Notes on Images Formed by Concave and Convex Mirrors and Determination of Focal Length | Grade 11 > Physics > Reflection at Plane and Curved Surfaces | KULLABS.COM

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#### Images Formed by Concave and Convex Mirrors

For concave mirror

For Convex Mirror

 S.no. Position of object Position and nature of image formed 1. In front of a convex mirror at any position, Image is erect, virtual, diminished and forms at other side of the object. Image formed behind the convex mirror when an object lies in front of mirror 2. If a converging beam is incident on convex mirror, Image is real, inverted and in front ofmirror. Real image formed by convex mirror in front of it when a converging beam is incident on it.

#### Determination of Focal Length of Spherical Mirrors

1. ##### Focal length of concave mirror by double in (u – v) method
It consists of a concave mirror two pins and three stands for pins and the mirror. The arrangement of pins and mirror on stands are adjusted as shown in figure.
First of all, the rough focal length of the mirror is found out by focusing a distance object on wall or in paper and measuring the distance between the mirror and sharp image. Now, the object pin is placed at a distance 2 f from or at centre of curvature C of the mirror and an inverted image will be formed by the mirror which lies at the position of the object. Exact positions are determined by moving the pin towards or away from the mirror removing parallax. Now, as the object is placed within pole P and C, the image will be formed beyond C and when an object is placed beyond C and when an object is placed beyond C, an image will be formed with in P and C. In each case, a real inverted image will be formed, the position of which is determined by using second pin on which the image inverted touching the tips of each other as shown in the figure. Hence it is noted as we move our eyes across the mirror object and image must be together. Thus we obtain a pair of object distance and image distance and on substituting in mirror formula we can determine focal length of the mirror.

1. ##### Focal length of convex mirror by plane mirror method It consists of a convex mirror mounted on a stand, and a plane mirror and a pin are adjusted on other two stands as shown in the figure. The plane mirror is held in front of the convex mirror with the top of the mirror lying on the principle axis of the convex mirror. An object pin is held in front of two mirrors and its position is adjusted such that there is no parallax between the image and the object seen through the two mirrors. The position of object pin, plane mirror and convex mirror are noted and we can determine
\begin{align*} \text {Object distance,} \: u &= PO \\ \text {Image distance,}\: v &= -PI = -(MI – PM) \\ &= (MO – PM) \\ \end{align*}
as MO = MI or u = v in plane mirror. Substituting these values, we can determine focal length of convex mirror.

Uses of Mirrors

 S.N Plane mirror Concave mirror Convex mirror 1. Plane mirrors are used as looking gas. Concave mirrors are used as shaving glass. The convex mirrors are used in cars that give the clear view of the vehicles approaching from behind. 2. Plane mirrors are used in Barber’s shop to see the back of your head. Concave mirrors are used as reflectors in car headlights, searchlights, torches, etc. Convex mirror can diverge the light over a larger area and, therefore, it is used as a street light reflector. 3. Plane mirrors are used to make periscope. Concave mirrors are used in projectors and telescopes. Convex mirror is used as an anti-shoplifting device. 4. Concave mirrors are used to observe the ear and nose of patients.

For concave mirror

• At infinity,

Image is real, inverted, small size and forms at principle focus.

• Beyond C,Image is real, inverted and smaller size. The image is formed between F and C.
• At C Image is real, inverted and smaller size. The image is formed between F and C.
• At F Image is real, inverted and highly magnified. The image is formed at infinity.

For Convex Mirror

• In front of a convex mirror at any position, Image is erect, virtual, diminished and forms at another side of the object.
• If a converging beam is an incident on a convex mirror,Image is real, inverted and in front of mirror.
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