Optic lenses - Research Paper Example

Optic Lenses The optical lens is a device capable of concentrating (converging) or diverging light. It is formed from a transparent material, most commonly curved (concave or convex) glass. According to the historical facts pertaining to lens gleaned from the Science Fair Project Encyclopedia, the earliest mention of a lens is found in the 424 B.C. play, “Clouds” by Aristophanes, in Ancient Greece which makes a reference to a ‘burning glass’ that is, a convex lens used to produce fire by focusing the sun’s rays.

Burning-glasses were known to the Roman Empire (1st Century A.D.) as well. The Roman emperor Nero in the 1st Century A.D. possibly used a lens in the form of a concave-shaped emerald to correct his myopia while watching the Gladiatorial games. Lenses are classified as either the converging type or the diverging type. The thickness at the centre of the shaped glass compared with the edges determines if it acts as a diverging or a converging lens. The point where the light passing through a lens converges or appears to diverge from, depending on the lens type, is known as the principal focus or the focal point of the lens and the distance between the principal focus and the optical center of the lens is the focal length of the lens (Abramowitz et al., 2003). The converging lens is also called convex lens or positive lens.

There are 3 main types of convex lens: plano-convex lens, double-convex lens, and concavo-convex or positive meniscus lens. The plano-convex lens is flat on one side and curved outward on the other. The double-convex lens has both its sides curved outward that is, the lens is thicker at the center than it is at the periphery and, thus, appears to be bulging outward in a hemispherical fashion with a constant curvature of radius. This type of lens causes light rays to converge, or concentrate, to form a real image which can be projected onto a screen.

The concavo-convex lens has a convex hemispherical shape on one side and a slightly concave shape on the other (Online Digital Education Connection, ODEC). The diverging lens is also known as the concave lens or negative lens. The properties of concave lenses are, in general, the opposite of convex lenses. The divergence occurs in a concave lens because the lens is thinner in the center and thicker on the periphery, which causes the incident light entering the lens to be refracted away from its center.

Concave lens can be of 3 types namely, plano-concave, double concave and convexo-concave. The plano-concave lens has a negative focal point and produces a virtual image. This type of lens is used in cameras (Online Digital Education Connection, ODEC). Imaging Properties In opposition to a collimated beam of light (a parallel rays of light) being focused on to the focal point, a point source of light placed at the focal point will be transformed into a collimated beam of light by the lens (Science Fair Project Encyclopedia).

In other words, an object at infinite distance (represented by a collimated beam of light) is focused as the image at the focal point. This is a real image. In a photographic camera, the image of an object placed in front of the camera lens (a positive lens) will be captured on a screen placed further behind the focal point of the lens as a real image. On the other hand, the image of an object placed at the focal point or closer than the focal point, will be at infinity. It will be a virtual image.

A virtual image cannot be projected on to a screen but the image is observed at its apparent calculated position when viewed through the lens. A magnifying glass works on this principle. The focal points on either side of the lens are equidistant from the center of the lens. Aberrations The image produced by a lens is not perfect as some amount of distortion or aberration is always introduced by the lens (Science Fair Project Encyclopedia). Spherical aberration occurs when the light rays parallel to but distant from the lens axis are focused at a slightly different place than the light rays close to the axis.

This causes a blurring of the image. Spherical aberration can be controlled by having a proper curvature of the lens surface. For example, a plano-convex lens when used to focus a collimated beam produces a sharper image when the convex side faces the beam of light. Coma is another type of aberration in which the aberrated image appears in a comet-like shape. Chromatic aberration is yet another type of aberration in which fringes of color are seen around the image. Other types of aberration are astigmatism, field curvature, barrel and pincushion distortion (Science Fair Project Encyclopedia).

Uses of the lens Diverging and converging lenses are both used in spectacles to correct vision defects. Myopia or nearsightedness is corrected with a concave lens, and hyperopia or farsightedness with a convex lens. A magnifying glass comprises of a single convex lens enclosed within a frame with a handle. A microscope, too, incorporates a single convex lens, whereas a refracting telescope has two convex lenses to magnify images in the sky. Binoculars use a pair of concave lenses. Lens composed of a very large number of flat surfaces located at slightly different angles is called a Fresnel lens (www.

britannica.com). In this type of lens, the surface consisting of a concentric series of simple lens sections assembled in proper relationship forms a thin lens with a short focal length and large diameter capable of concentrating light into a relatively narrow beam. Hence, Fresnel lenses are used in lighthouses and searchlights, car headlights, strobe lights, and solar energy concentrators. References Abramowitz M., Neaves SH & Davidson MW., 2003. Introduction to lenses. Accessed on 1 May 2011 from http://micro.magnet.fsu.

edu/optics/lightandcolor/lenses.html Online Digital Education Connection ODEC. Lenses and Vision: types of lenses. Accessed on 1 May 2011 from http://www.odec.ca/projects/2005/dong5a0/public_html/lenses.html www.britannica.com. Accessed on 1 May 2011 from http://www.britannica.com/EBchecked/topic/219816/Fresnel-lens). Science Fair Project Encyclopedia. Lens (optics). Accessed on 1 May 2011 from http://www.all-science-fair-projects.com/science_fair_projects_encyclopedia/ Lens_%28optics%29