Educational film studio http://shumstudios.com There are two types of images: Virtual images, which per the name, are only imagined, appearing to exist. An example is a (flat) mirror, where although one's image is perceived behind the mirror, if someone went behind the mirror, there'd be nothing there. It can also be produced by convex mirrors, which has a reflecting surface which is convex (bulges outward). It can also be produced by diverging lens, which have a center of lens thinner than either side, and are therefore convex. Convex therefore reflects light, diverging (spreading) it Real images, which actually exists. It can be produced by concave mirrors, which has a reflecting surface which is concave (bulges inward). It can also be produced by converging lens, which have a center of lens thicker than either side, and are therefore concave. Concave therefore reflects light toward a focal point, converging (concentrating) it, such as a projector sending an image onto a cinema screen Radius of curvature is based upon a principle that any curve, can be extrapolated to a circle. The radius of that circle, is the radius of curvature therefore, of that curve. For this reason, sharper curves have a smaller radius of curvature; and vice versa. The focal point is the point at which light converges at. Evidently, this will only be found in a converging/concave mirror, as diverging/convex mirrors do not focus light. All light rays parallel [to the ground] will reflect off the mirror, at f=12R, where f is the focal length away from the mirror: In converging/concave mirrors, its distance is ½ the radius of curvature away from the mirror, from the same side of the mirror. Therefore, the focal point in the converging mirror, is in front of the mirror; and the rays therefore reflect to it In diverging/convex mirrors, its distance is ½ the radius of curvature away from the mirror, from the opposite side of the mirror. Therefore, the focal point in diverging mirrors, is behind the mirror; and the rays therefore reflect away from it Figuring the focal point in lens is less intuitive, and requires the Lensmaker's equation, which states 1f=(n1n2−1)(1r1−1r2), and n is the refractive index of the lens material, and r is the radius of curvature [for either side of the lens]. Note therefore, the refractive index of the lens material and the atmosphere, will alter the focal point. Specifically, the greater the difference between these indices, the shorter the focal point, since these are inversely proportional in the formula. For example, because the combination of a magnifying glass and air, has a greater difference than a magnifying glass and water, the former will have a shorter focal point, meaning its effect will be more pronounced, and therefore will work better in air [than water]. Note also, the greater the distance between the radius of curvature [for either side of the lens], meaning the thicker the lens, the shorter the focal distance [and per , meaning its effect will be more pronounced], as these are also inversely proportional in the formula. [Focal] power is the reciprocal of the focal point, P=1f, and is the degree to which a lens bends light rays. The unit for optical power is the diopter. Ray tracing/diagrams show the mirror or lens, focal point, an object [which emits or reflects photons], and its image [where the ray converges]. Note that these diagrams use a spherical profile, although only a parabolic reflector can only truly make a focal point. This is because spherical mirrors suffer from spherical aberration, which results in an imperfectly produced image. Each object [point] can be associated with three rays: Path parallel to the ground, which as just stated , reflects to the focal point of a concave mirror [or refracts to the focal point of a converging lens], or away from the focal point of a convex mirror [or refracts away from the focal point of a diverging lens] Path aimed at center of lens or mirror, reflecting off the mirror, as if it were striking a flat mirror [or refracts through the lens as if it were not present] Path aimed at, or from; and focal point, or pseudo focal point. The pseudo focal point is a point of the same distance from the focal point, but on the opposite side of the lens Using only the first two rays, you can figure where the image will meet, extrapolating those lines on the other side of the mirror, if necessary. The latter ray is selected by going through a pseudo/focal point which forms a ray parallel to the ground, and meets up at the focal point [where the other two rays meet].

• published: 06 Jul 2014
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