Thin converging lens

3.2.2 Refraction of light

3.2.4 Dispersion of light

Thin converging lens

  1. Principle

    1. A thin converging lens is a horizontally- & vertically-symmetrical lens that is thicker across the middle, called the principle axis of the lens, than at the edges, causing rays of light parallel to the axis to converge to one point on it
    2. The principal focus, or focus point, is the point at which the rays of light parallel to the principal axis are focused upon; a lens has two foci, one on each side
    3. The focal length is the perpendicular distance from the principal focus to the plane of the lens

  2. Real image

    1. A real image is formed when light rays originating from a point on one side of the lens focuses on a single point on the other side of the lens, which happens when the point is further from the lens than the focal length
    2. This can be constructed by finding the intersection point of tho three rays formed using the rules of refraction for lenses:
      1. An incident ray parallel to the principal axis is refracted so that it travels through the focal point on the opposite side of the lens
      2. An incident ray travelling through the focal point on the way to the lens is refracted so that it travels parallel to the principal axis
      3. An incident ray travelling through the centre of the lens is effectively unrefracted

  3. Virtual image

    1. A virtual image is the apparent origin of light rays when the rays from a point are refracted so that the diverge, which happens when the point is nearer to the lens that the focal length
    2. They can be constructed in a similar way to real images:
      1. An incident ray parallel to the principal axis is refracted so that it appeared to have travelled through the focal point on the same side of the lens
      2. An incident ray travelling through the focal point on the way to the lens is refracted so that it travels parallel to the principal axis
      3. An incident ray travelling through the centre of the lens is effectively unrefracted
    3. Due to the divergence of rays originating from nearer than the focal length, the image appears larger, so the lens can be used as a magnifying glass