Question

. An object is placed at 25cm from a convex lens of focal length 20 cm, find the position of the image.​

Answer 1

Question : -

An object is placed at 25cm from a convex lens of focal length 20cm, find the position of the image.

ANSWER

Given : -

Object is placed at 25cm from a convex lens of focal length 20cm

Required to find :-

• Position of the image ?

Formula used :-

(1)/(f) = (1)/(v) - (1)/(u)

Here,

• f = focal length
• v = image distance
• u = object distance

Solution :-

From given information we can conclude that;

Object distance (u) = 25cm

Focal length (f) = 20cm

Lens used - Convex / bi-convex lens

Now,

By applying sign convention we have;

• u = - 25cm
• f = 20cm

Applying the formula;

(1)/(f) = (1)/(v) - (1)/(u)

(1)/(f) + (1)/(u) = (1)/(v)

(1)/(20) + (1)/(-25) = (1)/(v)

(1)/(20) - (1)/(25) = (1)/(v)

(5 - 4)/(100) = (1)/(v)

(1)/(100) = (1)/(v)

=> v = 100

Therefore,

• Image distance (v) = 100 cm

But,

Radius of curvature = 2 x f

R = 2 x 20

R = 40 cm

=> v > R

=> Image distance > Radius of curvature

Image properties

1. Image if formed beyond centre of curvature
2. Image is real, inverted.
3. Image is magnified .

Answer 2

Given :-

An object is placed at 25cm from a convex lens of focal length 20 cm

To Find :-

Position

Solution :-

We know that

1/f = 1/u + 1/v

1/20 = 1/-25 + 1/v

1/20 - 1/25 = 1/v

5 - 4/100 = 1/v

1/100 = 1/v

100(1) = 1(v)

100 = v

Properties

Formation - Beyond the center of curvature

Type - Real and inverted