Question

9. A Convex lens produces a
real and magnified image when
an object is 10 cm from the lens
and the image is formed at 30
cm. The focal length of lens is
O 1.5 cm
O 7.25 cm
O 10 cm
O 14.5 cm

​

Answer 1

12th

Physics

Ray Optics and Optical Instruments

Combination of Mirror and Lenses

A convex lens of focal leng...

PHYSICS

A convex lens of focal length 30cm produces 5 times magnified real image of an object. What is the object distance?

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ANSWER

Focal length of the convex lens f=30cm

A convex lens forms a magnified real image when the image is inverted i.e. magnification must be negative.

Magnification of the lens m=−5

Using lens formula

v

1

−

u

1

=

f

1

We get

f

v

=1−m

∴

30

v

=1−(−5)

⟹ v=180cm

Using m=

u

v

(for lens)

∴ −5=

u

180

⟹ u=−36cm

Thus the object distance must be 36cm in front of the lens.

Answer 2

Answer:

The focal length of the lens is 7.5 cm.

Explanation:

We will use the lens formula to solve this question. Which is given as,

$\mathrm{\frac{1}{f}= \frac{1}{v}-\frac{1}{u}}$         (1)

Where,

f=focal length of the lens

v=image distance from the lens

u=object distance from the lens

From the question we have,

The object distance from the lens(u)=-10 cm

The image distance from the lens(v)= 30 cm   ("+" as the lens produces real image)

Now by inserting the value of "u" and "v" in equation (1) we get;

$\mathrm{\frac{1}{f}= \frac{1}{30}-\frac{1}{-10}}$

$\mathrm{\frac{1}{f}= \frac{1}{30}+\frac{1}{10}}$

$\mathrm{\frac{1}{f}= \frac{1+3}{30}}$

$\mathrm{\frac{1}{f}= \frac{4}{30}}$

$\mathrm{f=\frac{30}{4} }$

$\mathrm{f=7.5\ cm }$           (2)

The focal length of the lens is 7.5cm i.e.none of the options is correct.

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