1. The focal length of a lens
© D Hoult 2009

2. object

3. object

6. real
image

14. real
image
real
image

18. rays from one point
on a distant object

19. rays from one point
on a distant object

20. rays from one point
on a distant object

21. rays from one point
on a distant object
Rays from one point on a distant object are very nearly

22. rays from one point
on a distant object
Rays from one point on a distant object are very nearly parallel

23. rays from one point
on a distant object
Rays from one point on a distant object are very nearly parallel
The focal length of a lens is

24. rays from one point
on a distant object
Rays from one point on a distant object are very nearly parallel
The focal length of a lens is the image distance when the object distance is 25 m or more

25. rays from one point
on a distant object
Rays from one point on a distant object are very nearly parallel
The focal length of a lens is the image distance when the object distance is 25 m or more