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Lens and defect of vision chapter SEE Nepal

Concave and Convex lens, Hypermetropia and Myopia

anzan_nepal
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Lens and defect of vision chapter SEE Nepal

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  1. Lens and defect of eyes Its now time to see the light….

  2. Types of Lenses • A lens is a curved transparent material that is smooth and regularly shaped so that when light strikes it, the light refracts in a predictable and useful way. • Made of transparent glass or very hard plastic

  3. When Light enters a Lens • Light undergoes two refractions: the first on entering the lens and the second on leaving the lens • The index of refraction of a lens is greater than that of air • When light passes from air into the lens, the light ray refracts away from the lens surface and towards the normal • When the light passes out of the lens at a an angle, the light rays refract again, this time bending away from the normal

  4. Basic Lens Shapes • Converging Lens (convex) • Diverging Lens (concave)

  5. Lens Terminology • Axis of Symmetry: an imaginary vertical line drawn through the optical centre of a lens • Principle Axis: an imaginary line drawn through the optical centre, perpendicular to the axis of symmetry • F and F’: Both kinds of lenses have two principal focuses. The focal point where the light either comes to a focus or appears to diverge from a focus is given the symbol F, while that on the opposite side is represented by F’ • Focal length (f): the distance from the axis of symmetry to the principal focus measured along the principal axis. There are two equal focal lengths since light behaves the same when travelling in either direction through a lens.

  6. The Converging Lens (Convex) • Secondary Pricipal focus (F’) • Optical Centre

  7. The Converging Lens • Thicker at the centre of the lens than at the edges • Convex lenses are able to form a real image on a screen • A common example is the magnifying glass

  8. Drawing a Convex Lens Ray Diagram • The first ray of a convex lens ray diagram travels from the tip of the object parallel to the principal axis. When it emerges from the lens it passes through the principal focus. • The second ray travels from the tip of the object through the optical centre of the lens and is not refracted. • Draw the real image where the rays appear to intersect.

  9. The Diverging Lense (Concave)

  10. Drawing a Concave Lens Ray Diagram • The first ray of a concave lens ray diagram travels from the tip of the object parallel to the principal axis. When it emerges from the lens, it appears to come from the principal focus. • The second ray travels from the tip of the object through the optical centre of the lens, and is not refracted. • Draw the virtual image where the rays appear to intersect.

  11. The Diverging Lens • Thinner in the centre than at the edges • The image formed is always upright, smaller than the object, and virtual • Image is always located on the object side of the lens

  12. Thin Lens Equations • Ray diagrams may help one determine the approximate location and size of the image but will not provide numerical information about image distance and image size • To obtain this type of numerical information, it is necessary to use the Lens Equation and the Magnification Equation. • Both equations use the variables illustrated in this diagram

  13. Lens Maker Formula • A thin lens is a lens that has a thickness that is slight compared to its focal length. You can assume that all refraction takes place at the axis of symmetry for ray diagrams. • The thin lens equation expresses the quantitative relationship between the object distance (u), the image distance (v), and the focal length (f). The equation is stated as follows: 1/f = 1/u + 1/v

  14. Sign Conventions • Keep in mind when working with this equation: • A concave lens has a negative (-) focal point and a negative (-) distance to the image • A convex lens has a positive focal point (+) and • A positive distance to the image, if the image is real • A negative distance to the image, if the image is virtual • Convex and concave lens’ have positive distances to the object

  15. Magnification Equation • The magnification equation relates the ratio of the image distance (V) and object distance (U) or the ratio of the image height (I) and object height (O). The magnification equation is stated as follows: M = I / O M = V/ U

  16. Iris : regulates size of the pupil Pupil: controls passage of light Ciliary muscle: controls shape of lens a/c to the distance objects from lens

  17. Accommodation of Eye • The ability of eye lens to adjust its focal length is called accommodation. • Near point: Nearest point from eye at which an object can be seen clearly by the eye. Its value for normal eyes is 25 cm. • Far point: Farthest point from eye at which an object can be seen clearly by the eye. Its value for normal eyes is infinity.

  18. Thank you

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