Lenses

• Real image


  Rays of light which make it up, pass through it. It can be formed on a screen.
• Virtual image


  Rays of light which make it up do not pass through it but appear to come from it. It cannot be formed on a screen.
• Optical Centre


  The exact centre of the lens.
• Principal Axis


  An imaginary line passing through the optical centre at right angles to the plane of the lens.
• Rays of light originally parallel and close to the principal axis passing through a converging lens


  Refracted so that they pass through a point on the principal axis known as the principal focus.
• Focal length


  The distance from the principal focus to the optical centre of the lens.
• Rays of light originally parallel and close to the principal axis passing through a diverging lens


  Spread out from a point on the principal axis known as the principal focus.
• Linear Magnification


  height of image/height of object
• Power of a lens

  1/f (f is in m)
• Far point


  The furthest point which can be seen clearly by the unaided eye.
• Near point


  The nearest point which can be seen clearly by the unaided eye.
• Myopia (short sight)


  A person who suffers from this is unable to see distant objects clearly.
• Long sight


  A person who suffers from this is unable to see near objects clearly.