Thin Lens Equation

• The focal length of an optical system is a measure of how strongly it focuses or diverges light; a shorter focal length means greater optical power
• Where the thickness of the lens is small compared to the curvature radius, the thin lens approximation applies and the system can be described by the thin lens equation

Let us assume that the thickness of the lens is small compared to its curvature. For such thin lenses, the path of the light through the lens itself contributes very little to the overall change in the direction of the light rays. Using the so-called thin-lens approximation, we can mathematically describe the properties of a converging lens with the well known thin lens equation.

The focal length f is positive for converging lenses and negative for diverging lenses. The value 1/f is known as the optical power of the lens. Lens power is measured in dioptres, which are units equal to inverse meters (m−1).

The object point and image point of a lens system are said to be conjugate points.

• Inclined plane with 3 different track terminals — 5491.11
• Holder for 50 mm diameter lenses and mirrors — 4414.02
• Slider for holder — 4417.01
• Prism table — 4418.29
• Set of 4 Biconvex Spherical Lenses (diam.50 mm),f = +50, +100, +150, +200 mm — 4445.00
• Set of seven diaphragms — 4414.12
• White metal screen, squared 140 mm, 10 mm diameter support — 4418.35
• Holder for slides and diaphragms — 4414.03
• Projector with 12V 20W halogen lamp — 4414.18
• Slider for Projectors — 4414.17

• Determine the image distance for a 5-cm tall object placed 10 cm from a double convex lens having a focal length of 15 cm.
• What can we say about the direction of the beam passing through a converging lens? And in diverging lens?
• What is a famous application of lenses, according to your everyday experience?