WAEC Physics 2013 Paper

Lens formula:

$\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}$

where f, d${}_o$ and d${}_i$ are the focal length, object distance and image distance respectively.

The distance from the lens to the focal point is called the focal length. For converging lenses, the focal length is always positive, while diverging lenses always have negative focal lengths.

⇒ $\frac{-1}{30} = \frac{1}{d_o} + \frac{1}{20}$

$\frac{1}{d_o} = \frac{-1}{30} - \frac{1}{20}$

$\frac{1}{d_o} = \frac{-2 + -3}{60}$

→ \frac{-5}{60}\) = \frac{-1}{12}\)

$d_o = -12\,\text{cm}$

The object distance -12cm = 12cm on the other side of the lens.