 The principal focus of a concave mirror is the specific point where light rays parallel to the principal axis converge after being reflected by the mirror. This focal point lies along the principal axis, in front of the mirror. The distance between the mirror’s surface (pole) and the principal focus is known as the focal length, which is a key characteristic in determining the mirror’s image-forming properties.

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## Definition of Principal Focus in Concave Mirrors

Convergence of Parallel Rays: The principal focus of a concave mirror is a unique point where light rays, initially parallel to the principal axis of the mirror, converge after reflection. This phenomenon is a fundamental characteristic of concave mirrors, distinguishing them from other reflective surfaces.

### Location of the Principal Focus

Position Relative to the Mirror: The principal focus is located along the principal axis, on the same side as the reflecting surface of the concave mirror. Its exact position depends on the curvature of the mirror. #### Focal Length

Distance from the Pole: The focal length is the distance between the mirror’s surface, specifically the pole (the central point on the mirror’s surface), and the principal focus. This distance is a critical parameter in optical calculations and mirror design.

#### Image Formation

Role in Image Projection: The principal focus is crucial in determining how images are formed by a concave mirror. Objects placed at different distances from the mirror produce images of varying nature (real or virtual) and size, influenced by the position of the principal focus.

##### Optical Significance

Importance in Optics: The concept of the principal focus is vital in optics, especially in designing optical instruments like telescopes and microscopes, where precise control over light paths is essential for accurate image formation.

###### Practical Applications

Use in Various Devices: Concave mirrors with well-defined principal focuses are used in various applications, such as in headlights of vehicles, reflecting telescopes, and certain types of cameras, where they help in focusing light effectively to form clear images.

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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?