An object is placed between a magnifying lens and its focal point. (a) Show that
ID: 3896656 • Letter: A
Question
An object is placed between a magnifying lens and its focal point.
(a) Show that if the object is between the lens and half the focal distance (i.e. p < 1?2 f ), then the image
will be between the lens and the focal point (i.e. q < f ). Will the image be real or virtual?
Inverted or upright?
(b) Suppose the object is placed a distance 1?2 f from the lens. What will the magnification be? What
will the angular magnification be, assuming the near point to be at N?
(c) Where should the object be placed if the image is to be a distance 2f from the lens? What would
the magnification be in this case? What is the angular magnification?
(d) Now suppose the object was put just inside the focal distance from the lens (set p = xf where x is
smaller than 1, but very close to 1). What would be the distance of the image from the lens? What would be the magnification? What would be the angular magnification?
Explanation / Answer
During this experiment record the positions of the lenses, objects, and images. Then,
using these data determine the object and image distances, etc. The positions are data
whereas the object and image distances, etc. are calculations.
Record the uncertainties in all your measurements as usual. The uncertainty in the
position of an image can be estimated as one-half the range over which the image is
judged to be in focus, whenever this value is larger than the reading uncertainty of the
screen position.
1) Determine the approximate focal length of the convex lens by focussing on a
screen an image of a distant object (at least 10 metres away), and calculating the
distance between the lens and screen.
2) Measure the object size. Set up the lens on the middle of the optical bench, with
the object on one side and the image screen on the other. Determine the object distance
p and image distance q for the following object positions and record in each case
whether the image is real or virtual, upright or inverted, larger or smaller than the object.
Also measure the image size (real images only!).
a. p > 2 f
b. p = 2f
c. 2f > p >f
d. p < f
For cases in which the image is virtual, positioning a pointer on the same side of the lens
as the object and viewing the object through the lens and the pointer above the lens may
find its location. Adjust the pointer until it coincides with the image: this position of zero
parallax is reached when the image and the pointer stay together as the viewer