Reflection and Refraction Object To determine focal lengths of lenses and mirrors and to determine the index of refraction of glass. Apparatus Lenses, optical bench, mirrors, light source, screen, plastic or glass disk. Theory Reflection and Refraction For reflection, the of incidence should equal the angle of reflection as shown in Figure i r 1. When a light ray is transmitted from one material into another the process is called refraction. Figure 1. Reflection off of a smooth, flat surface termed specular reflection. – It is customary to measure all angles with respect to the normal, which is perpendicular to the interface between the media at the point of incidence. Snell’s law describes what happens at such an interface sin = sin n1 n2 θ1 θ2 where and are the indices of refraction of the two materials, is the incident angle, and n1 n2 θ1 is the refracted angle. The index of refraction is defined as the ratio of the speed of light in θ2 vacuum (space) over the speed of light in the material in question. Mathematically, For n c/v. ≡ air, 1, and for water = 1.33, whereas often = 1.5 for glass. It has also been found that n n n n ≃ will vary as a function of wavelength (or color) of light. Thus, white light entering a second material will have its different colors refracted at different angles and often the separate colors will become observable. A geometrical derivation for Snell’s Law can be found in most Physics texts. In general, when light hits the interface between two different media, some of the light will be reflected and some will be refracted, with each obeying the above laws, accordingly. This is shown in Figure 2. The amount reflected versus refracted depends on several variables, including the indices of refraction and the incident angle. In Snell’s law, if > then the refracted ray will bend away from the normal, meaning that n1 n2, > As is increased, also increases, until = 90◦. Under these conditions no light is θ2 θ1. θ1 θ2 θ2 refracted into the second material and the angle is referred to as the critical angle of θ1
Reflection and Refraction Object To determine focal lengths of lenses and mirrors and to determine the index of refraction of glass. Apparatus Lenses, optical bench, mirrors, light source, screen, plastic or glass disk. Theory Reflection and Refraction For reflection, the of incidence i should equal the angle of reflection r as shown in Figure 1. When a light ray is transmitted from one material into another the process is called refraction. Figure 1. Reflection off of a smooth, flat surface – termed specular reflection. It is customary to measure all angles with respect to the normal, which is perpendicular to the interface between the media at the point of incidence. Snell’s law describes what happens at such an interface n1 sin θ1 = n2 sin θ2 where n1 and n2 are the indices of refraction of the two materials, θ1 is the incident angle, and θ2 is the refracted angle. The index of refraction is defined as the ratio of the speed of light in vacuum (space) over the speed of light in the material in question. Mathematically, n ≡ c/v. For air, n ≃ 1, and for water n = 1.33, whereas often n = 1.5 for glass. It has also been found that n will vary as a function of wavelength (or color) of light. Thus, white light entering a second material will have its different colors refracted at different angles and often the separate colors will become observable. A geometrical derivation for Snell’s Law can be found in most Physics texts. In general, when light hits the interface between two different media, some of the light will be reflected and some will be refracted, with each obeying the above laws, accordingly. This is shown in Figure 2. The amount reflected versus refracted depends on several variables, including the indices of refraction and the incident angle. In Snell’s law, if n1 > n2, then the refracted ray will bend away from the normal, meaning that θ2 > θ1. As θ1 is increased, θ2 also increases, until θ2 = 90◦. Under these conditions no light is refracted into the second material and the angle θ1 is referred to as the critical angle of