Question
A concave lens has focal length of 15 cm. At what, distance should the object from the lens be placed so that it forms an image at 10 cm from the lens? Also, find the magnification of the lens.
Solution
A concave lens always forms a virtual, erect image on the same side of the object.
Given,
Image distance, v = -10 cm
Focal length, f = - 15 cm [f is -ve for a concave lens]
Object distance, u = ?
Now, using lens formula,
we have,
i.e.,
Thus the object should be placed at a distance of 30 cm from the lens on the left side.
Now,
Magnification,
Since, magnification is positive, we can say that the image is erect and virtual.
The size of the image is reduced to one-third in size than the object after refraction.
Given,
Image distance, v = -10 cm
Focal length, f = - 15 cm [f is -ve for a concave lens]
Object distance, u = ?
Now, using lens formula,
we have,
i.e.,
Thus the object should be placed at a distance of 30 cm from the lens on the left side.
Now,
Magnification,
Since, magnification is positive, we can say that the image is erect and virtual.
The size of the image is reduced to one-third in size than the object after refraction.
