Correct Answer - Option 2 : 4 D
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).
\(P = \frac{1}{f\,(m)}=\frac{100}{f\,(cm)}\)
where P is the power of the lens and f is the focal length of the lens.
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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.
CALCULATION:
Given - focal length f = 25 cm
- Even if a hole of aperture 5 cm is made the focal length would remain the same
- Then the power of the lens is given by
\(\Rightarrow P = \frac{100}{f}\)
\(\Rightarrow P = \frac{100}{25} = 4 D\)
- Hence, option 2 is the answer