To reduce the magnification from +1∕2 to +1∕3, the object should be moved further to the spherical mirror.
Let the object distance be u and the image distance be v. The magnification is given by: magnification =−v∕u
Since the initial magnification is +1∕2, we have: +1∕2=−v∕20‌cm
Solving for v, we get: v=−10‌cm
Substituting this value of v into the magnification equation, we get,
+1∕3=−(−10)∕u
Simplifying, we get: u=30‌cm
Therefore, the object should be placed at a distance of 30‌cm from the spherical mirror to reduce the magnification to +1∕3.