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Announcements • Happy Groundhog Day! • Pick up old labs at the end of class • Why not start preparing now for the final exam? ÆRead through the old lab again, making sure you know the answers to the quiz questions ÆTake another look at the lecture notes (on the web) and see if they now make more sense • We were lenient in the grading • In the future, be sure to ÆAnswer question explicitly being as quantitative as possible ÆShow your calculations ÆRarely (if ever) will more that 3 sig figs be appropriate Light – Some History • Early 1800’s: Light is a wave traveling through the aether (not quite right) • 1865 James Clerk Maxwell: Light is a wave of electromagnetic radiation (yes) • 1905 Albert Einstein: Light is both a particle and a wave (amazing but true) wavelength, λ PARTICLE WAVE Electromagnetic Spectrum • All electromagnetic waves travel through vacuum with a speed c (3 X 108 m/s) • The visible portion of the spectrum forms a tiny portion of the total EM spectrum • For all EM waves, c=λf (true for any type of wave); λ = c/f Views of Crab Nebula X-ray optical infra-red increasing wavelength radio Reflection and Refraction When ever light travels from one medium to another, two things can happen: Æ Reflection Æ Refraction Reflection • Draw a line normal (perpendicular) to the surface • Consider the angle between the incident ray and the reflected ray • The angle of reflection is equal to the angle of incidence: θ1 = θ1’ Æ just what you’d expect if light were a stream of particles bouncing off the surface Specular and Diffuse Reflection Refraction • Some of it is transmitted (refracted) into the 2nd medium • Not all of the light is reflected at the surface • Note that the refracted angle is not equal to the incident angle • In fact, the angles are related to the velocities of light in the two media Æ sin θ1/sin θ2 = v1/v2 Refraction Can be understood from wave properties of light. The angle changes because light travels SLOWER inside some materials than others. The speed of light inside any materials is always less than in vacuum, v(inside) < c. Refraction Snell’s law • Light travels at a speed c in vacuum, but slower in other media. • Define index of refraction n = c/v, where v is the speed that light travels in a given medium (glass, water, etc). • sin θ1/sin θ2 = v1/v2 can be rewritten as Æsin θ1/sin θ2 = n2/n1 Æn1 sin θ1 = n2 sin θ2 Refraction The angle can change in either direction, depending on the relative index of refraction (or speed) of each material. Reflection and refraction Fig. 22.6b, p.691 Total Internal Reflection When light travels from a medium with a larger index of refraction to a lower index of refraction, it bends out At some incident angle, the refracted angle is 90o ÆFor any angles greater than that, no light makes it into the refracted medium ÆNO REFRACTED LIGHT; 100% of it is reflected Fiber Optics Used for example for fiber optic transmission of phone calls Cladding has a lower index of refraction than core so for any angles greater than the critical angle, the light is totally internally reflected