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an object is kept at a distance of 18cm,20cm,22cm,30cm,from lens of power +5D.

(i)in which case would you get a magnified image.

(ii)which magnified imagecan be get on the screen

Power=+5D

so the lens is convex

P=1/f

f=1/5 m

f=20 cm

so in each case

1/v-1/u=1/f

1/v=1/u +1/f

v=uf/(u+f)

v=20u/(20+u)

now u varies from 

-18,-20, -22,-30 cm

so,

v=-180 cm

if u=-18 cm

so, magnification is (v/u)=+10

similarly

v=+220 cm if u=-22 cm

so magnification (v/u)=-10

v=+60 cm ,if u=-30 cm

so magnification is (v/u)=-2

if u=-20 cm

v=infinity

so the magnification=-infinity

now magnification is positive in 1st case

where u=-18 cm

all the other images can be obtained on screen

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Power of a lens is the degree to which a lens can diverge or converge the light passing through it. It is the reciprocal of focal length. Its unit is dioptre or m⁻¹.

b)power of lens = 5D
P = 1/f
⇒ f = 1/P = 1/5 = 0.2m = 20cm
So focal length = 20cm

you will get a magnified image when object is between f and 2f. So you will get magnified image in case of 22cm and 30 cm.
In case of 18cm, virtual, magnified image will be formed.
In case of 20cm, image will be formed at infinity.

So you can get magnified image on a screen in case of 22cm and 30cm.

c)
-used in spectacles to help people with visual imparment
- used in telescopes, microscopes etc.

 
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