Draw a convex lens with principal optical axis, focus and focal length.

What shape does a piece of ice have to be in order to set fire to paper?

Looking through the windows of old houses, objects seem stretched out and distorted. Why?

How can you measure the focal length of a convex lens using sunlight?

Draw the path of the rays through the lenses.

What is the optical power of a lens whose focal length is $50cm$ away from the lens?

A compound lens with a focal length of $40cm$ consists of a convex lens with a focal length of $25cm$ and a concave lens. What is the optical power of a concave lens?