Constructing images with a thin lens
To construct an image three convenient rays are used: a ray parallel to the principal optical axis (after the lens it passes through the focus), a ray through the focus (after the lens it is parallel to the axis) and a ray through the lens centre (not refracted). Their intersection is the image point. The lens formula is 1/F = 1/d + 1/f (d – object to lens, f – image to lens) and the linear magnification is Γ = f/d = H/h. For a converging lens with d > 2F the image is real, inverted and reduced; at d = 2F it is the same size; for F < d < 2F it is enlarged; at d = F no image forms; for d < F the image is virtual, upright and enlarged (magnifier). A diverging lens always gives a virtual, upright, reduced image; for it F and f are taken as negative. A real image can be seen on a screen, a virtual one only through the lens with the eye.
Draw in your notebook: a lens with F = 4 squares and objects at d = 6, 8 and 3 squares; construct the image with three rays for each. Name the kind of image in each case and check with the formula.