Waves - 6.2.5 Lenses (Physics Only) (GCSE Physics AQA)

 Lenses (Physics Only)

Convex Lenses

• Lenses form images. We can use lenses in everyday life to form images. These lenses work by refracting light (a principle that we covered earlier in this chapter). When a lens refracts light, it will form an image.
• Convex lenses use a principal focus. If the lens is convex (), then any parallel rays of light will brought to focus at a certain point. This point is called the principal focus. As you can see from the diagram, all the rays are brought together at one point, which is the principal focus.
• There is a focal length. Since we have a lens and a principal focus, we also have a length called the focal length. This is the distance between the lens and the principal focus, or in the diagram, this is the length from the letter ‘L’ to the letter ‘F’.
• We can use ray diagrams to represent this. In order to represent the effects of a convex lens, we can use a ray diagram such as the one below.

Images Produced by Concave Lenses

• Images can be real or virtual. When we talk about the images produced by a lens, we always refer to real images and virtual images. If the lens is convex, then the image produced can be real or virtual.
• Concave lenses form virtual images. If a lens is concave, then the image produced is always virtual.

Representing Convex and Concave Lenses

We can represent convex and concave lenses in the following way.

Convex VS Concave Lenses

We can construct ray diagrams to illustrate the similarities and differences between convex and concave lenses, as shown below.

Magnification

Magnification is a ratio and so has no units.

Calculating Magnification

We can calculate magnification using the following formula:

Where:

• Image height in cm or mm (if the object height is in mm, then the image height must be in mm).
• Object height in cm or mm (if the image height is in cm, then the object height should be in cm).

Magnification has no units since it is a ratio.

Question: An object that is 5cm tall produced an image 250cm tall. What is the magnification?

1. Write out the equation.

In this instance, our equation will be magnification = image height / object height.

2. Substitute in the numbers.

magnification = image height / object height
magnification = 250 / 5
magnification = 50