What is meant by power of lens? What does its sign ( ve or -ve) indicate ? State its si unit. How is this unit related to focal length of lens ?

Question

Answer

Power of lens is ability to converge or diverge the beam of light ray. Positive power of lens indicates converging ability of a lens as in the case of convex lens, and negative power of lens indicate its diverging ability as in the case of concave lens.

The S.I unit of power is diopter.
Power is expressed as inverse of focal length, f (in metre).
Power in diopters = 1/focal length.(in metres).

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Answer

The ability of a lens to converge or diverge the rays is called the power of a lens
P=1/f
Its si unit is dioptre (d)
The power of a lens is related to its focal length as for greater convergent light, lens of high power and smaller focal length is needed i.e, the lens of focal length 20cm is needed for the same

Answer

Power of Lens:
The power of a lens is a measure of its ability to converge or diverge light. It is denoted by the symbol P and is defined as the reciprocal of the focal length of the lens. The power of a lens is given by the formula:

P = 1/f

Where P is the power of the lens in diopters (D) and f is the focal length of the lens in meters.

Sign of Power:
The sign of the power of a lens indicates whether the lens is converging or diverging. A positive sign (+) indicates a converging lens, while a negative sign (-) indicates a diverging lens.

SI Unit:
The SI unit of power is the diopter (D), which is equal to 1/meter. It represents the inverse of the focal length of the lens.

Relationship with Focal Length:
The relationship between power and focal length is inverse. This means that as the power of a lens increases, its focal length decreases, and vice versa.

When a lens has a high power, it means that it is able to bend light rays more strongly and bring them to a focus at a shorter distance. This corresponds to a shorter focal length. On the other hand, a lens with a low power bends light rays less strongly and requires a longer focal length to bring them to a focus.

The relationship is mathematically expressed as:

P = 1/f

Where P is the power of the lens and f is the focal length of the lens. As the power increases, the focal length decreases, and as the power decreases, the focal length increases.

This relationship is important in understanding the behavior of lenses and their ability to form images. It allows us to determine the power of a lens based on its focal length, or vice versa. Knowing the power of a lens helps in selecting the appropriate lens for correcting vision problems or designing optical systems.

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