a convex lens has a focal length of 20cm .find the position of the object for which the image will be half the size of the object.
Answers
Answered by
10
Answer:
-20
Explanation:
Given:
Focal length = 20 cm
Magnification =
To find:
- Position of the object for which magnification is
So now using the formula:
Cross multiplying:
-2V=u
Using the other formula which says:
Focal length = f = 20 cm (given)
We derived and got
And we have to find u
u = -20 cm
It has to be placed at a position of 20 cm for the image to be half size of the object
Answered by
19
Given :
- Focal length of Convex lens is 20 cm
- Magnification is -1/2
To Find :
- Object Distance when magnification is 1/2
Solution :
Use formula for magnification of lens :
➠ m = -v/u
⇒1/2 = -v/u
⇒-v = u/2
⇒v = -u/2 ...(1)
___________________________
Now, use lens formula :
➠1/f = 1/v + 1/u
⇒1/20 = 1/-u/2 + 1/u
⇒1/20 = -2/u + 1/u
⇒1/20 = -1/u
⇒-u = 20
⇒u = - 20
Object is placed at distance of 20 cm from lens, negative sign indicates that object is placed on left side of lens .
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