The focal length of a thin biconvex lens is 20cm. When an object is moved from a distance of 25 cm to 50 cm in front of it, the magnification of its image changes from m₁ to m₂. The ratio m₁/m₂ is ______
Physics · Mixed Chapter
JEE Mains

Question

The focal length of a thin biconvex lens is 20cm. When an object is moved from a distance of 25 cm to 50 cm in front of it, the magnification of its image changes from m₁ to m₂. The ratio m₁/m₂ is ______

Physics · Mixed Chapter

G1 Physics · Accepted Answer

Step 1: For a thin converging lens, v=fu/(u−f) and the magnitude of lateral magnification is |m|=v/u=f/(u−f). Step 2: When object distance u₁=25 cm and f=20 cm, |m₁|=20/(25−20)=4. Step 3: At u₂=50 cm, |m₂|=20/(50−20)=2/3. Step 4: |m₁/m₂|=4/(2/3)=6. Answer: 6.