5. Describe how lenses focus light

Prior Learning

Introduce the basic types of lenses, convex and concave, and their distinctive shapes. The students should understand the difference between the two types and their effects on light.

Starter

Ask the students to think of five objects that contain lenses. They may be able to find some examples in the room.

Main Teaching

Read or watch through the presentation, looking in particular at how a convex lens bends light towards a focal point. Consider why different lenses have different focal points. Draw accurate ray diagrams to show how light behaves in a convex lens.

Career Film: Crossrail Engineers: Rebecca Hughes - Apprentice Field Engineer

Expert Film: Jon Badgery demonstrates how lenses can be used to bend and focus light.

Mission Assignment

Build a Refracting Telescope

Firstly, the students must find the focal length of their lenses. This works best when done directly under fluorescent tubes.

1. Hold the lens directly under the florescent lighting above a clear plain desk. Make sure it is flat.
2. Move the lens up and down until the image of the lights is clear and focused on the surface. It should cast a sharp image of the lighting unit.
3. Use a set square to keep the ruler perpendicular to the desk and measure the distance from the desk. This is the focal length of that lens.

Then, to build the telescope, you need to:

1. Using modelling clay, stick lens 1 its focal distance away from the end of the ruler.
2. Add together the focal lengths of lens 1 and 2.
3. Stick lens 2 this distance apart from lens 1 on the ruler.
4. Bring the ruler with the lenses to the end of a table and look through your telescope.
5. Experiment looking at objects that are close and some which are further away.

Mission to Write - Eye of the Tiger
The students can look at the image on the associated handout and write about how two lenses work. The first one is the lens in the eye of the tiger itself. The students may want to do this after the lesson 'Describe the parts of the human eye' as this focuses on how animal and human eyes work.

The second one to write about is how the lens on the camera works by detecting light.  Students should be encouraged to use the lesson’s keywords and check the accuracy of their spelling, punctuation and grammar.

Differentiated tasks (Support/Challenge)

Support: Before/instead of completing the telescope activity, ask the students to build a simple magnifying glass. Provide each student with a small convex lens (eg, a magnifying glass lens) and a small object such as a coin or a printed text. Ask them to hold the lens between their fingers and position it over the object. Instruct them to move the lens closer to or farther away from the object until they can see a clear, magnified image. Encourage the students to observe how the lens changes the size and clarity of the object.

Challenge: Ask the students to research and present information on different types of lenses used in refracting telescopes. They can explore the advantages and disadvantages of various lens materials, shapes, and designs. This will deepen their understanding of optics and give them a broader perspective on telescope construction.