Lenses

Lenses

1. Converging lenses

   • Parallel rays of light that as through a converging lens are refracted so that they converge to a point.
     • This point is called the prinicipal focus. The distance from the centre of the lens to the principal focus is the focal length.
   • Because the light can pass through the lens in either direction, there is a principal focus on either side of the lens.
   • If the object is further away from the lens than the principal focus, and inverted, real image is formed. The size of the image depends on the position of the object. The nearer the object is to the lens, the larger the image.
   • If the object is nearer to the lens than the principal focus, an upright, virtual image is formed behind the object. The image is magnified - the lens acts as a magnifying glass.
   • The magnification can be calculated using:
     • Magnification = Image Height / Object height

2. Diverging lenses

   • Parallel rays of light that pass through a diverging (concave) lens are refracted so that they diverge away from a point. This point is called the principal focus.
   • The distance from the centre of the lens to the principal focus is the focus length.
   • Because the light can pass through the lens in either direction, there is a principal focus on either side of the lens.
   • The image produced by a diverging (concave) lens is always virtual.