Question

A converging lens of focal length 5 cm is placed at a distance of 20 cm from a screen. How far from the lens should an object be placed so as to form its real image on the screen ? Full calculation answer . Subject physics

Answer 1

Focal length of the convex lens (f)  = 5 cm

Image distance (u) = 20 cm

Let the object distance from the lens be u.

Now, using the lens formula:

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

$\frac{1}{u} =\frac{1}{20} -\frac{1}{5}$

$\frac{1}{u} = \frac{1 - 4}{20}$

$u = -\frac{20}{3}$ cm

u = - 6.33 cm

Hence, the object should be kept 6.33 cm in front of the lens.

Answer 2

Answer:

Explanation:

f = 5 cm

u = - 20 cm

v = + v (Since image is real)

Lens formula : 1/v – 1/u = 1/f

1/v – 1/-20 = 1/5

1/v = 1/5 – 1/20 = (4 – 1)/20 = 3/20

V = 6.66

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