Recall. Young s Double Slit Review. Preflight 21.1, 21.2 L 11/4/2010. Multiple Slits (Diffraction Grating N slits with spacing d)

Transcription

1 //00 Physics 6: ecture Diffraction, Gratings, Resolving Poer Textbook sections Recall Interference (at least coherent aves) Constructive (full avelength ifference) Destructive (½ avelength ifference) ight ( source, but ifferent paths) Thin Films Double/multiple slit Diffraction/single slit (toay) Young s Double Slit Revie Multiple Slits (Diffraction Grating N slits ith spacing ) Path length ifference Which conition gives estructive interference? here m = 0, or, or,... ) ) = sin Path length ifference - = sin = Path length ifference - = sin = Path length ifference - = sin = Constructive interference for all paths hen Preflight.,. 9I 0 Three slit interference All rays are interfering constructively at the point shon. If the intensity from ray is I 0, hat is the combine intensity of all rays? ) I 0 ) I 0 ) 9 I 0 % 65 % % When rays an are interfering estructively, is the intensity from the three rays a minimum? ) Yes ) No 5 % 9 % I 0 sin =

2 //00 For many slits, maxima are still at sin = m As no. of slits increases bright fringes become narroer an brighter. slits (N=) Multiple Slit Interference (Diffraction Grating) 0 slits (N=0) Peak location epens on avelength! Diffraction Grating N slitsith spacing * screen VERY far aay intensity 0 sin intensity 0 sin Constructive Interference Maxima are at: sin = m Same as for Young s Double Slit! Single Slit Interference?! Diffraction Rays Wall shao Screen ith opening (or obstacle ithout screen) bright This is not hat is actually seen! Diffraction/ Huygens Every point on a ave front acts as a source of tiny avelets that move forar. st minima ight aves originating at ifferent points ithin opening travel ifferent istances to all, an can interfere! Physics 6: ecture, Slie We ill see maxima an minima on the all.

3 //00 Single Slit Diffraction Single Slit Diffraction W W sin When sin = rays an interfere estructively. sin( ) sin( ) When = rays an ill interfere estructively. Rays an also start W/ apart an have the same path length ifference. Uner this conition, every ray originating in top half of slit interferes estructively ith the corresponing ray originating in bottom half. st minimum a: Rays an also start / apart an have the same path length ifference. Uner this conition, every ray originating in top quarter of slit interferes estructively ith the corresponing ray originating in secon quarter. n minimum at Single Slit Diffraction Summary Conition for halves of slit to estructively interfere sin( ) = Preflights.,.5 A laser is shone at a screen through a very small hole. If you make the hole even smaller, the spot on the screen ill get: Conition for quarters of slit to estructively interfere sin( ) = () arger () Smaller Conition for sixths of slit to estructively interfere sin( ) = Which raing correctly epicts the pattern of light on the screen? All together sin = m (m=,,, ) THIS FORMUA OCATES MINIMA!! Narroer slit => broaer pattern Note: interference only occurs hen > Preflight. () () () () Diffraction from Circular Aperture Diffraction from Circular Aperture st iffraction minimum st iffraction minimum Diameter D Diameter D light light Maxima an minima ill be a series of bright an ark rings on screen Maxima an minima ill be a series of brightan ark rings on screen First iffraction minimum is at: First iffraction minimum is at sin =. D

4 //00 Intensity from Circular Aperture I. D First iffraction minimum Physics 6: ecture, Slie9 sin These objects are just resolve To objects are just resolve hen the maximum of one is at the minimum of the other. Resolving Poer To see to objects istinctly, nee objects» min Pointillism Georges Seurat objects is angle beteen objects an aperture: objects tan objects /y min min is minimum angular separation that aperture can resolve: sin min min =. /D D y Improve resolution by increasing objects or ecreasing min Preflight.6 Astronaut Joe is staning on a istant planet ith binary suns. He ants to see them but knos it s angerous to look straight at them. So he ecies to buil a pinhole camera by poking a hole in a car. ight from both suns shines through the hole onto a secon car. But hen the camera is built, Astronaut Joe can only see one spot on the secon car! To see the to suns clearly, shoul he make the pinhole larger or smaller? To see the to suns clearly, shoul he make the pinhole larger or smaller? larger smaller Ho oes the maximum resolving poer of your eye change hen the brightness of the room is ecrease.. Increases. Remains constant. Decreases 0% 0% 0%

5 //00 Recap. Interference: Coherent aves Full avelength ifference = Constructive ½ avelength ifference = Destructive Multiple Slits Constructive sin() = m (m=,, ) Destructive sin() = (m + /) slit only More slits = brighter max, arker mins Huygens Principle: Each point on ave front acts as coherent source an can interfere. Single Slit: Destructive: sin() = m (m=,, ) Resolution: Max from at Min from 5