Correct Answer - Option 1 : 25 cm
CONCEPT:
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Power of Lens: The inverse of the focal length is known as the power of the lens.
- It shows the bending strength for the light ray of the lens.
- The unit of power of a lens is Dioptre when the focal length of the lens is taken in meter (m).
\(P = \frac{1}{f}\)
where P is the power of the lens and f is the focal length of the lens.
- In optical systems composed of lenses, the position, magnitude, and errors of the image depending upon the refractive indices of the glass.
- If white light is employed, all the images are formed and they cause confusion, named chromatic aberration. The absence of this error is termed achromatism, and an optical system so corrected is termed achromatic.
Calculation:
The power (P) of the combination of two lenses is given by:
P = P1 + P2
Where P1 and P2 are the power of two lenses.
P1 = 12 D, P2 = - 8 D
P = 12 D - 8 D = 4 D
Now focal length f = 1 / P
\(f =\frac{1}{4D} =\frac{1}{4}m = 0.25m\)
Focal length = 0.25m = 25 cm
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Concave lens: It is a diverging lens that diverges the parallel beam of light.
- It can also gather light from all directions and project it as a parallel beam.
- The focal length of the concave lens is negative.
- It has a virtual focus from the diverging rays of light that seem to converge.
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Convex lens: The lens whose refracting surface is upside is called a convex lens.
- The convex lens is also called a converging lens.
- The focal length of a convex lens is positive.
