Physics · Ch 6 — Optics
Simple Microscope
Simple Microscope
A simple microscope is a single magnifying (converging) lens of small focal length, used to obtain an erect, magnified, virtual image; the object is placed between the pole and the focus of the lens, and viewed from the other side. Two kinds of focusing are possible: near point focusing, where the image forms at the near point (the least distance of distinct vision, 25 cm for a normal eye), a comfortable but slightly strained viewing condition; and normal focusin …
What this figure shows. An object placed between a converging lens and its focus is viewed through the lens with the eye focused on its own near point; the virtual, magnified image is shown formed at distance D (the standard 25 cm near point) from the eye. This is the near-point-focusing configuration, giving the eye a comfortable if slightly strained view, and the magnification formula m = 1 + D/f i …
What this figure shows. Panel (a) shows the unaided eye viewing an object of height h directly at the near point D, subtending angle theta-zero approximately equal to h/D. Panel (b) shows the same object viewed instead through a simple magnifying lens, now placed so the image forms at infinity (the object sits exactly at the lens's focus); the eye, relaxed and focused at infinity, sees the image subtending the larger angle theta-i approximately equal to h/f. The angular magnification m = theta-i/the …
Worked out. A person with near point D = 25 cm uses a convex lens of focal length f = 5 cm as a magnifying glass. For the closest usable object distance (image at the near point, v = -25 cm), the lens equation gives u = -25/6 = -4.167 cm; for the farthest usable object distance (image at infinity, normal focusing), the lens equation gives u = -f = -5 cm. So the book must be held somewhere between 4.167 cm and 5 cm from the lens for the text to stay in focus. The corresponding magnifications are m = 1 + D/f = 1 + 25/5 = 6 (near-point focusing, the maximum magnification) and m = D/f = 25/5 = 5 (normal focusing, the minimum, more comfortable magnification) -- illustrating the small but real trade-off between maximum magnification and comfort …