Question
Physics Question on Ray optics and optical instruments
Two convex lenses of focal lengths 20 cm and 30 cm are placed in contact with each other co-axially. The focal length of the combination is:
A
60 cm
B
10 cm
C
12 cm
D
40 cm
Answer
12 cm
Explanation
Solution
The correct option is: (C) : 12 cm.
When two convex lenses are placed in contact with each other, their effective focal length (eff) can be calculated using the lens maker's formula:
f eff1=f 11+f 21
Where:
- f 1 is the focal length of the first lens.
- f 2 is the focal length of the second lens.
Given that f 1=20 cm and f 2=30 cm, substitute these values into the formula:
f eff1=201+301=603+602=605
Now, invert both sides of the equation to find f f eff:
f eff=560=12 cm