1. Lenses This Presentation was used for Year 12 students.
Note: This PowerPoint is one of over 60 that were developed and used at John Paul College, Rotorua between 1996 and 2002 by D Bradley. They were primary designed as teacher directed resources.

2. The word “lens” comes from the Latin word for lentils.
Picture of lentils.

3. Lenses Diverging Lens Or Double Concave lens Converging Lens Or
Virtual Focal point!
Diverging Lens Or
Double Concave lens
Converging Lens Or
Double Convex lens F F f f

4. Thin Lenses
Thin lenses are those whose thickness is small compared to their radius of curvature. They may be either converging (a) or diverging (b).

5. Double Convex Lens
Focal Point

6. Lens = refraction x2

7. Light passes through a lens
Ray Tracing for Lenses
Light passes through a lens
There is a focal point on both sides of a lens
Converging Lens:
Ray #1: Parallel to the axis Refracts through F
Ray #2: Through F Refracts parallel to axis
Ray #3: Through Center of lens undeflected

8. Ray diagrams Inverted Real Enlarged Double Convex
Converging Thin Lens: do > f
Inverted
Real
Enlarged
Real Image
Ray diagrams

9. Double Convex Thin Lens: do > fdi
Similar Triangles ho f f hi

10. Converging Lens Practical Activity
Equipment:
meter ruler
candle
Converging lens (f ~ 20 cm ???)
Object location
do > 2f
do = 2f
2f > do > f
For the 3 object locations listed
~ Sketch a ray diagram
~ state the nature of the image
~ record di (so as to be able to calculate f)
do = f
do < f
Investigate the image produced for

11. Example: Camera

12. Example: Projector

13. See 5 Lens applet

14. Find the focal length of a converging lens by holding it up to a window.
(See how far away from the lens you need to hold a piece of paper to focus the image on the paper.)

15. A camera with a focal length of 50 mm takes a photograph of a 100 m tall building from 350 m away. How tall is the image on the film?

16. Converging Thin Lens: do < f
Double Convex
Converging Thin Lens: do < f f
f ‘
Virtual
Upright
Enlarged R1 R2 do di

17. Example: Magnifying Glass
Web Link: Ray tracing

18. Convex thin lens: do < f
Similar Triangles ho hi di
negative do

19. A lens is used to make an image; three light rays are drawn out of the infinite number coming from the arrowhead. The image given in this sketch could be seen: f
image f
object
By placing a screen at the image point.
Without a screen by looking back at the lens.
By both techniques (1) and (2).
Only if the lens is big enough.
None of the above answers is 100% correct.

20. A screen is placed at the position of the image, and a “sharp” image appears on the screen. Next, Jennifer moves the screen a SHORT distance TOWARD the lens. The image would appear: f
image f
object
Smaller and “sharper”.
Smaller and “fuzzier”.
Larger and “sharper”.
Larger and “fuzzier”.
Would disappear.

21. Finally, Jennifer blocks half the lens, as shown, with a piece of paper. What happens to the image?
object
Paper blocks half of lens
It disappears.
Only half of it is still seen
It looks the same, but gets slightly dimmer.
It gets fuzzy.
It depends on what part of the lens is blocked.

22. Object distance > 2f: Image is real, smaller, and inverted
Results: Ray Tracing for Converging Lenses (in each case, draw in the 3 rays for practice)
Object distance > 2f: Image is real, smaller, and inverted F 2F
Object between f and 2f: Image is real, larger, inverted F 2F
Object between f and lens: Image virtual, larger, upright F 2F