A birdwatcher takes a small refracting spotting scope on a trek
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 birdwatcher takes a small refracting spotting scope on a trek, together with a new pair of spectacles.
((a)) The scope's objective is the aperture stop: a converging thin lens of focal length fo=300 mm and diameter D=60 mm. The eyepiece is a converging thin lens of focal length fe=20 mm, placed so that the tube is afocal (lens separation fo+fe). Using the thin lens equation 1/so+1/si=1/f under CV1 (real object and real image both positive, left to right), treat the objective rim as the object for the eyepiece and find the position of the exit pupil (its distance from the eyepiece, real or virtual) and its diameter.
((b)) On the trip the birdwatcher finds her far point has receded to 80 cm. Find the dioptric power of the spectacle lens that images a distant object at her far point. The lens is ground from crown glass of refractive index 1.50, with its front surface of radius R1=+50 cm (CV1: R positive when the centre of curvature lies to the right of the vertex). Use the lensmaker's equation D=(n−1)(1/R1−1/R2) to find R2.