6.2.4 Lenses

In this lesson, we will explore the properties and behaviour of convex and concave lenses. We will learn about the principal focusWhat the writer draws attention to at a given moment (e.g., setting, character, detail)., focal length, and the use of ray diagrams to illustrate image formation. Additionally, we will delve into the concept of magnificationThe number of times larger an image appears compared to the real object. and how it can be calculated using the image and object height measurements. A lens forms an image by refracting light.

Convex Lenses

A convex lens is thicker at the centre and thinner at the edges.

• Principal Focus: Parallel rays of light passing through a convex lens converge and come to a focus at a specific point called the principal focus.
• Focal Length: The distance from the lens to the principal focus is referred to as the focal length.
• Ray Diagrams: Ray diagrams are graphical representations used to illustrate the formation of images by convex lenses.

Image Formation by Convex Lenses:

• Real Image: If the object is placed beyond the focal point, a real image is formed on the opposite side of the lens. It can be projected onto a screen and is inverted.
• Virtual Image: If the object is placed within the focal length, a virtual image is formed on the same side as the object. It cannot be projected onto a screen and is upright.

Concave Lenses

• A concave lens is thinner at the centre and thicker at the edges.
• Principal Focus: When parallel rays of light pass through a concave lens, they appear to diverge. Extending the diverging rays backward gives the illusion of them coming from a common point, known as the virtual principal focus.
• Ray Diagrams: Similar to convex lenses, ray diagrams can be used to demonstrate image formation with concave lenses.

Image Formation by Concave Lens:

• Virtual Image: A concave lens always forms a virtual image. The image is erect, reduced in size, and cannot be projected onto a screen.

Ray Diagrams for Lenses

Ray diagrams are graphical tools that help visualise the path of light rays and determine the characteristics of the image formed by a lens.

• For convex lenses, at least two rays (e.g., parallel ray and central ray) are drawn to intersect at the principal focus or appear to diverge from it.

• For concave lenses, the rays appear to diverge from the virtual principal focus or can be extended backward to show their path.

Magnification Calculation

Magnification is a measure of how much larger or smaller an image appears compared to the object. It can be calculated using the equation:

magnification = image height / object height

The magnification is a ratio and has no units. Both the image height and object height measurements should be taken in either millimetres (mm) or centimetres (cm).

Conclusion

Lenses play a vital role in forming images through the refraction of light. Convex lenses converge light rays to a principal focus, creating real or virtual images depending on the object's position. Concave lenses cause light rays to appear to diverge, resulting in the formation of virtual images. Ray diagrams are useful tools to illustrate the image formation process for both convex and concave lenses. Magnification, a measure of image sizeThe measured size of an object as seen under a microscope. compared to the object, can be calculated using the image and object height measurements.