PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Geometrical optics and imaging · Geometrical optics and imaging
Optics and Photonics, Ch Stops, pupils and optical instruments
Problem
A reader's near point has receded to 600 mm, so a book held at a comfortable 250 mm can no longer be focused. Two remedies are considered. Treat every lens as thin, the eye as placed close behind the lens, and rays as paraxial; a virtual image lies on the object side with si<0.
((a)) Reading spectacles: what power (in dioptres) images a page at 250 mm from the lens at the reader's near point, 600 mm in front of it?
((b)) A hand magnifier instead, used with a relaxed eye (final image at infinity), is to make the print subtend four times the angle it subtends when the page is held at this reader's own near point of 600 mm. What focal length is needed?
((c)) The maker engraves on that same lens the angular magnification computed with the standard near point d0=250 mm. What number is engraved? Which of the two numbers describes what this reader gains?