Constructing the image of an object

Method of construction

To construct the image A'B' of an object AB positioned perpendicular to the optical axis (A on the axis, B at the vertex), draw at least two of the three specific rays emanating from B. Their point of intersection gives B'. The image A' then lies on the optical axis, directly above B'.

Practical steps

1. Position the object AB, the optical centre O, and the focal points F and F'.
2. Draw a ray from B parallel to the optical axis -> it emerges through F'.
3. Draw a ray from B passing through O -> it is not deflected.
4. The intersection of the two emerging rays gives B’.
5. Project B’ perpendicularly onto the optical axis to obtain A’.

Nature of the image

Depending on the position of the object relative to F, the nature of the image changes:

Position of the object	Image obtained
Beyond 2F	Real, inverted, smaller
Between F and 2F	Real, inverted, smaller
Between O and F	Virtual, upright, enlarged (magnifying glass effect)

A real image is formed where the rays actually intersect (it can be captured on a screen). A virtual image is formed at the point where the rays appear to converge, without ever actually intersecting there (it cannot be projected onto a screen, only observed with the naked eye or using an instrument).

Practical example

An object placed 15 cm from a lens with a focal length f' = 5 cm is beyond 2F' (10 cm): the image will be real, inverted and smaller than the object, as in a camera.

Common pitfall

Forgetting to invert the image when it is real: in this case, if B is above the axis, B’ ends up below it.