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BME 314 Lecture 20 2010

# BME 314 Lecture 20 2010 - BiomedicalOptics Krish Roy PhD...

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Biomedical Optics Krish Roy, PhD Associate Professor Biomedical Engineering

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Optics:  Light, What is it? Optics - Study of Light, its nature, and the manner in which it interacts with matter Light – Wave or stream of particles? There is experimental evidence for both interpretations, although they can sometimes seem contradictory. What is a wave? A perturbation with a periodic spatial pattern which propagates, or travels in space. E.g. Sound, or a ripple through water surface What are the perturbed quantities in these cases?
Waves and their Properties  Waves start as a disturbance (source) and propagate energy, perhaps to a detector. transverse waves - the vibration is perpendicular to the direction of propagation (e.g. ripples on water). longitudinal waves - the vibration is in the same direction as the propagation of the wave (e.g. sound). Light is a kind of transverse wave so we will focus on these.

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Wave properties Suppose a wave travels down a rope at a velocity v (ms -1 ) The length of one full cycle of the wave is called the wavelength λ Traveling at velocity v it takes a time T seconds given by where T is called the period of the wave. v v v v v t = 0 t = T t = T /4 t = T /2 t = 3 T /4 x x = λ v λ = T
Wave properties Usually we will deal with continuous waves. Here the distance between crests is the easiest way of measuring the wavelength. The number of crests that pass an arbitrary point P in one second is termed the frequency, f, and is usually expressed in Hertz (Hz), or cycles/second. If one crest passes point P in time T seconds, then the number of crests passing in one second is the frequency f (2) Combining Eqs. (1) and (2) , the velocity, wavelength and frequency are related by (3) v P λ A T f 1 = f λ = v

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Wave properties of light:  Interference If two waves come together in the same place they simply add together but the result depends on the phase difference. Two waves in phase (coherent): φ = 0. Constuctive interference + Two waves Out of phase φ = π radians Destructive interference +
Wave properties of light:  Polarization In 3 dimensions a transverse wave may oscillate in any plane perpendicular to the direction of propagation. For light this means the direction in which its electric field oscillates. There are two independent possibilities usually referred to as vertical and horizontal polarization. Light beams with perpendicular polarizations will not interfere with each other. Most light is a mixture of polarizations and is said to be un-polarized. In 3 dimensions a transverse wave of known velocity is completely specified by four quantities: Wavelength or frequency. Amplitude. Phase. Polarization.

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Wave properties of light: Diffraction  and Huygen’s Principle Defined in part by the Huygens-Fresnel Principle: every unobstructed point of a wavefront, at a given instant, serves as a source of spherical secondary wavelets (with the same frequency as the primary wave). The amplitude of
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BME 314 Lecture 20 2010 - BiomedicalOptics Krish Roy PhD...

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