 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.

#### Concave Lenses

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.

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Questions of 10th Science Chapter 9 in Detail

 Define the principal focus of a concave mirror. The radius of curvature of a spherical mirror is 20 cm. What is its focal length? Name a mirror that can give an erect and enlarged image of an object. Why do we prefer a convex mirror as a rear-view mirror in vehicles? Find the focal length of a convex mirror whose radius of curvature is 32 cm. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located? A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why? Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? Find out, from Table, the medium having highest optical density. Also find the medium with lowest optical density. You are given kerosene, turpentine and water. In which of these does the light travel fastest? The refractive index of diamond is 2.42. What is the meaning of this statement? Define 1 dioptre of power of a lens. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Find the power of a concave lens of focal length 2 m. We wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? Name the type of mirror used in the following situations. (a) Headlights of a car. (b) Side/rear-view mirror of a vehicle. (c) Solar furnace. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. What is the position, size and the nature of the image formed. A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find the position and nature of the image. The magnification produced by a plane mirror is +1. What does this mean? An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size. An object of size 7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm. At what distance from the mirror should a screen be placed, so that a sharp focussed image can be obtained? Find the focal length of a lens of power – 2.0 D. What type of lens is this? A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?