Chapter 35 - Refraction

Transcription

1 Chapter 35 - Refractio

2 Objectives: After completig this module, you should be able to: Defie ad apply the cocept of the idex of refractio ad discuss its effect o the velocity ad wavelegth of light. Apply Sell s law to the solutio of problems ivolvig the refractio of light. Determie the chages i velocity ad/or wavelegth of light after refractio. Defie ad apply the cocepts of total iteral reflectio ad the critical agle of icidece.

3 Refractio Refractio is the bedig of light as it passes from oe medium ito aother. Note: the agle of icidece q A i air ad the agle of refractio q A i water are each measured with the ormal N. q A q w N Water refractio The icidet ad refracted rays lie i the same plae ad are reversible.

4 Refractio Distorts Visio Water Water The eye, believig that light travels i straight lies, sees objects closer to the surface due to refractio. Such distortios are commo.

5 The Idex of Refractio The idex of refractio for a material is the ratio of the velocity of light i a vacuum (3 x 10 8 m/s) to the velocity through the material. Idex of refractio c v c c v v Examples: = 1; glass = 1.5; Water = 1.33

6 Example 1. Light travels from air ( = 1) ito glass, where its velocity reduces to oly 2 x 10 8 m/s. What is the idex of refractio for glass? v air = c Glass v G = 2 x 10 8 m/s c v 8 3 x 10 m/s 8 2 x 10 m/s For glass: = 1.50 If the medium were water: W = The you should show that the velocity i water would be reduced from c to 2.26 x 10 8 m/s.

7 Aalogy for Refractio 3 x 10 8 m/s Pavemet Glass 2 x 10 8 m/s Sad v s < v p 3 x 10 8 m/s Light beds ito glass the returs alog origial path much as a rollig axle would whe ecouterig a strip of mud.

8 A Simplified Form of the Law Sice the idices of refractio for may commo substaces are usually available, Sell s law is ofte writte i the followig maer: siq siq v v siq siq The product of the idex of refractio ad the sie of the agle is the same i the refracted medium as for the icidet medium.

9 Example 3. Light travels through a block of glass, the remerges ito air. Fid agle of emergece for give iformatio. First fid q G iside glass: 50 0 Glass q G q G =1.5 q siq siq A A G G A si q A (1.0)si 50 siqg 1.50 G q G = q e = 50 0 Apply to each iterface: A siqa G siqg A siqa Same as etrace agle! 0

10 The May Forms of Sell s Law: Refractio is affected by the idex of refractio, the velocity, ad the wavelegth. I geeral: Sell s Law: siq siq v v All the ratios are equal. It is helpful to recogize that oly the idex differs i the ratio order.

11 Example 4: A helium eo laser emits a beam of wavelegth 632 m i air ( A = 1). What is the wavelegth iside a slab of glass ( G = 1.5)? Glass q G G = 1.5; A = 632 m A G AA ; G G A G q q G q =1.5 G (1.0)(632 m) m Note that the light, if see iside the glass, would be blue. Of course it still appears red because it returs to air before strikig the eye.

12 Dispersio by a Prism Red Orage Yellow Gree Blue Idigo Violet Dispersio is the separatio of white light ito its various spectral compoets. The colors are refracted at differet agles due to the differet idexes of refractio.

13 Total Iteral Reflectio Whe light passes at a agle from a medium of higher idex to oe of lower idex, the emergig ray beds away from the ormal. light q c 90 0 Critical agle i = r Water Whe the agle reaches a certai maximum, it will be reflected iterally. The critical agle q c is the limitig agle of icidece i a deser medium that results i a agle of refractio equal to 90 0.

14 Example 5. Fid the critical agle of icidece from water to air. For critical agle, q A = 90 0 A = 1.0; W = 1.33 siq siq W C A A A siqc 0 si 90 (1)(1) w 1.33 Critical agle: q c = Critical agle q c 90 0 Water I geeral, for media where 1 > 2 we fid that: siq C 1 2

15 Summary Medium c = 3 x 10 8 m/s v Idex of refractio c v Refractio is affected by the idex of refractio, the velocity, ad the wavelegth. I geeral: Sell s Law: siq siq v v

16 Summary (Cot.) The critical agle q c is the limitig agle of icidece i a deser medium that results i a agle of refractio equal to Critical agle 2 q c > 2 I geeral, for media where 1 > 2 we fid that: siq C 1 2