Differences between lenses and mirrors

• Light reflects from a mirror. Light goes through, and is refracted by, a lens.
  
  
• Lenses have two focal points, one on either side of the lens.
  
  
• A concave mirror converges light to a focal point. For lenses, light converges to a point for a convex lens. A convex mirror diverges light, as does a concave lens.

Any lens that is thicker in the center than the ends is a convex lens. Any lens thicker at the ends than in the center is a concave lens.

Similarities between lenses and mirrors

• The equations we used for mirrors all work for lenses.
  
  
• A convex lens acts a lot like a concave mirror. Both converge parallel rays to a focal point, have positive focal lengths, and form images with similar characteristics.
  
  
• A concave lens acts a lot like a convex mirror. Both diverge parallel rays away from a focal point, have negative focal lengths, and form only virtual, smaller images.

The sign convention is just a little different. Because the light goes through the lens positive image distances (and real images) are on the opposite side of the lens from the object. Negative image distances are for virtual images, again, but those are on the same side of the lens as the object.