http://www.ck12.org Chapter 20. Geometric Optics
FIGURE 20.19
A convex lens with the object in front of
the focal point, F’.- The object distance is positive.
- The image distance is positive if the image is on the opposite side of the lens from the object. If the image is
on the same side as the object, the image distance is negative. - An image that forms on the opposite side of the lens from the object is a real, inverted image. An image that
forms on the same side of the lens as the object is a virtual, upright image. - The object height is always positive. The image height is positive when upright and negative when inverted
(as referenced to the object).
Concluding remarks for converging lenses
An object placed beyond the focal point will always form real and inverted images.
An object placed closer to the lens than the focal point will always form virtual, upright, and enlarged images.
Illustrative Example 21.4.1
An object of height 12.0 cm is placed 30.0 cm from a converging lens of focal length 10.0 cm (seeFigure20.18.).
a. Find the image position.
Solution:
We are givenho= 12. 0 cm,do= 30. 0 cm,and f= 10. 0 cm
Using the Lens Equation,
1
do+
1
di=
1
f→
1
30. 0
+
1
di=
1
10. 0
→
1
di=
1
10. 0
−
1
30. 0
=
30. 0 − 10. 0
300 cm→
di= 15. 0 cmSince the image distance is positive, the image is real.
b. What is the magnification?
Solution:.