Geometrical Optics Review Phys 375 Mirrors Spherical mirror – a section of a sphere Principal Axis – A line drawn from the center of the sphere (C) to the center of the spherical segment. Note that C is a distance R from the spherical segment. C fC fR Rays from infinity parallel to the principal axis all go through the focal point, f. C f An object a distance sO in front of the mirror will create an image a distance si from the mirror according to: fssiO111=+ where f is the focal length of the mirror: f = R/2, where R is the radius of curvature of the spherical mirror. The lateral magnification is M = - si/sO. M < 0 means that the image is inverted, M > 0 means that the image is upright. Whenever light actually passes through a point, the image formed there is real. Otherwise the image is virtual (that is virtual images are formed by rays of light that appear to diverge from a point, even if they did not originate there – see below.) Sign Convention for Mirrors: Front orreal sideMirror SurfaceBack orvirtual sidesO> 0si> 0sO< 0si< 0 For a concave mirror: If the object is in front with sO > f, the image is also in front (real) but is inverted. If the object is in front with sO < f, the image is behind (virtual) and upright. f < 0 for convex mirror f > 0 for concave mirror Ray-tracing rules for mirrors are summarized on page 32 of P3.Lenses Lenses are made up of refracting spherical surfaces or flat (R Æ ∞ spherical) surfaces. A principal axis can also be defined for lenses, as above. The lens equation is identical to the mirror equation above! fssiO111=+ A group of rays approaching parallel to the optic axis will converge at the focal point of the lens. This defines the focal length f for the lens. Sign Conventions: si > 0 when the image is on the opposite side of the lens from the object. si < 0 when the image is on the same side of the lens as the object. f > 0 for a converging lens (Thicker in the middle than at the edges) f < 0 for a diverging lens (Thinner in the middle than at the edges) FrontLens SurfaceBacksO> 0si< 0sO< 0si> 0FrontLens SurfaceBacksO> 0si< 0sO< 0si> 0fssiO111=+ The lens-maker’s equation is: ()⎟⎟⎠⎞⎜⎜⎝⎛−−=211111RRnf where n is the index of refraction of the lens, and R1 and R2 are the radii of curvature of the front and back surfaces of the lens, respectively. As before, the lateral magnification is M = - si/sOSome examples of ray tracing and image formation with thin lenses: Ray-tracing rules for thin lenses are summarized on pages 36-37 of
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