LectureInterference.pptVIP

Lecture 16Interference Wave Optics Young’s Double Slit Experiment Wavelength Measurement Problem with Double Slit Interference from Reflection Interference in Thin Films Newton Rings Summary Interference is one of the wave-like properties of EM waves. Interference can be constructive or destructive. Reflection in a material with a higher index of refraction inserts a 180o phase change in the reflected wave. * Chapter 24.1 ? 24.4 Outline Conditions for Interference Experiments Showing Interference Interference in Thin Films Geometric optics considers light as rays. Wave optics applies to wave properties of light. Conditions for interference: The waves must maintain a constant phase shift between them (coherency) The wave must have identical wavelengths Interference Demonstration Interference is a superposition of two or more waves. It can be constructive or destructive. Demonstration Double slit interference Constructive interference: the path difference is an integer multiple of the wavelength : d sin ? = m ?. Destructive interference, the path difference is an odd multiple of ?/2 : d sin ? = (m+1/2) ?. Assumption: The two waves travel in parallel lines (the wave source at infinity). d ? distance between the slits, m ? the order number, ? ? angle between the slit line and the wave direction, ? ? wavelength. The double slit experiment allows to measure ?. Assumption: ? is small, so sin ? ? tan ? L is the distance between the slits and the screen, y is the distance between the normal to the screen and the image position. y = L tan ? ? L sin ? d sin ? = m ? ?L y = ? m d For bright fringes Problem: The double slit is at 0.80 m from the screen. If the distance between alternating bright fringes is 0.95 cm and ? = 580 nm, what is the slit separation (d)? 1 nm = 10?9 m L = 80 cm dy = 0.95 cm ?L yb = ? m d ?L dy = ? d ? ?L d = ? dy d = 4.9 10?5 m = 49 microns (?m) Interference can be produced by reflection: Lloyd’s