Physics quiz question: object just outside converging lens focal point

Physics 205B Quiz 2, spring semester 2019
Cuesta College, San Luis Obispo, CA

A converging lens has a focal length of +10.0 cm. An object 0.50 cm in height is placed at a distance of 12.5 cm in front of the lens, which will produce a(n) __________ image.
(A) diminished, real.
(B) enlarged, real.
(C) diminished, virtual.
(D) enlarged, virtual.
(E) (No image would be produced.)

Correct answer (highlight to unhide): (B)

Solving for the location of the image d_i:

(1/d_o) + (1/d_i) = (1/f),

(1/d_i) = (1/f) – (1/d_o),

(1/d_i) = (1/(+10.0 cm)) – (1/(+12.5 cm)) = +0.0200 cm^–1,

d_i = 1/(0.02 cm^–1) = +50.0 cm,

where the positive sign by convention makes this a real image located to the right side of the lens (the side of the lens opposite the original object). The linear magnification m is given by:

m = h_i/h_o = –d_i/d_o = –(+50.0 cm)/(12.5 cm) = –4.00,

which means that this real image is enlarged, being four times the size of the original object (and will be inverted, due to the negative sign).

Sections 30882, 30883
Exam code: quiz02BTLn
(A) : 10 students
(B) : 23 students
(C) : 3 students
(D) : 4 students
(E) : 1 student

Success level: 56%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.53