Question

two lens of power 3d and -5d are placed In contact to form a composite lens.an object is placed at a distance 50 cm from this composite lens.find the position of the image?

Answer 1

Answer:

-2.5cm

Explanation:

total power = 1/focal length

Answer 2

The position of the image is 18 cm.

Explanation:

The relation between focal length(f) and power(P) is given as,

$f=\frac{1}{P}$

As in question, two lens of power 3 D and -5 D given.

Therefore, total focal length of composite lens

$f =\frac{1}{3D} +(\frac{1}{-5D} )$

$f=\frac{2}{15} m$

To calculate the position of image use lens formula as,

$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$

Here, v is the image distance and u is the object distace.

Given, $u = -50 cm=\frac{1}{2} m$.

substitute the values, we get

$\frac{1}{\frac{2}{15} } =\frac{1}{v} -\frac{1}{\frac{1}{2} }$

$\frac{1}{v}=(\frac{15}{2}-2) m$

$v =\frac{2}{11}m$

v = 0.18 m = 18 cm.

Thus, the position of the image is 18 cm.

#Learn More: lens formula.

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