Cambridge IGCSE Physics 0625

3.2.3 Thin Lenses - Image Characteristics

This section describes the characteristics of images formed by converging lenses (also known as convex lenses). We will explore how the position of the object relative to the lens affects the image formed.

Image Characteristics

The characteristics of an image formed by a converging lens depend on the relative position of the object and the lens. Key characteristics include:

• Real or Virtual: Whether the image can be projected onto a screen or not.
• Inverted or Everted: Whether the image is upside down compared to the object.
• Upright or Erect: Whether the image is right-side up compared to the object.
• Magnified or Diminished: Whether the image is larger or smaller than the object.

Image Formation - Ray Diagram

To understand how images are formed, we use ray diagrams. Consider a converging lens. A ray diagram involves tracing two or three rays from a point on the object, which then refract through the lens to form the image.

Object Position and Image Characteristics

The following table summarizes the image characteristics for different object positions relative to the lens:

Object Position	Image Position	Image Type	Image Orientation	Magnification
Object beyond 2f (where f is the focal length)	Between 2f and infinity	Real	Inverted	Diminished
Object at 2f	At 2f	Real	Inverted	Same size as object
Object between f and 2f	Beyond 2f	Real	Inverted	Magnified
Object at f	At infinity	None (image not formed)	N/A	N/A
Object between the lens and f	Behind the lens (virtual image)	Virtual	Upright	Magnified

Magnification

The magnification (M) of an image is defined as the ratio of the image height (h') to the object height (h):

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

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

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

Therefore, we can express magnification as:

$$M = -\frac{v}{u}$$

The negative sign indicates that the image is inverted.

Virtual Images

When the object is placed between the lens and the focal point (0 < u < f), the image formed is virtual. Virtual images cannot be projected onto a screen. They are formed by the apparent intersection of light rays.

Summary

By understanding the relationship between object position, focal length, and the lens formula, we can predict the characteristics of images formed by converging lenses. This knowledge is crucial in applications such as magnifying glasses and cameras.