Simple Lenses

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Introductory Note: Optics and the use of lenses and other optical components to control the passage of light or other electro-magnetic energy through systems is generally studied within the scientific discipline of physics. Optical system design is a complex subject, detailed knowledge of which is not necessary for a basic understanding of how the eye works. However, because the lens is a very important part of the eye, and problems with eyesight (vision) are often treated with the use of spectacles or contact lenses, this introduction to two very simple types of lenses is included as background information.

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.

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.

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.

Note: Understanding of parts of the nervous system (such as the eye / visual system) are required for many exams - e.g. GCSE Physics, GCSE Biology, and AS and "A"-Level Biology and Human Biology. Check your syllabus or ask your teacher to find out about the level of detail required.