General Physics II Geometrical Optics: Reflection & Refraction of Light Waves • Wave front: The line or surface defined by adjacent points of a wave that are in phase • Page 1 Ray = arrow drawn perpendicular to any wave front Waves...continued... Light incident on an object Absorption • Reflection (bounces) – See it – Mirrors • Refraction (bends) – Lenses • Often some of each 5 Page 2 Reflection and Refraction • When a light wave strikes a smooth interface of two transparent media (such as air, glass, water etc.), the wave is in general partly reflected and partly refracted (transmitted). The Reflected Ray • The angle of incidence θ i is equal to the angle of reflection θ r for all wavelengths and for any pair of material. Page 3 Types of Reflection Refraction The speed of a light wave is different in matter than it is in vacuum (≈ air). Light Beam Normal Line Fast c v= n AIR WATER Where n = “index of refraction” of the material Slow Page 4 Think of a day at the beach… What's the fastest path to the ball knowing you can run faster than you can swim? This is better Not the quickest route… Physics 212 Lecture 25, Refraction How is the angle of refraction related to the angle of incidence? Unlike reflection, θ 1 cannot equal θ 2 !! » Why?? Remember v = fλ » n1 ≠ n2 ⇒ v1 ≠ v2 (But the frequencies (f1, f2) must be the same ⇒ therefore, the wavelengths must be different!) Therefore, θ 2 must be different from θ 1 !! λ1 v1 n 2 = = λ 2 v 2 n1 Page 5 It works in either direction! Incident Ray Refracted Ray Refracted Ray Incident Ray Which of the following ray diagrams could represent the passage of light from air through glass and back to air? (nair=1 and nglass=1.5) (a) (b) (c) air air air glass glass glass air air air Page 6 Dispersion • The index of refraction of a material depends on wavelength. Example: The Rainbow Raindrops refract, reflect and disperse sunlight. Rainbows will always appear opposite of the Sun in the sky. You cannot run from or run to a rainbow! Total Internal Reflection Consider light moving from glass (n1=1.5) to air (n2=1.0) Light is bent away from the normal as it enters the air. As θ1 gets bigger, θ2 gets bigger, but θ2 can never get bigger than 90° !! refracted ray n2 θ2 AIR n1 reflected ray GLASS θr θ1 incident ray So, from Snell’s Law, if sin θ1 > (n2 / n1), we have NO refracted ray; we have TOTAL INTERNAL REFLECTION. Page 7 Total Internal Reflection: The Critical Angle When θ 2 is 90o then θ1 is called the critical angle (θ c). Furthermore when θ1 > θ c , all the light is reflected (total internal reflection). In Summary… Page 8 End of Geometrical Optics: Reflection & Refraction of Light Lecture Page 9