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The Nature of Light • Because of the way we see everyday objects, Newton and many others always believed light was a stream of particles (Chapters 25 and 26). • Diffraction, when light bends around the edges of objects, countered the particle theory and lead to the theory that light acts as a wave (Chapter 27). • It was the Photoelectric Effect that led to the quantization of the energy of a light wave. These packets of light are called photons and they carry the energy: E = hf • h is Planck’s constant, h=6.63x10-34 Js • Light exhibits the characteristics of a wave in some situations and the characteristics of a particle in other situations – This is now referred to as wave-particle duality The Ray Model • For chapters 25 and 26, we will model light as a stream of particles. • By doing so, we make it possible to sketch out ray diagrams. • These diagrams illustrate how light waves (drawn as rays of light) travel between objects and observers. Specular vs. Diffuse Reflection • For simplicity, we will assume all surfaces are flawless and flat, as in example (a). • This is called specular reflection. Law of Reflection The angle of reflection is equal to the angle of incidence 1 1 ' These angles are always measured from the normal line. Refraction • When light traveling through a transparent medium encounters a boundary for another transparent medium, refraction occurs. sin 2 v2 const. sin 1 v1 The Index of Refraction • Light passing from one medium to another is refracted because the speed of light is different in the two media. • Light travels at its fastest speed in a vacuum. c 3.0 x10 m / s 8 • The index of refraction is defined as the ratio: c n v Refraction Scenarios • (a) As light moves from air into glass, the ray is refracted towards the normal. • (b) As light moves from glass into air, the ray is refracted away from the normal. Working to the Law of Refraction • As light travels from one medium into another, the frequency of the light does not change. v1 f1 ,v 2 f2 f v1 1 v2 2 Working to the Law of Refraction 1 v1 c n1 n2 2 v2 c n2 n1 1n1 2 n2 air n n The Law of Refraction (Snell’s Law) • Combine: sin 2 v2 sin 1 v1 v1 n2 v2 n1 sin 1 n2 sin 2 n1 n1 sin 1 n2 sin 2 The Spectrum of Electromagnetic Waves (24.7) • Wave Types – Radio waves – Microwaves (1 mm – 30 cm) – Infrared waves (1 mm – 700 nm) – Visible light (700 nm – 400 nm) – Ultraviolet light (400 nm – 0.6 nm) – X-rays (10 nm – 0.1 pm) – Gamma rays (0.1 nm – 10 fm) Dispersion and Prisms • The index of refraction depends on the wavelength of the light • Therefore, the angle of refraction depends on the wavelength of the light. • This dependence is called dispersion. Dispersion and Prisms Dispersion and Prisms • Dispersion of light into a spectrum is demonstrated through the formation of a rainbow. • Research has indicated that the angles of highest intensity (brightest rainbows) are 40o and 42o Total Internal Reflection • This occurs when light travels from a medium with a high index of refraction to a medium with a lower index of refraction. • There is a particular angle, called the critical angle, at which the refracted light ray moves parallel to the boundary (ray 4, in picture). • At angles greater than the critical angle, no ray is refracted and total internal reflection occurs (ray 5) sin c n2 n1 Fiber Optics – Total Internal Reflection