1. Starter
Number the sentences in the correct order to explain why we see the pencil as bent in the water. 5 2 3 1 4 6

2. Learning Goals
By the end of this lesson you should be able to…
Describe the path of light when it enters a thin converging or a thin diverging lens.
Describe the images formed by convex and concave lenses.
Draw simple ray diagrams to show the principle axis, focal point, focal length and focus.
Explain the effect of increasing the thickness of a lens on its strength
Apply the equation for magnification of a lens.

3. What have these all got in common?

4. Diverging Converging Causes rays to disperse from a central focus.
Brings parallel rays into focus (causes them to converge to a point)

5. Will each lens be diverging or converging?

6. What do you notice about the lenses which are diverging / converging?

7. All diverging lenses are wider at the edges than in the centre.
All converging lenses are wider in the centre than at the edges.

8. Light travelling through a lens is refracted twice; once when it enters the lens and once when it leaves.
In the case of a thin lens, it is possible to neglect this double bending.

9. What effect does a convex lens have on parallel rays of light?
Principal Axis
Principal Axis F
Focal point
They will converge to a point which will call the focal point (F). If you put a screen at the focal point you would focus a real image onto the screen.

10. What effect does a convex lens have on parallel rays of light?
Principal Axis
Principal Axis F
Focal Length
The distance from the center of the lens to the focal point is called the focal length of the lens (f).

12. What effect does a convex lens have on diverging rays of light?
If a light is placed at the focal point of a convex lens it will produce parallel light

13. What effect does a concave lens have on parallel rays of light?
Principal Axis F
Causes them to disperse, so that they will never converge.
Thus when we talk about the focal point for a diverging lens it is a virtual image and must be found by extending the diverging rays backwards.

14. What effect does a concave lens have on parallel rays of light?
Focal Length
The distance from the centre of the lens to the focal point is called the focal length of the lens (f).

15. Real
Virtual
Which type of image do you need in a projector or camera?

16. Converging / diverging lens Principal axis Principal focus
Continue the 3 rays on each diagram through the lens and label the following:
Converging / diverging lens
Principal axis
Principal focus
Focal length

17. Experiment: Finding the focal length of a lens
Experiment: Finding the focal length of a lens. Method Find the focal length (F) of each of the lenses by focussing a distant object ‘at infinity’ onto the white screen. The distance between the midpoint of the lens and the screen is the focal length.

18. SAFETY
Be careful the ray boxes get very hot when on, so do not touch the bulbs.

19. Experiment: Investigating the image size and position with a convex lens.
Method: Using the lens with focal length F=15cm investigate the image formed to complete the table. Use the words, enlarged, same size, diminished, upright or inverted.
Object position
Object height
Image Height
Magnification (image height / object height)
Description of image and position
Further than 2F
Diminished, real, inverted Between 2F and F
At 2F
Between 2F and F
At F
Closer than F

20. Object position
Type of image
Real/Virtual
Inverted/ Non-inverted
Position of image
Further than 2F
Smaller / Diminished
Real
Inverted
Between 2F and F
At 2F
Same sized
Larger / Enlarged
At F
More than 7F
Closer than F
Larger/ Enlarged
Virtual
Non-inverted/ Upright
On the same side as the object

21. Lens Simulation http://ophysics.com/l12.html
Move position of rhs Principal Focus to lhs to convert to concave lens.

22. Extension
Using p152/3 (Complete Physics) draw a ray diagram to show how a convex lens produces an image if the object is at 2F and as the object moves closer or further away from the lens.

23. Modelling a lens worksheet

24. Diverging or Converging Lens