1. Draw ray diagrams:
An object is standing 3 m away from a convex lens with focal length 2 m.
An object is standing 2 m away from a convex lens with focal length 5 m.
An object is standing in front of a concave lens.

2. Descarte’s Formula Newton’s Formula
and
Newton’s Formula
(with lenses)

3. Descartes’ Formula (with lenses)
f = focal length
di = distance between image and lens
do = distance between object and lens
Note:
f is negative if the lens is concave (diverging)
di is negative if the image is virtual

4. Newton’s Formula (with lenses)
Si = distance between image and the nearest focus
So = distance between object and the nearest focus
Note:
Si and So are always positive – you cannot tell which side of the lens the image/object is.

5. Magnification Factor m = magnification factor hi = height of the image
ho = height of the object
Remember:
m < 1 if the image is diminished
m > 1 if the image is enlarged

6. Using Descartes’ Formula – EXAMPLES
Find the position of the image of an object located 40 m in front of a convex lens with focal length of 32 m.
An object 5 m high is placed 15 m away from a concave lens with focal length of 20 m. Find the position AND the height of the image.

7. Using Newton’s Formula – EXAMPLES
An object 2 m high is placed 10 m from a concave lens with focal length of 18 m. Find the position AND the height of the image.
An object 6 m high is placed 30 m away from a convex lens with focal length 40 m. Find the position, nature and size of the image formed.

9. Lenses formulae worksheet
Homework Booklet: Lenses (Q21 ~ 25)
Textbook: Activity 6D