Happy Tuesday! Get ready for warm up #4 Get out paper for notes. Title them: “Snell’s Law and Refraction” Essential Question: What is Snell’s Law? Quiz = Thursday Raytheon information

Warm up #4 A. In your own words, write down what happens to light when it changes the medium through which it is traveling B. The hardest mineral is diamond, with an index of refraction of At what speed does light travel through a diamond? n = 2.42 c = 3 x 10 8 m/s v = ?

Review: Index of Refraction Ratio of speed of light in a vacuum (3 x 10 8 m/s) to speed of light in a transparent substance is the index of refraction (n). The larger the n, the slower light travels n air = 1 (memorize this!) n = index of refraction (no units) c = speed of light (3 x 10 8 m/s) v = velocity of light in the transparent substance (m/s)

What does the index of refraction mean? It tells you how much light will slow down in a specific medium The higher the “n”, the slower light will travel an “n” of 2 means that light travels half as fast

Snell’s Law

Air Water Normal Normal –line that is perpendicular to the surface boundary Incident ray Incident ray – the Initial ray of light: Light originates from this direction Refracted ray Refracted Ray – ray that undergoes a change in speed and direction because it Is traveling through A different medium

2 angles are formed:   = initial angle, or incident angle  angle    refracted angle

Compare Side-by-Side Water Air normal Incident ray ΘiΘi Refracted ray ΘrΘr Air Water normal Incident ray ΘiΘi Refracted ray ΘrΘr Less dense  More dense Light slows down Bends toward normal More dense  Less dense Light speeds up Bends away from normal

Snell’s Law Williford Snell determined the relationship between angle of incidence (θ i ) and angle of refraction (θ 2 ) “Snell’s Law” = “Law of Refraction” n 1 = index of refraction light is traveling FROM θi = angle of incidence (measured from normal to incident ray) n 2 = index of refraction light is traveling TO θr = angle of refraction (measured from normal to refracted ray)

Snell’s Law Sample Problem Ex: Light strikes water (n = 1.33) from air at an angle of 36 degrees to the normal. What is the angle of refraction?

Critical Angle As light emerges from more optically dense medium to less optically dense medium it ________ (speeds up, slows down, stays same) and bends _________(toward, away from) normal. –SPEEDS UP –Bends AWAY Critical Angle (Θ c )– the angle of incidence that produces a 90°angle of refraction

Critical Angle Critical Angle – the angle of incidence that produces a 90°angle of refraction Air n = 1 Water n = 1.33 ΘcΘc Θ r = 90°

Critical Angle Using Snell’s Law…. n 1 sin Θ c = n 2 sin 90° Since sin 90°=1, you will get: n 1 sin Θ c = n 2 Therefore….

Sample Problem What is the critical angle for light emerging from a diamond (n = 2.42) into air?

Total Internal Reflection Total Internal Reflection – Occurs when a light ray strikes the surface at an angle GREATER THAN the critical angle This is how fiber optics transfer info along glass fiber