JOHN WANGOMBE BAHATI PCEA SECONDARY SCHOOL

THIN LENSES

Built up of parts of prisms in contact.

Ray diagrams To obtain the position and size of an image by geometric/graphical construction, draw at least two rays from the top of the object. Draw a parallel ray to the principal axis. The ray passes through the principal focus after refraction in convex lens. It appears to come from the focus in concave lens. Draw another ray passing through the centre of the lens. It goes straight without bending. A ray passing through the principal focus is refracted by the lens such that it is parallel to the principal axis.

Ray diagram of a convex lens.

Ray diagram of a concave lens.

Image formation by converging lens using graphical construction Ray diagrams are drawn to scale using graph papers. The scale used is simple. The larger the diagram the more accurate the results. Example An object 200mm in height is placed 300mm from a convex lens of focal length 100mm. find graphically, the position, size and nature of the image.

Method Draw a principal axis Mark by a line perpendicular to the axis indicating the lens L. Choose a simple horizontal scale and mark focal length F, principal focus on both sides of lens position of object X. Draw object standing on and right angles to the axis at X with a simple scale.

From O, draw a ray parallel to the principal axis meeting lens at A. Let the tip of the object be O. From, draw a ray passing through F. From O, draw another ray passing through the optical center P of the lens.

Observation The two rays intersect at I, the tip of the image. Conclusion Measure the distance of the image from the lens. Measure the size of the image. What is the nature of the image?

Assignment An object 50mm high is placed 200mm from the converging lens of focal length 100mm.Find graphically i.The position of the image ii.The size of the image iii.Nature of the image