1 / 18

180 likes | 567 Vues

Chapter 24 Wave Optics. Diffraction Grating Interference by Thin Films Polarization. Q. d. Q. Extra distance m l. sin Q = m l /d or dsin Q = m l. m =0,1,2,3, . . . Constructive inference m =1/2,3/2,5/2, . . . Destructive inference.

Télécharger la présentation
## Chapter 24 Wave Optics

**An Image/Link below is provided (as is) to download presentation**
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.
Content is provided to you AS IS for your information and personal use only.
Download presentation by click this link.
While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

**Chapter 24Wave Optics**Diffraction Grating Interference by Thin Films Polarization**Q**d Q Extra distance ml sinQ=ml/d or dsinQ=ml m=0,1,2,3, . . . Constructive inference m=1/2,3/2,5/2, . . . Destructive inference**Example: In a double-slit experiment it is found that blue**light of wavelength 460 nm gives a second-order maximum at a certain location on the screen. What wavelength of visible light would have a minimum at the same location? Answer:For constructive interference d sinQ=ml=2x460nm=920nm For destructive interference of the other light, we have d sinQ=(m’+1/2)l When the two angle are equal, then 920nm=(m’+1/2)l l=1.84x103 nm for m’=0 l=613 nm for m’=1 l=368 nm for m’=2 The only wavelength here that is visible is 613 nm**When a light wave travels from one medium to another, its**frequency does not change, but its wavelength does: l2/l1=v2t/v1t=v2/v1=n1/n2 (v=c/n) n1/n2 = l2/l1= sin(r)/sin(i) The shorter l, the larger refraction angle**1 mm**0.589 mm 1 m Example: Monochromatic yellow light illuminates two narrow slits 1 mm apart the screen is 1 m from the slits, and the distance from the central bright light to the next bright line is found to be 0.589 mm. Find the wave length of the light. Solution:d sinQ=ml, where m=1or l=d sinQ/1= (1x10-3m)(5.89x10-4m)/1m= 5.89x10-7 m =589 nm**There are two difficulties in using a double slit for**measuring wavelengths. 1. The bright lines on the screen are actually extremely faint and an intense light source is therefore required; 2. The lines are relatively broad and it is hard to locate their center accurately.**Diffraction Grating**A diffraction grating that consists of a large number of parallel slits overcomes both of these difficulties. A diffraction grating uses interference to disperse light. It is often an important component in optical instrumentation for wavelength determinations.**dsinQ=ml**m=0,1,2,3, . . . Constructive inference m=1/2,3/2,5/2, . . . Destructive inference**For a diffraction grating, the intensity falls away from**these maxima much more rapidly than that for a double slit. Because there are so many slits to act as sources, any angle other than those for maxima will be dark or nearly dark.**Example:Visible light includes wavelengths from 4x10-7 m to**7x10-7m. Find the angular width of the first-order spectrum produced by a grating ruled with 800 lines/cm. Solution: The slit space d that corresponding to 800 line/cm is d=(10-2 m/cm)/(8x103 lines/cm)=1.25x10-6 m Since m=1, sinQb=lb/d = 4x10-7m/1.25x10-6m = 0.32, Qb=19o sinQr=lr/d = 7x10-7m/1.25x10-6m = 0.56, Qr=34o The total width of the spectrum is therefore 34o-19o=15o The angle can be measured to very high accuracy, so the wavelength of a line can be determined to high accuracy using l=d sinQ /m**Question: A characteristic property of the spectra produced**by a diffraction grating is • the sharpness of the bright lines • diffuseness of the bright lines • absence of bright lines • absence of dark lines Answer: a**Question: The greater the number of lines that are ruled on**a grating of given width, • The shorter the wavelengths that can be diffracted • The longer the wavelengths that can be diffracted • The narrower the spectrum that is produced • The broader the spectrum that is produced Answer: d**Spectrometer and Spectroscopy**using a grating or prism**A prism also disperses light**n1/n2 = l2/l1= sin(r)/sin(i)**Question: White light strikes (a) a diffraction grating, and**(b) a prism. A rainbow appears on a screen just below the direction of horizontal incident beam in each case. What is the color of the top of the rainbow in each case? Answer: (a) Violet for diffraction grating (ml=dsinQ) (b) Red for prism (n1/n2 = l2/l1)**Single-slit Diffraction**Light from all parts of the slit travels the same distance and arrives "in phase" so there is a bright central maximum.**A Single-slit Diffraction Intensity**A single slit diffraction pattern has a bright central maximum surrounded by much smaller maxima.

More Related