The Nature of Light. Chapter 22. Geometric Optics Using a Ray Approximation. Ray Approximation

Transcription

1 The Nature of Light Chapter Reflectio ad Refractio of Light Sectios: 5, 8 Problems: 6, 7, 4, 30, 34, 38 Particles of light are called photos Each photo has a particular eergy E = h ƒ h is Plack s costat h = 6.63 x 0-34 J s Ecompasses both atures of light Iteracts like a particle Has a give frequecy like a wave Geometric Optics Usig a Ray Approximatio Light travels i a straight-lie path i a homogeeous medium util it ecouters a boudary betwee two differet media The ray approximatio is used to represet beams of light A ray of light is a imagiary lie draw alog the directio of travel of the light beams Ray Approximatio A wave frot is a surface passig through poits of a wave that have the same phase ad amplitude The rays, correspodig to the directio of the wave motio, are perpedicular to the wave frots

2 Reflectio of Light A ray of light, the icidet ray, travels i a medium Whe it ecouters a boudary with a secod medium, part of the icidet ray is reflected back ito the first medium This meas it is directed backward ito the first medium Specular Reflectio Specular reflectio is reflectio from a smooth surface The reflected rays are parallel to each other All reflectio i this text is assumed to be specular Diffuse reflectio is reflectio from a rough surface The reflected rays travel i a variety of directios Diffuse reflectio makes the road easy to see at ight Diffuse Reflectio Law of Reflectio The ormal is a lie perpedicular to the surface It is at the poit where the icidet ray strikes the surface The icidet ray makes a agle of θ with the ormal The reflected ray makes a agle of θ with the ormal The agle of reflectio is equal to the agle of icidece θ = θ

3 Refractio of Light Whe a ray of light travelig through a trasparet medium ecouters a boudary leadig ito aother trasparet medium, part of the ray is reflected ad part of the ray eters the secod medium The ray that eters the secod medium is bet at the boudary This bedig of the ray is called refractio Refractio of Light, cot The icidet ray, the reflected ray, the refracted ray, ad the ormal all lie o the same plae The agle of refractio, θ, depeds o the properties of the medium Followig the Reflected ad Refracted Rays Ray is the icidet ray Ray is the reflected ray Ray 3 is refracted ito the lucite Ray 4 is iterally reflected i the lucite Ray 5 is refracted as it eters the air from the lucite More About Refractio The agle of refractio depeds upo the material ad the agle of icidece siθ v = = costat siθ v The path of the light through the refractig surface is reversible

4 Refractio Details, Light may refract ito a material where its speed is lower The agle of refractio is less tha the agle of icidece The ray beds toward the ormal Refractio Details, Light may refract ito a material where its speed is higher The agle of refractio is greater tha the agle of icidece The ray beds away from the ormal The Idex of Refractio Whe light passes from oe medium to aother, it is refracted because the speed of light is differet i the two media The idex of refractio,, of a medium ca be defied speed of light i a vacuum = = speed of light i a medium For a vacuum, = For other media, > is a uitless ratio c v Frequecy Betwee Media As light travels from oe medium to aother, its frequecy does ot chage Both the wave speed ad the wavelegth do chage The wavefrots do ot pile up, or are created or destroyed at the boudary, so ƒ must stay the same

5 Idex of Refractio Exteded The frequecy stays the same as the wave travels from oe medium to the other v = ƒ λ The ratio of the idices of refractio of the two media ca be expressed as various ratios Sell s Law of Refractio si θ = si θ θ is the agle of icidece 30.0 i this diagram λ λ = v v = c c = θ is the agle of refractio QUICK QUIZ.3 QUICK QUIZ.3 ANSWER A material has a idex of refractio that icreases cotiuously from top to bottom. Of the three paths show i the figure below, which path will a light ray follow as it passes through the material? (b). Whe light goes from oe material ito oe havig a higher idex of refractio, it refracts toward the ormal lie of the boudary betwee the two materials. If, as the light travels through the ew material, the idex of refractio cotiues to icrease, the light ray will refract more ad more toward the ormal lie.

6 QUICK QUIZ.4 QUICK QUIZ.4 ANSWER As light travels from vacuum ( = ) to a medium such as glass ( > ), which of the followig properties remais the same: (a) wavelegth, (b) wave speed, or (c) frequecy? (c). Both the wave speed ad the wavelegth decrease as the idex of refractio icreases. The frequecy is uchaged. Dispersio The idex of refractio i aythig except a vacuum depeds o the wavelegth of the light This depedece of o λ is called dispersio Sell s Law idicates that the agle of refractio whe light eters a material depeds o the wavelegth of the light Variatio of Idex of Refractio with Wavelegth The idex of refractio for a material usually decreases with icreasig wavelegth Violet light refracts more tha red light whe passig from air ito a material

7 Refractio i a Prism The amout the ray is bet away from its origial directio is called the agle of deviatio, δ Sice all the colors have differet agles of deviatio, they will spread out ito a spectrum Violet deviates the most Red deviates the least Prism Spectrometer A prism spectrometer uses a prism to cause the wavelegths to separate The istrumet is commoly used to study wavelegths emitted by a light source Total Iteral Reflectio Total iteral reflectio ca occur whe light attempts to move from a medium with a high idex of refractio to oe with a lower idex of refractio Ray 5 shows iteral reflectio A particular agle of icidece will result i a agle of refractio of 90 This agle of icidece is called the critical agle si θ = > Critical Agle for

8 Critical Agle, cot For agles of icidece greater tha the critical agle, the beam is etirely reflected at the boudary This ray obeys the Law of Reflectio at the boudary Total iteral reflectio occurs oly whe light attempts to move from a medium of higher idex of refractio to a medium of lower idex of refractio A applicatio of iteral reflectio Plastic or glass rods are used to pipe light from oe place to aother Applicatios iclude medical use of fiber optic cables for diagosis ad correctio of medical problems Telecommuicatios Fiber Optics