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.

**Let’s discuss in detail**

## 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.

Discuss this question in detail or visit to Class 10 Science Chapter 9 for all questions.

**Questions of 10th Science Chapter 9 in Detail**