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.

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

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

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.

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

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.

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.

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