Lens

The power of the lens equal to reciprocated of the focal length in meter or 100/f for focal length in cm (unit : dioptri/ diopter)
Convex Lenses
Thicker in the center than edges, lens that converges (brings together) light rays.
There are 3 kinds of convex lenses : plan(o)convex, biconvex and concav(o)convex
Concave Lenses
Lens that is thicker at the edges, diverges light rays. There are 3 kinds of concave lenses plan(o)concave, biconcave and convex(o)concave

Ray tracing for convex lens

Ray #1: Light ray comes from top of object; travels parallel to principal axis will be refracted thru focal point.
Ray #2: Light ray comes from top through center of lens is undeviated.
Ray #3 : Light ray comes to the focal point will be refracted to the principal axis
Special for convex lens, we can categorized 5 zone of object and image which simplify us to know the characteristics of the image:
1. Object in zone 1 (from the lens to the focal point) will create image in zone 4 (at the same zone as the object) which means virtual, magnified and upright
2. Object in zone 2 (from the focal point to the 2 times of focal point) will create image in zone 3 (from the 2 times of focal point or more) which means real, magnified and upside down
3. Object in zone 3 (from the 2 times of focal point or more) will create image in zone 2 (from the focal point to the 2 times of focal point ) which means real, reduced and upside down
4. Object in the focal point will create infinity image
5. Object in the two times of focal point will create the image at the same place which means real, same size and upside down.

Concave lens and its ray tracing

Ray #1: Light ray comes from top of object will travelparallel to principal axis will be refracted, as if it comes from the focal point.
Ray #2 : Light ray comes from top of object travels through center of lens will be undeviated.
Ray #3 : Light ray to the focal point will be refracted parallel to the principal axis.

Now compare your ray diagram with below formula

where f for the focal length, s for the distance of the object and s1 for the distance of the image. Do not forget to value virtual image and virtual focal point as a negative value. Then you will find the same result of calculation and diagram. For the magnification you can find by dividing the image distance with object distance or image height with object height.

Lens Exercise
1. Draw the ray diagram when an object was placed in front of convex lens at zone 1
2. Draw the ray diagram when an object was placed in front of convex lens at zone 2
3. Draw the ray diagram when an object was placed in front of convex lens at zone 3
4. Draw the ray diagram when an object was placed in front of convex lens at focal point.
5. Draw the ray diagram when an object was placed in front of concave lens at zone 1
6. Draw the ray diagram when an object was placed in front of concave lens at zone 2
7. Draw the ray diagram when an object was placed in front of concave lens at zone 3
8. Draw the ray diagram when an object was placed in front of concave lens at focal point.