Question

How far from a piece of paper must you hold your father’s 2.25 Dreading glasses to try to burn a hole in the paper with sunlight?

Short answer

The focal point to burn a hole in the paper with sunlight isf=444mm.

Step-by-step solution

Concept Introduction

The focal length is the distance between a convex lens or a concave mirror and the focal point of a lens or mirror. It is the point at which two parallel light beams meet or converge. Depending on the lens and mirror (concave or convex), the focal length varies with the sign (positive or negative).

Information Provided

Power of the reading glass: 2.25 D

Focal length of the glasses

To burn the paper, the paper has to be at the focal point of the lens. The power in dioptres of lenses is inversely proportional to the focal length;

$P=\frac{1}{f}$.

Rearranging the expression and substituting the values –

$$\begin{gathered} f=\frac{1}{P} \\ =\frac{1}{2.25D} \\ =0.444m \\ =0.444\times 1000mm \\ =444mm \end{gathered}$$

Therefore, the value for focal length is obtained as f=444 mm.