Given here, u=−40cm,v=−120cm
Using mirror formula, v1+u1=f1
120−1−401=f1⇒f=−30cm
Now, differentiating mirror equation, we get
v2−1dv−u21du=−f21df
Also, the least count of scale, dv=du=201cm
Putting in above equation,
∴(120)2201+(40)2201=(30)2df
On solving, we get
df=321cm
∴K=32.