Simple Lenses

This follows the page introducing the principle of refraction of light, which also includes a statement of the Law of Refraction and an explanations of why lenses are useful for re-directing light using refraction.

Summary of conclusions from the previous page:

Parameters controlling how light is refracted (i.e. re-directed through surfaces between two different media) include:

  1. The refractive indices of the materials on either side of the surface, and
  2. The angle of incidence at the interface between the two media.

Re. Lenses: Choice of the material from which a lens is made determines its refractive index and specification of the curvature of the surfaces of a lens and its orientation determine the range of angles of incidence with which light arrives at that lens from any particular point in space.

Types of Lenses - Convex and Concave

Lenses can be described and classified in many different ways - including according to the curvature of their surfaces. The first distinction to understand is the difference between convex lenses and concave lenses.

Convex Lens
(also described as a converging lens)

Concave Lens
(also described as a diverging lens)

Focal Length and Focal Point (F)

Each lens has a focal point that is usually labelled F on diagrams.

The distance between the focal point (F) and the centre of the lens is called the focal length of that lens.


Convex (Converging) Lens:

The Focal Point (F) of a convex lens is the point through which rays incident (on the lens) parallel to the principal axis pass after refraction by the convex lens.

Concave (Diverging) Lens:

The Focal Point (F) of a concave lens is the point from which rays incident (on the lens) parallel to the principal axis appear to have come after refraction by the concave lens.

The above definitions are illustrated on ray-diagrams of light passing through convex and concave lenses.

The focal length is an important property of a lens. It is a simple way of conveying information about the effect that lens has on light passing through it.

Based on certain assumptions about lenses, incl. e.g. them being reasonably 'thin' and made of a material, such as a glass, whose refractive index is not unusual (e.g. significantly different in different areas of the lens), simple ray diagrams can be drawn to illustrate the general behaviour of lenses based on their basic shape (convex or concave) and their focal length alone - that is, without the need for multiple calculations involving angles of incidence and refraction and refractive indices - see more about refraction.

Note that even when optical paths through lenses can be drawn and explained without reference to the refractive index of the lens(es), re-direction of light passing through lenses is due to the effect of refraction.

The above ray diagrams for concave vs convex lenses represent examples of light passing through simple convex and concave lenses.

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