1. Lenses

2. Lenses
You have two lenses on your bench, one that is thick in the middle (converging) and one that is thin in the middle (diverging). Try looking through them.

3. Converging and diverging lenses
WHY?

4. Principal axis
The straight line that goes through the centre of the lens at right angles to the lens surface

5. Principal focus
Rays parallel to the principal axis, after being refracted by the lens, will all pass through a point on the principal axis called the principal focus.

6. Focal length
The distance from the principal focus to the centre of the lens (denoted by the symbol f)

7. Finding the focal length - a quick practical
Use the “distant light” method to show the students how to find the focal length. Give them a number of unmarked lenses that they have to find the focal length to.

8. Real and virtual images
A real image is where the rays actually pass through the image and it can be projected and seen on a screen

9. Real and virtual images
A virtual image is where no rays of light pass through, only their mathematical extensions. It cannot be displayed on a screen

10. Let’s try it! – Investigating images practical

11. Let’s construct some ray diagrams to find out!
Why?
Let’s construct some ray diagrams to find out!

12. Drawing ray diagrams

13. Drawing ray diagrams

14. Drawing ray diagrams

15. Drawing ray diagrams

16. Drawing ray diagrams
You can check by drawing a ray backwards from the object parallel to the axis which has passed through the first principal focus.

17. Drawing ray diagrams

18. Now construct the following 5 diagrams
An object at greater than 2F
an object at 2F
an object less than 2F (between F and 2F)
an object at F
an object at less than F

21. Virtual image

22. Let’s try some questions