To get a magnification of 0.2X for any object, you use a convex lens that has a focal length of 7.5mm. How far must you place the object from the lens? 

 

Answer:

4.5 cm (45 mm)

Step-by-step explanation

For a lens we can write

1/?f?=1/v?−1/u??

??1/v?=1/f?+1/u?=f+u?/uf?

??v=uf?/u+f?

So magnification by lens is given as

?m=v/u???=? ?uf/u+f/u??=f/u+f??

As a diminished image is formed when the objec tis beyond 2f so here image will be diminished and inverted. So

Here, m = -0.2 and f = 7.5mm

S we can write

-0.2 =7.5/ ?7.5+u??

???−0.2u−1.5=7.5?−0.2u=9?u=−45mm?

Or -4.5cm

So object should be placed at 4.5 cm in front of lens.