Question

the principle axis of a convex lens is along x axis the coordinate of point object and image are (-20cm,2cm) and(25cm,-4cm) find it's focal length.

Answer 1

Focal length: The distance between the center of a lens and its focus is called the focal length.

Given:

• X axis coordinate of the object (-20cm, 2cm)
• X axis coordinate of the image (25cm, -4cm)

Solution:

According to lens formula,

1 / f = 1 / v - 1 / u

The distance of object distance u = -20 cm

The distance of image distance v = 25 cm

The focal length,

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

f = ( - 20 x 25 ) / ( -20 - ( -25 ) )

f ≅ 10cm

Hence, the focal length is 10cm.