Radiation & Matter 3: Refraction

Transcription

1 Rdition & Mtter 3: Refrction Refrction AIM The effects of refrction (the chnge of direction tht tkes plce when light psses fro ir into glss) y hve been et during n erlier study of Physics. The i of this unit is to derive reltionship between the ngle of incidence nd the ngle of refrction when light psses fro one ediu into nother, nd to consider how the ount of bending depends on the frequency (colour) of the light. The bility to esure nd know the bending power or terils is prticulrly iportnt to lens kers nd opticl instruent designers. OBJECTIVES At the end of this unit you should be ble to: stte tht the rtio to ediu. is constnt when light psses obliquely fro ediu stte tht the bsolute refrctive index n of ediu is the rtio in vcuu (or ir s n pproxition) nd θ is in the ediu. crry out clcultions using this reltionship. where θ is describe the principles of ethod for esuring the bsolute refrctive index of glss for onochrotic light. stte tht the refrctive index depends on the frequency of the incident light. stte tht the frequency of wve is unltered by chnge in ediu. stte nd use the reltionships for refrction of wve fro ediu to ediu : n = = v v = λ λ Strthven Acdey - - Rdition nd Mtter

2 Activity 5 Refrctive index Ai: To find how chnging the incident ngle ffects the refrcted ngle when light enters Perspex block. Apprtus: RyZer light box set to give ry 9V power supply seicirculr Perspex block ruler protrctor Norl B θ θ C A Initil set up: Plce the block on sheet of white pper nd trce its outline. Using the ruler nd protrctor, drw the norl t the idpoint of the stright edge (B). Obtining results: Drw in the line AB, king the ngle θ equl to 0 degrees. Put the block on the pper. Shine ry of light long line AB nd rk the direction of the refrcted ry. Reove the block nd drw in the line BC. Mesure the ngle θ. Repet these steps for other vlues of the incident ngle. Clculte nd for ech row, nd the rtio. Drw grph of vs. θ ( ) θ ( ) Strthven Acdey - - Rdition nd Mtter

3 REFRACTION OF LIGHT Refrctive index By vrying the ngle θ, reltionship between θ nd θ g cn be found. Experient shows tht is constnt. This constnt is clled the reflective index n of the ediu. g g = n The vlues given in dt books re clled bsolute refrctive indices. These re the rtios of the sine of the ngle in vcuu, not ir, to the sine of the ngle in the ediu. However, for ost prcticl purposes we cn use ir. = n θ = ngle in ir esured reltive to norl θ = ngle in ediu esured reltive to norl. The refrctive index esures the effect ediu hs on light. The greter the refrctive index, the greter the chnge in speed nd direction. The refrctive index of ediu is the se whether light oves fro ir into the ediu or vice vers. The bsolute refrctive index is lwys vlue greter thn (or equl to). Exple Using infortion fro the digr, find the refrctive index of the cler plstic. All ngles ust be esured fro the norl. θ = = 60 θ = = 36 n = = sin 60 sin 36 =.47 Strthven Acdey Rdition nd Mtter

4 Refrctive index nd frequency of light The refrctive index of ediu depends upon the frequency (colour) of the incident light. When light enters glss pris, it seprtes into its coponent colours nd produces spectru. This hppens becuse ech colour (frequency) is refrcted by different ount. Since violet is refrcted ore thn red (i.e. it hs chnged speed nd direction by greter ount), it follows tht the refrctive index for violet light ust be greter thn the refrctive index for red light. Refrctive index nd reltionship with v, λ nd θ n = = v v = λ λ In generl, fro ediu to ediu : = v v = λ λ Exple Clculte the speed of light in glss of refrctive index.50. v v = n =.50 =.50 v v v = 0 v 8 s - Strthven Acdey Rdition nd Mtter

5 Refrction of light 8. Clculte the refrctive index n of ech of the terils below: 9. Clculte the issing ngle in ech of the following digrs: 0. The refrctive index of the teril shown in the digr below is.35. ) Clculte the ngle r. b) Find the velocity of the light in the teril.. A ry of light of wvelength psses fro ir to glss s shown below. ) Clculte the refrctive index of the glss. b) Clculte the speed of light in the glss. c) Clculte the wvelength of the light in the glss. d) Clculte the frequency of the light in ir. e) Stte the frequency of the light in the glss. Strthven Acdey Rdition nd Mtter

6 . A ry of light of wvelength 500 n psses fro ir into perspex of refrctive index.50 s shown. ) Clculte the ngle r. b) Clculte the speed of light in the perspex. c) Clculte the wvelength of light in perspex. 3. The refrctive index for red light in crown glss is.53 nd for violet light it is.53. ) Using this infortion, explin why white light cn produce spectru when pssed through crown glss. A ry of white light psses through sei-circulr block of crown glss s shown nd produces spectru. b) Which exit ry is red nd which ry is violet? c) Clculte the refrcted ngle in ir for ech of the exit rys. d) Find ngle x, the ngle between the red nd violet rys. 4. A ry of white light is dispersed by pris producing spectru, S. The ngle x is found to be 0.7. If the refrctive index for red light is.5, clculte the refrctive index for blue light. Strthven Acdey Rdition nd Mtter