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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? Draw the ray diagram.


Focal length, f = - 15 cm    [f is - ve for a concave lens]
Image distance, v = - 10 cm [Concave lens forms virtual image on same side as the object, so v is - ve]
As,                            1 over straight f space equals space 1 over straight v minus 1 over straight u
therefore               1 over straight u space equals space 1 over straight v minus 1 over straight f space equals space fraction numerator 1 over denominator negative 10 end fraction minus fraction numerator 1 over denominator negative 15 end fraction space equals space fraction numerator negative 3 plus 2 over denominator 30 end fraction space equals negative 1 over 30
               Object distance, u = -30 cm. Ans. 

Drawing the ray diagram: Using a scale of 1: 5, we get v = - 2 cm, f = - 3 cm. We draw the ray diagram as follows:
(i) Draw the principal axis (a horizontal line).
(ii) Draw a convex lens, keeping principal centre (C) on the principal axis.
(iii) Mark points F and B on the left side of lens at a distance of 3 cm and 2 cm respectively.
(iv) Join any point D (nearly at the top of lens) and F by a dotted line.
(v) Draw a line AD, parallel to principal axis.

Focal length, f = - 15 cm    [f is - ve for a concave lens]Image d

(vi) Draw a line A'B', perpendicular to principal axis from B'.
(vii) Draw a line CA', backwards, so that it meets the line from D parallel to principal axis at A.
(viii) Draw a line AB, perpendicular (downwards) from A to meet the principal axis at B.
(ix) The AB is position of object. Measure distance BC. It will be found to be equal to 6 cm.
Thus, object is placed at a distance of 6 cm × 5 = 30 cm from the lens.



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Name a mirror that can give an erect and enlarged image of an object.

Concave mirror has the property of giving an erect and enlarged image of an object. 
468 Views

The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

Given, 
Radius of curvature, R = 20 cm 

Relationship between radius and focal length is as :

          Focal length = 12× Radius of curvature 

                       f = 202=10 cm is the focal length of the mirror. 

259 Views

Define the principal focus of a concave mirror.


Principal focus is that point on the principal axis at which, a beam of light which is incident parallel to the principal axis converges after reflection from the concave mirror.
574 Views

Why do we prefer a convex mirror as a rear-view mirror in vehicles?


Uses of convex mirrors: Drivers use convex mirror as a rear-view mirror in automobiles because of the following two reasons:
1. A convex mirror always forms an erect, virtual and diminished image of an object placed anywhere in front of it.
2. A convex mirror has a wider field of view than a plane mirror of the same size, as shown in Fig.
Thus convex mirrors enable the driver to view much larger traffic behind him than would be possible with a plane mirror. The main disadvantage of a convex mirror is that it does not give the correct distance and the speed of the vehicle approaching from behind.

Uses of convex mirrors: Drivers use convex mirror as a rear-view mirr
Fig. Field of view of (a) a plane mirror (b) a convex mirror

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