Exercice 2

A magnifying glass of commercial magnification 5 has a focal distance f ' of 250/5 = 50 mm. The lens is convergent. The object and the image are real and from each side of the lens. z (object) is negative and z' (image) positive. We can write the two equation system with two unknown quantities :

Conjugate relationship :

After simplification, we obtain the second degree equation in z' :

Roots are 30 and 300.

For z'= 60, z = -300 and gy= z'/z = -1/5 the image is smaller.

For z'= 300, z = 60 and gy= -5 The image is greater. This is the solution.

Negative magnification means that the image is inversed in relation to the object.