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CONCEPT:
- Focal length: The distance from the lens to its focal point is called the focal length.
The focal length of a lens is given by the lens maker formula:
\(\frac{1}{f}=(μ-1)(\frac{1}{R_1}-\frac{1}{R_2})\)
where f is the focal length of the lens, μ is the refractive index of the lens, and R1 and R2 are the radiuses of curvature.
- For converging lenses or convex lenses, the focal length is always positive.
- For diverging lenses or concave lenses, the focal length is always negative.
EXPLANATION:
- For the concave lens, the focal length is always negative.
- A concave lens diverges the rays incident on it and a convex lens converges the rays incident on it.
- We know that the focal length is in the negative x-direction for a concave lens while it is in the positive x-direction for the convex lens.
- So f is negative for a concave lens and positive for a convex lens.
- It is a sign convention.
- Hence the correct answer is option 1.
Additional Information
The Sign convention for spherical lenses are as follows:
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Is focal length of convex lens negative when it is surrounded by a medium with greater refractive index than that of lens?
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