Snell's Law: Refraction, Exams of Law

Download Snell's Law: Refraction and more Exams Law in PDF only on Docsity! Snell’s 11-1 SNELL’S LAW Objective: To investigate refraction at a boundary of media and use this to determine the index of refraction n. Prelab: 1. Read this lab and Taylor Chapter 7 2. Using figure 1c and 1d below as a reference, redraw each diagram and indicate the angles 1, 2, and indices n1, and n2. For both diagrams, indicate which has the larger n value. Apparatus: Laser, D-shaped water lens, protractor, coffee creamer, mystery fluid. Introduction: Light is a wave. For many situations using lenses and mirrors we can simplify our analysis using geometric optics. Geometric optics rests on these simple assumptions: 1. Light travels in straight lines, called rays; 2. Light rays cross each other with no interference between them; 3. Whenever rays strike the interface between two transparent media in which the speed of light is different (e.g., airglass, glassair, airwater, etc.), a portion of the light is reflected and a portion is transmitted. 4. The transmitted light rays bend by an amount that depends on the two speeds and on the angle of incidence 1. This bending of light rays is called refraction and it is our focus in lab today. These 4 assumptions have their limits: We know from diffraction that the first assumption does not work when light passes through apertures the size of the wavelength of light. The second assumption is related to the superposition principle that is true for all waves. The fourth assumption is known as “Snell’s law,” and it states that the angle of refraction is related to the angle of incidence via (1) Fig. 1a. Normal Incidence. Air to Water. Reflected ray not shown. air water incident light transmitted light incident light reflected light refracted light Fig. 1c. Light traveling from air into water. air water incident light reflected light refracted light Fig. 1d. Light traveling from water into air. air water incident light transmitted light air Fig. 1b. Normal Incidence. Water to Air. Reflected ray not shown. water nI sin(qI ) = nR sin(qR ) Snell’s 11-2 with and being the index of refraction for the material on the incident side and refracted side, respectively. The index of refraction 𝑛 ≡ 𝑐 𝑣 is the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v). (Note that ≡ means “equal by definition”.) The geometry is shown in Fig. (1) for light passing from a less dense medium (e.g. air) to more dense (e.g. water). Fig. 1: Snell’s Law. Angles are measured relative to the normal (dotted line). Part I: Data Refraction refers to the bending of light when it crosses an interface between two materials. We will discuss this from a wave point of view in class, but for now we will consider it experimentally. (1) Arrange your laser and D shaped water dish as explained by your instructor. We are interested in looking at refraction as the light passes from water-to-air. Using Eq. (1) show that the light does not bend when it crosses a surface along the normal (perpendicular) to the surface. The beam must hit the dish at normal incidence – perpendicular to the surface- so there is no bending of the light at the air-to-water interface. Align the dish so that its straight edge is along the protractor axis and the dish is centered on the protractor. Align the laser so that it is centered and perpendicular to the axis of the protractor. (What happens if either of these two conditions is not met? Is there an experimental technique you can use to minimize the effect of even a small misalignment?) Qualitatively describe what happens to air as water increases from 0 to 90 degrees. Now measure air as a function of water. For each point, estimate the uncertainty in your measured values. nI nR Normal to the surface Surface of interest air water I R