When a beam of light meet the boundary between two different media, some light is reflected back to the same media (law of reflection) some light is refracted to a different media (law of refraction). You might want to check out another java applet The Transmission of Wave through Dense media -- Reflection and Refraction

The same laws applied when light wave impinging the surface of a lens. This java applet let you play with a thick lens. For thin lens and mirror please check out Thin Lens (converging/diverging lens/mirrors) java applet

How to play:

You can enter the index of refraction of the lens into the Text Field.

The gray ellipse area represents the thick lens.

You can click the left mouse button within one of four cyan circle around the lens

and drag the mouse button to change the shape of the lens.

You can click the circle at the center of the ellipse and drag the mouse left/right to move the lens horizontally. Light beam entering from the left of the screen. You can click at the beam and drag it up/down. Hit Reset button for default conditions.

Approximate indices of refraction of various substance