Thin Lenses

3.2.3 Using a Single Lens as a Magnifying Glass

A magnifying glass is a simple example of a converging lens used to produce a magnified image of an object. This section will describe how a single lens works as a magnifying glass, focusing on the principles of refraction and image formation.

How a Magnifying Glass Works

A magnifying glass is a convex lens with a long focal length. When an object is placed closer to the lens than its focal length, the lens forms a virtual, upright, and magnified image. This is because the light rays from the object refract through the lens and appear to diverge from a point behind the lens, creating the magnified image.

The Formation of Images with a Magnifying Glass

When an object is placed between the focal point ($f$) and the principal focus ($2f$) of a converging lens:

• The image formed is virtual.
• The image is upright.
• The image is magnified.

The magnification ($m$) of the image formed by a thin lens is given by the formula:

$$m = \frac{h'}{h}$$

where:

• $h'$ is the height of the image.
• $h$ is the height of the object.

The magnification is also related to the object distance ($u$) and image distance ($v$) by the lens formula:

$$\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$$

Experiment: Using a Magnifying Glass

A simple experiment can be conducted to demonstrate the use of a magnifying glass:

1. Hold the magnifying glass close to your eye.
2. Hold an object (e.g., a small piece of paper or text) between the magnifying glass and your eye.
3. Slowly move the object closer to the magnifying glass until you see a magnified image.
4. Observe how the distance between the object and the magnifying glass affects the size and clarity of the image.

Applications of Magnifying Glasses

Magnifying glasses are used in various applications, including:

• Reading small print.
• Examining small objects (e.g., insects, coins).
• Repair work.
• Jewellery inspection.

Table: Properties of Images Formed by a Magnifying Glass

Property	Description
Image Type	Virtual
Image Orientation	Upright
Image Size	Magnified
Object Distance	Between the focal point and the principal focus ($f$)