q36.5

A. The dispersion cannot be determined without additional information

B. The grating with 100 lines/mm produces the greater dispersion.

C. Both gratings produce the same dispersion.

D. The grating with 200 lines/mm produces the greater dispersion.

© 2012 Pearson Education, Inc.

Consider two diffraction gratings. One grating has 100 lines/mm, and the other one has 200 lines/mm. Both gratings are illuminated with a beam of the same monochromatic light. Which grating produces the greater dispersion?

A36.5

A. The dispersion cannot be determined without additional information

B. The grating with 100 lines/mm produces the greater dispersion.

C. Both gratings produce the same dispersion.

D. The grating with 200 lines/mm produces the greater dispersion.

© 2012 Pearson Education, Inc.

X-ray diffractionX-ray diffraction• When x rays pass through a crystal, the crystal behaves like When x rays pass through a crystal, the crystal behaves like

a diffraction grating, causing a diffraction grating, causing x-ray diffractionx-ray diffraction. Figure 36.20 . Figure 36.20 below illustrates this phenomenon.below illustrates this phenomenon.

© 2012 Pearson Education, Inc.

A simple model of x-ray diffractionA simple model of x-ray diffraction

• Constructive interference is essentially the reason the Constructive interference is essentially the reason the angle of reflection equals the angle of incidence.angle of reflection equals the angle of incidence.

• The The Bragg conditionBragg condition for constructive interference is for constructive interference is 22dd sin sin = m = m..

© 2012 Pearson Education, Inc.

Circular aperturesCircular apertures

• An aperture of An aperture of anyany shape forms a diffraction pattern. shape forms a diffraction pattern.• Figures 36.25 and 36.26 below illustrate diffraction by a circular Figures 36.25 and 36.26 below illustrate diffraction by a circular

aperture. The aperture. The airy diskairy disk is the central bright spot. is the central bright spot.• The first dark ring occurs at an angle given by sinThe first dark ring occurs at an angle given by sin11 = 1.22 = 1.22 /D/D..

© 2012 Pearson Education, Inc.

Diffraction and image formationDiffraction and image formation

• Diffraction limits the Diffraction limits the resolution of optical resolution of optical equipment, such as equipment, such as telescopes. telescopes.

• The larger the aperture, the The larger the aperture, the better the resolution. Figure better the resolution. Figure 36.27 (right) illustrates this 36.27 (right) illustrates this effect. effect.

© 2012 Pearson Education, Inc.

Bigger telescope, better resolutionBigger telescope, better resolution

• Because of diffraction, large-diameter telescopes, such as Because of diffraction, large-diameter telescopes, such as the VLA radio telescope below, give sharper images than the VLA radio telescope below, give sharper images than small ones.small ones.

© 2012 Pearson Education, Inc.

You use a telescope lens to form an image of two closely spaced, distant stars. Which of the following will increase the resolving power?

Q36.6

A. Use a filter so that only the blue light from the stars enters the lens.

B. Use a filter so that only the red light from the stars enters the lens.

C. Use a lens of smaller diameter.

D. more than one of the above

© 2012 Pearson Education, Inc.

You use a telescope lens to form an image of two closely spaced, distant stars. Which of the following will increase the resolving power?

A36.6

A. Use a filter so that only the blue light from the stars enters the lens.

B. Use a filter so that only the red light from the stars enters the lens.

C. Use a lens of smaller diameter.

D. more than one of the above

© 2012 Pearson Education, Inc.

What is holography?What is holography?

• By using a beam splitter and mirrors, coherent laser light By using a beam splitter and mirrors, coherent laser light illuminates an object from different perspectives. illuminates an object from different perspectives. Interference effects provide the depth that makes a three-Interference effects provide the depth that makes a three-dimensional image from two-dimensional views. Figure dimensional image from two-dimensional views. Figure 36.28 below illustrates this process.36.28 below illustrates this process.

© 2012 Pearson Education, Inc.

How does holography How does holography work?work?• Initially an interference pattern is created in film, when light Initially an interference pattern is created in film, when light

shines from behind on that film, it recreates the original image.shines from behind on that film, it recreates the original image.

© 2012 Pearson Education, Inc.

An example of holographyAn example of holography

• Figure 36.32 below shows photographs of a holographic Figure 36.32 below shows photographs of a holographic image from two different angles, showing the changing image from two different angles, showing the changing perspective.perspective.