Lenses are transparent objects made of glass or plastic with at least one curved side that refracts light.
Before analysing lenses, it is necessary to understand the vocabulary used in relation to them.
Note : Image in English coming soon.
Vocabulary linked to lenses
| Incident ray | A ray of light directed towards a surface. |
| Refracted ray | A ray of light that has been deviated by a surface. |
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Principal focal point (or primary focus) (F) |
The point on the principal axis where incident rays parallel to the principal axis converge, or the point from which the rays appear to originate. |
| Secondary focal point (or object focus) (F') | The point on the principal axis on the opposite side of the lens from the focal point. |
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Optical centre (O) |
Centre of the lens. |
| Principal axis | Line passing through the optical centre and perpendicular to the lens. |
| Focal lenght | Distance between the focus and the optical centre. |
| Ray of curvature (R) | Segment linking the centre of curvature to the corresponding surface of the lens. |
The vocabulary used above also applies to diverging lenses. However, the principal focal point and the secondary focal point are reversed because, by definition, the principal focal point is the point where the parallel rays appear to originate from.r.
Note : Image in English coming soon.
When studying lenses, it is wrong to claim that the centre of curvature is twice the focal length. Unlike mirrors, where this relationship is always true, the same cannot be said for the lens. In lens diagrams, we will generally represent the principal focal point and the object focal point, as well as points at locations twice the focal length. These points are sometimes labelled 2F.
There are two types of lenses.
A converging lens is a lens that refracts parallel light rays so that they are brought closer to the principal axis.
Converging lenses can be identified by touch, since the centre of the lens feels thicker than the ends.
There are three types of converging lenses.
| Biconvex | Plano-convex |
Converging meniscus |
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The shape of the lens will bring the rays closer to the principal axis and make them converge in a point.
A converging lens is symbolized by a double arrow pointing outwards. This symbol comes from a construction in which prisms are placed side by side.
A converging lens has the property of gathering parallel rays that pass through it towards its focus.
When an object is in front of a converging lens, there are three main rays that can be used to determine the position of the image.
- A ray parallel to the principal axis is deflected by the lens as it passes through the image focus (principal focal point).
- A ray passing through the optical centre of the lens is not deflected.
- A ray passing through the object focus (secondary focus) is deflected parallel to the main axis.
A diverging lens is a lens that refracts parallel light rays away from the principal axis.
Diverging lenses can be identified by touch, since the centre of the lens feels thinner than its ends.
There are three types of diverging lenses.
| Biconcave | Plano-concave |
Diverging meniscus |
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A diverging lens has the complete opposite effect from a converging lens. It tends to move the rays passing through it away from the principal axis. As a result, the rays passing through it will not converge at a single point, but instead will disperse.
A diverging lens is symbolized by a double arrow with the ends pointing inwards. This symbol comes from a construction of prisms joined together.
A diverging lens has the property of gathering together the extensions of the parallel rays that pass through it towards its focus.
When an object is in front of a diverging lens, there are three main rays that can be used to determine the position of the image.
- A ray parallel to the principal axis is deflected by the lens, appearing to come from the primary focus (principal focal point).
- A ray passing through the optical centre of the lens is not deflected.
- A ray directed towards the object focus (secondary focal point) is deflected parallel to the main axis.
Chromatic aberration is an optical aberration that produces a blurred image due to the decomposition of white light into several bands of colour.
Chromatic aberration is a defect in lenses. These lenses disperse the light that passes through them. The consequence of this defect is to create a coloured halo around the objects observed, which diminishes the quality of the observation.
As can be seen in the illustration below, the incident rays are dispersed once they have passed through the lens. As a result, the lens will have a distinct focus for each colour |(F_b, F_g, F_r)|.
To correct this imperfection, a second lens is placed next to the lens. This will correct the chromatic aberration. In the case of a converging lens, a diverging lens is used. In the case of a diverging lens, a converging lens is used.