The power of a lens is calculated as the inverse of its focal length in meters. For a concave lens with a focal length of 2 meters, the power P is given by P = 1/f, where f is the focal length. Therefore, P = 1/2 = 0.5 dioptres. Since it’s a concave lens, its power is negative, so the power is -0.5 dioptres.
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Introduction to Lens Power
Understanding Optical Power: The power of a lens is a crucial concept in optics, representing the lens’s ability to converge or diverge light. It is measured in dioptres and is inversely proportional to the focal length of the lens.
Focal Length of Lenses
Defining Focal Distance: The focal length of a lens is the distance from the lens to its focus. For a concave lens, which diverges light, the focal length is considered negative, reflecting its diverging effect on light rays.
Diverging Light Rays: A concave lens is a type of lens that diverges light rays. This means that instead of converging light to a point, a concave lens spreads out light rays, making them appear to originate from a point behind the lens.
Calculating the Power of a Concave Lens
Application of the Formula: The power of a lens is calculated using the formula P = 1/f, where P is the power in dioptres and f is the focal length in meters. For a concave lens with a focal length of 2 meters, the calculation is straightforward.
Result of the Calculation
Determining Lens Power: Substituting the focal length into the formula, P = 1/2, we find that the power of the lens is 0.5 dioptres. However, since it is a concave lens, its power is negative, making the power -0.5 dioptres.
Understanding the Optical Power of Concave Lenses: Therefore, a concave lens with a focal length of 2 meters has an optical power of -0.5 dioptres. This negative value is indicative of the lens’s diverging properties and is essential in applications where spreading out light rays is required, such as in certain types of eyeglasses.