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:
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:
60 cm
10 cm
12 cm
40 cm
The Correct Option is C
Solution and Explanation
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:
feff1=f11+f21
Where:
- f1 is the focal length of the first lens.
- f2 is the focal length of the second lens.
Given that f1=20 cm and f2=30 cm, substitute these values into the formula:
feff1=201+301=603+602=605
Now, invert both sides of the equation to find ffeff:
feff=560=12 cm
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Concepts Used:
Ray Optics and Optical Instruments
Optics, deals with the determination of behaviour and the properties of light, along with its interactions with the matter and also with the instruments that are used to detect it.
Ray optics is also known as the geometrical optics and it is a branch of science which describes light propagation.
Reflection is the change in direction of light at an interface in-between two different media so that the wave-front returns into a medium from which it was originated.
Speed of light is the rate at which the light travels in free space.
A phenomenal change in image formed when the light is passed from one medium to another which is called Refraction.
Total Internal Reflection is the reflection of light when the light ray enters into a rarer medium from a denser medium and the angle of incidence is higher than the critical angle of incidence then that light ray will be reflected back to the denser medium.
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