S4.10.6 For the adult
Lenses
Held far from a window, a magnifying glass puts a small, upside down picture of the window on a sheet of paper. Why is the picture upside down?
What this chapter teaches
A converging lens brings parallel rays to its focus. Three rays fix an image, and 1/f = 1/u + 1/v gives its distance. Short sight needs a diverging lens and long sight needs a converging one. The power of a lens is 1 ÷ f, with f in metres, in dioptres.
Three questions to ask
There is no one right wording. Listen for the idea.
- What are the three rays used to draw the image in a converging lens?
- What kind of lens helps a short sighted person, and why?
- A lens has a focal length of 50 cm. How do you work out its power?
After the chapter, the learner can
- Draw ray diagrams for a converging lens
- Explain how lenses correct short sight and long sight
What they copy into their notebook
Three rays: parallel through the far focus, through the centre straight, through the near focus out parallel. 1/f = 1/u + 1/v. Short sight: diverging. Long sight: converging. P = 1 ÷ f.