The power of a lens (P) is given in dioptres (D) and is related to the focal length (f) by the formula P = 1/f, where f is in meters. For a lens with a power of -2.0 D, the focal length is f = 1/−2.0 = −0.5 meters or -50 cm. The negative sign indicates that the lens is a concave lens, which diverges light rays.
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Introduction to Lens Power and Focal Length
Understanding Lens Power: The power of a lens is a measure of its ability to converge or diverge light, expressed in dioptres (D). It is inversely related to the focal length of the lens, providing a quick way to understand a lens’s optical properties.
Power of the Given Lens
Specified Lens Power: The lens in question has a power of -2.0 dioptres. The negative sign in the power indicates a specific type of lens, which we can determine by calculating its focal length.
Calculating Focal Length
Using the Power Formula: The relationship between power (P) and focal length (f) is given by P = 1/f, where f is in meters. To find the focal length of a lens with a power of -2.0 D, we rearrange this formula to f = 1/P.
Determining the Focal Length
Focal Length Calculation: Substituting the power value into the formula, f = 1/−2.0, we find that the focal length is -0.5 meters, or -50 cm. The negative value is crucial in identifying the type of lens.
Identifying the Type of Lens
Nature of the Lens: The negative focal length indicates that the lens is a concave lens. Concave lenses are diverging lenses, characterized by their ability to spread out light rays passing through them.
Characteristics of the Lens: Therefore, a lens with a power of -2.0 dioptres has a focal length of -50 cm and is a concave lens. This type of lens is commonly used in applications where light needs to be diverged, such as in certain types of eyeglasses for correcting vision.