# An 80 mm focal-length lens is used to focus an image for a camera, a) how far is the image from the lens if the object to be photographed is 10 m away? b) If the object is 3.0 m away? c) If the...

An 80 mm focal-length lens is used to focus an image for a camera, a) how far is the image from the lens if the object to be photographed is 10 m away? b) If the object is 3.0 m away? c) If the object is 50 cm away? D) If the maximum distance from the lens to the image is 120 mm, what is the closest object on which this lens could focus?

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### 2 Answers

In Camera we use convex lense.

for convex lense as convention u is (-),vand f are (+)

and lense formula is

1/v-1/u=1/f

a. f=80mm=.08m

u= 10 m

Thus `1/v=1/.08+1/10`

`=1008/80`

`1/v=1008/80`

`v=80/1008`

`=.079m ` behind the lense.

b. same u= 3m

`1/v=1/.08+1/3`

`1/v=308/24`

`v=.078m ` behind the lense

C.u=50cm=.5m

`1/v=1/.08+1/.5`

`1/v=116/8`

`v=8/116x^(m+n)`

D. v=120 mm

v=.12m

1/.12=1/.08+1/u

100/12-100/8=1/u

-400/96=1/u

u=-96/400

=-.24 m

Ans.

(a) 81 mm

(b) 82mm

(c) 95mm

(d) 240 mm = 24 cm