(2012 Zigong, Sichuan, Question 31) When an object is moved from 12 cm to 18 cm from a convex lens, the position of the light screen is adjusted so that the focal length of this lens is always (B．10 c

(2012 Zigong, Sichuan, Question 31) When an object is moved from 12 cm to 18 cm from a convex lens, the position of the light screen is adjusted so that the focal length of this lens is always (B．10 cm). The focal length of this convex lens may be (B. 10 cm)

Derivation process is as follows:

∵ object from 12 cm from the convex lens to 18 cm from the convex lens in the process of adjusting the position of the light screen, can always be inverted on the light screen to get the magnified image

∴ According to the law of imaging of convex lenses can be obtained:

f <12 <2f

< p>f<18<2f

Arranging the above two equations, we can get:

6

9

∴ 9

Looking at the question, we see that the question gives the following four options:

A.6cm B.10 cm C.16 cm D.20 cm

It is easy to conclude that only B is correct.

I hope this helps, if you have any questions, please ask~~~

Good luck with your studies! (*^__^*)