Question

A needle of length 6 cm is held in front of a spherical lens of focal length 15 cm Image of this needle is brought to sharp focus on a screen kept 25 cm away from the lens. Find position of object, nature and size of the image. ​

Answer 1

Answer:

We get the image distance due to the lens using lens formula,

v

1

−

u

1

=

f

1

or

v

1

=

15

1

−

30

1

or

v=30cm in front of the convex mirror, which means 20cm behind the mirror. Assuming mirror to reflect from both the sides and this image will act as virtual object for mirror and second image, I

2

will be formed at 20 cm in front of mirror. That is 10 cm in front of lens.

Understanding the convention used while taking +ve and -ve sign for image and object distance, the direction along which ray of light travels is taken as +ve.

So, here, the ray of light is coming from behind the mirror towards lens, for the second image, I

2

with respect to lens,

u=+10cm;f=15cm

So, applying lens formula,

v

1

−

u

1

=

f

1

or

v

1

−

10

1

=

15

1

or

v=6cm from lens (in front of lens)

So this image is at a distance of 16 (10 + 6) cm in front of mirror.

Explanation:

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