Light and Optics Propagation of light Electromagnetic waves (light) in vacuum and matter Reflection and refraction of light Huygens principle

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1 Light nd Optics Propgtion of light Electromgnetic wves (light) in vcuum nd mtter Reflection nd refrction of light Huygens principle Polristion of light Geometric optics Plne nd curved mirrors Thin lenses Interference Double slits Diffrction Single slit Double slits Grtings Luke Wilson Room E17)

2 Light nd Optics Propgtion of light Electromgnetic wves (light) in vcuum nd mtter Reflection nd refrction of light Huygens principle Polristion of light Geometric optics Plne nd curved mirrors Thin lenses Interference Double slits Diffrction Single slit Double slits Luke Wilson Room E17)

3 Wves or prticles? both! The nture of light Light propgtion use wves Light bsorption/ emission use prticles (photons) We will not try to nswer the question Wht is photon? * Light is one form of electromgnetic (EM) rdition Accelerting chrges emit EM rdition. All bodies emit EM rdition due to therml motion we re surrounded by it! Light sources incndescent lmp, fluorescent lmp, LED, lser *see pdf document on web pge for interesting red

4 Electromgnetic Rdition Visible light is n exmple of ELECTROMAGNETIC RADIATION:

5 Converting between energy nd wvelength Visible light pprox. wvelength rnge λ nm To convert to energy use E hc λ 140 ( ev ) λ So, λ 400nm, E 3.10 ev λ 700nm, E 1.77 ev Remember, longer wvelengths give lower energies

6 Electromgnetic Wves Existence predicted by Jmes Clerk Mxwell (1865) Consist of crossed time-vrying electric nd mgnetic fields Trnsverse wve, both electric nd mgnetic fields oscillte in direction perpendiculr to propgtion direction No medium is necessry: Electromgnetic wves cn propgte through vcuum Constnt speed of propgtion through vcuum: c 3 x 10 8 ms -1

7 Electromgnetic Wves

8 Electromgnetic Wves It cn be shown from MAXWELL S EQUATIONS of Electromgnetism (See second yer course) tht the electric nd mgnetic fields obey the wve equtions: 0 0 t E x E y y ε µ 0 0 t B x B z z ε µ ), ( 1 ), ( t t x y x t x y v stndrd liner wve eqution ε µ c

9 Electromgnetic Wves E y B z E sin( kx ωt 0 B sin( kx ωt 0 ) ) Where E 0 nd B 0 re relted by: E 0 cb 0 INTENSITY of n EM wve E 0

10 Polristion Light from ordinry sources (e.g. light bulbs) is usully unpolrised We cn mke linerly polrised light by e.g. pssing unpolrised light through filter, commonly Polroid We define the direction of polristion to be the direction of the electric field vector E, E x, t je0 cos kx ωt e.g. ( ) ( ) is polrised in the y-direction

11 Speed of light in mteril Constnt speed of propgtion through vcuum: c 3 x 10 8 ms -1 But, when trvelling through mteril, light slows down v c n n is the refrctive index of the mteril. (NB refrctive index depends on the wvelength of the light)

12 Dispersion The speed of light in vcuum is the sme for ll wvelengths In mtter this is not the cse, with both v (λ) nd n (λ) - this is known s dispersion

14 Wves nd rys Wvefronts At ny instnt, ll points on wve front re t the sme prt of their cyclicl vrition Rys An imginry line long the direction of trvel of wve Geometric optics ry description (e.g. lenses) Physicl optics wve description (e.g. interference)

Reflection nd refrction When light wve strikes smooth* interfce between two trnsprent mterils, the wve is prtly reflected nd prtly refrcted (trnsmitted).

15 Reflection nd refrction When light wve strikes smooth* interfce between two trnsprent mterils, the wve is prtly reflected nd prtly refrcted (trnsmitted). More light reflected for lrger refrctive index contrst, e.g. 4% for ir/glss (n1.5), 17% for ir/dimond (n.4) from Fresnel equtions (you do not need to know these) * We only consider speculr reflection

16 Lws of reflection nd refrction 1. Incident, reflected nd refrcted rys nd norml to the surfce re ll in the sme plne.. Angle of reflection θ r is equl to the ngle of incidence. θ r θ 3. The rtio of the sines of the ngles θ nd θ b (mesured from the norml to the surfce) is equl to the inverse rtio of the two indices of refrction, OR n n b b (Snell s Lw)

17 Refrction Angles mesured from the norml

18 Refrction - exmple A flt-sided, glss fish tnk is filled with wter. A light ry in ir strikes the glss t 35 ngle to the surfce norml. Wht is the ngle of the light ry when ()It enters the glss? (b)it enters the wter? If the light ry entered the surfce of the wter with the sme ngle, wht is the refrcted ngle? [n ir 1.00, n glss 1.5, n wter 1.33] 35 ( 1.00) n θ n ( 1.5) sin. ( 1.33) θ n g ( 1.00) sin35 ( 1.33) θ n g w w sin g n 5.5 n g w w 5.5 g w w g w w

19 Angle of devition Minimum ngle of devition δ when light psses through prism symmetriclly At ech prism fce devition is α α δ n n b b n b sin A θ A + α so sin A + α sin A + α nsin A sin A + δ nsin A

20 Refrctive index effects on wve chrcteristics Strt with the eqution v fλ Frequency f DOES NOT chnge when pssing from one mteril to nother the boundry surfce does not crete or destroy wves. Wves slow down in higher refrctive index mterils i.e. v reduces Therefore, the wvelength λ of light lso reduces f c v n So, nd gives λ0 λ v c λ λ 0 n

21 Summry Light is n exmple of electromgnetic (EM) rdition n importnt one! EM rdition consists of crossed time-vrying electric nd mgnetic fields Light slows down nd the wvelength reduces when it enters mteril. The frequency does not chnge. Divide by n (the refrctive index) to clculte the new speed/ wvelength. Don t forget n (λ)! The ngle of reflectnce is equl to the ngle of incidence. For mterils nd b, Snell s lw gives us the ngle of refrction: n n b b

1 Which of the following summarises the change in wave characteristics on going from infra-red to ultraviolet in the electromagnetic spectrum?

1 Which of the following summarises the change in wave characteristics on going from infra-red to ultraviolet in the electromagnetic spectrum? Which of the following summrises the chnge in wve chrcteristics on going from infr-red to ultrviolet in the electromgnetic spectrum? frequency speed (in vcuum) decreses decreses decreses remins constnt