lenses

converging lens image (object farther than F)

image is real (back side of lens), inverted

converging lens image (object closer than F)

image is virtual (front side of lens), upright, larger

diverging lens image

image is virtual (in front of lens), upright, smaller

converging lens focal length

positive

divergine lens focal length

negative

converging lens ray diagram (object is farther than focal point)

(blue) refraction of ray parallel to principal axis passes through focal point behind lens, (red) ray intersects principle axis at center of lens and reflects back on itself, (green) refraction of ray diagonally intersecting bottom of lens is parallel to to principal axis behind lens

converging lens ray diagram (object is closer than focal point)

image is behind object, all lines converge at image; (blue) refraction of ray parallel to principal axis intersects focal point behind lens, (red) line appears to originate from image and intersects principal axis through center of lens, (green) line appears to originate from image and is parallel to principal axis

diverging lens ray diagram

(blue) refraction of ray parallel to principal axis reflects upward away and appears to originate from the focal point in front of the lens, (red) intersects principal axis at center of lens and reflects back on itself, (green) ray diagonal to principal axis refracts parallel to optical axis behind lens