Question

*PLEASE HELP*

When an object is placed in front
of a convex lens, it creates a virtual
image at -12.8 cm with a
magnification of 2.85. What is the
focal length of the lens?
(Mind your minus signs.)
(Unit = cm)

Answer 1

6.92

Step-by-step explanation:

2.85=-(-12.8/x)

do=4.49

1/f= 1/4.49 + 1/-12.8

f=6.92

Answer 2

Answer: The focal length of the lens = 3.32 cm.

Step-by-step explanation:

Lens formula :
`\frac1f=\frac1v-\frac1u` (i)

f= focal length , v=image distance , u =object distance.

magnification: m =
`(v)/(u)` (ii )

Given: v= -12.8 cm , m =2.85

Put values in (ii), we get

`2.85=(-12.8)/(u)\\\\\Rightarrow\ u=(-12.8)/(2.85)\\\\\Rightarrow\ u=-4.49\ cm`

substitute values of u , v in (i)

`\frac1f=\frac1{-12.8}-(1)/(4.49)\\\\\Rightarrow\frac1f=-0.30084214922\\\\\Rightarrow\ f=(-1)/(-0.30084214922)\approx3.32\ cm`

Hence, the focal length of the lens = 3.32 cm.