Experiment #17: Refraction, Lab Reports of Physics

Download Experiment #17: Refraction and more Lab Reports Physics in PDF only on Docsity! Experiment #17: Refraction OBJECTIVES The transmission of light across a boundary between two media is accompanied by a change in both the speed and wavelength of the wave. This can result in a change of direction at the boundary, a phenomenon known as refraction. In this experiment you measure the change in direction of light beams as they refract or reflect at a boundary to determine the index of refraction of a transparent object. The objectives of this experiment are as follows: 1. To measure the angles of incidence and refraction at a boundary between media 2. To observe total internal reflection at a boundary between media 3. To calculate the critical angle of a boundary between media THEORY The index of refraction is a property of transparent substances that has been independently discovered several times, but is attributed to Willebrord Snellius whose name is associated with the law (you can't make this stuff up). Mathematically, Snell's law describes the relationship between the angle of incidence of a beam of light as it intersects a new transparent medium and the angle of refraction as enters that transparent medium. Figure 6.1: Refraction overview Snell's law quantifies the relationship that is observed in Figure 6.1: n1∙ sin θ1 = n2∙ sin θ2 where n1 is the index of refraction of medium 1, n2 is the index of refraction medium 2, θ1 is the angle that the light ray makes with respect to the normal in medium 1, θ2 is the angle that the light ray makes with respect to the normal in medium 2. The index of refraction of any medium (ni) is the ratio of the speed of light in vacuum (c) to the speed of light in that medium (vi), as shown in equation 6.2. 𝑛𝑖 = 𝑐 𝑣𝑖 where c = 3.00×108 m/s (the accepted value for the speed of light in vacuum, a constant). A very good approximation for the refractive index of air is 1.00, i.e. nair=1.00. On observation, it can easily be seen that as light travels from a lighter medium to a denser one (i.e. n1<n2), the refracted light ray bends towards the normal. Conversely, when light travels from a denser medium to a lighter one (i.e. n1>n2), the refracted light ray bends away from the normal. But θ1 θ2 medium 1 refractive index n1 medium 2 refractive index n2 normal incident ray refracted ray (6.1) (6.2) refracted ray internally reflected ray incident ray The critical angle, θc normal medium 1, n1> n2, medium 2 when you think about it, how much “away from the normal” is possible? One can only get as far as 90° without leaving the medium! When the refracted ray exceeds 90°, it’s not refraction anymore, instead light is reflected back into the same medium it started from, and this phenomenon is known as total internal reflection. Note that this only happens for light traveling from a denser medium to a lighter one (see figure 6.2 below). Figure 6.2: Total internal reflection ACCEPTED VALUES The glass used in this experiment is made of Lucite. The accepted value for the refractive index of Lucite is 1.50. The mystery media have no accepted value for their refractive indices. It is up to the experimenter to determine their values! DATA Medium Measurement Magnitude (°) Refractive index Air Angle of incidence (40°< θi < 60°) na= Glass Angle of refraction Air Angle of incidence (60°< θi < 90°) nb= Glass Angle of refraction Glass Critical Angle (θc) nc= Air Angle of incidence (θi) nA= Mystery Medium A Angle of refraction Air Angle of incidence (θi) nB= Mystery Medium B Angle of refraction ANALYSIS 1. Use equation 6.1 to calculate the refractive index of glass in the first three scenarios on the data table (na, nb, and nc). 2. Find the average experimental value for the refractive index of Lucite, n. 3. Calculate the error (as a percentage) in your average experimental value calculated above. 4. Calculate the speed of light in Lucite. 5. Use equation 6.1 to calculate the refractive index of “Mystery A” and “Mystery B” media. The critical angle (θc) is the angle of incidence for which the angle of refraction is 90°. Beyond the critical angle, 100% of the incident light is reflected back into the same medium.