Question

When an object is moving along the principal axis of a lens at object distances 8 cm and 16 cm , the images being thrice the size of the object can be found. Find the type and focal length of the lens

Answer 1

We have object distance of -16 cm and the magnification is -3. Since magnification is

$$m=\frac{i}{o}\rightarrow i=mo=(-16)(-3)=48\text{ cm}.$$

According to thin lens equation:

$$\frac{1}f=\frac{1}i+\frac{1}o,\space\space\space\space\space\space\frac{1}f=\frac{1}{48}+\frac{1}{(-16)},\\\space\\ f=12\text{ cm}.$$

This is a convex lens because a concave (diverging) lens cannot produce magnified images (i.e. magnification greater than unity).