Thin lenses

• Converging lenses are thicker at their center than at their edges, while diverging lenses are thinner at their centers than at their edges.
• A converging lens has a positive focal length, while a diverging lens has a negative focal length.
• A lens is a transparent material that has at least one curved surface
• A lens bends light towards the thicker parts of the lens
• A convex lens is thicker at the middle
• A concave lens is thinner at the middle
• A convex lens is a converging lens
• A concave lens is a diverging lens
• The centre of curvature is the centre of the sphere of which the lens is part of
• The radius of curvature is the radius of the sphere of which the lens is part of
• Principal axis is the imaginary line passing through the centre of curvatures
• Optical centre is the centre of the lens
• Principal focus(focal point) of a convex lens is the point on the principal axis where rays that are parallel to the principal axis appear to converge after refraction by the lens
• Principal focus(focal point) of a concave lens is a point on the principal axis where rays that are parallel to the principal axis appear to diverge from after refraction by the lens
• Focal length is the distance between the focal point and the optical centre
• The focal length of convex is real(+)while the focal length of concave is virtual(-)
• Focal plane is a plane that is perpendicular to the principal axis and it passes through the focal point
• In convex lens the parallel rays that are not parallel to the principal axis converge at the focal plane
• In concave lens the parallel rays that are not parallel to the principal axis appear to diverge from the focal plane
• Rays passing through the optical centre are not deviated
• In convex lens the closer the object moves to the lens the further the image moves from the lens
• In concave lens the closer the object moves to the lens the smaller the image gets
• Images in concave lens are upright, virtual and diminished
• Only a convex lens can form a magnified virtual image
• Magnification is the ratio of the size of the image to the size of the object
• m=hi/ho (hi-height of image, ho-height of object)
• m=V/U (v- image distance, u-object distance)
• Power of a lens is the measure of the refracting ability of a lens
• The SI unit of power of lens is Dioptres (D)
• power= 1/f (f- focal length in metres)
• Applications of lenses

  • Human eye
  • Camera
  • Magnifying glass
  • compound microscope
• Defects of the human eye

  • Myopia(short sightedness)
  • Hypermetropia(long sightedness)
• Myopia is when the eye can only see clearly when objects are closer
• Hypermetropia is when the eye can only see objects clearly when they are far away
• Accommodation as applied on the human eye

  Adjustments made by the lens to accommodate different image distances on the retina
• Near point


  The closest point which the human eye can clearly focus
• Far point


  The furthest point which the eye can clearly focus
• Myopia(short sighted) defects

  • short focal length
  • long eyeball
• The lens used to correct myopia is

  Concave (diverging lens)
• The eye naturally has a convex lens