Question

(a) Why magnification is taken negative for real images and positive for virtual images?
(b) Why convex mirror is used in rear view mirrors and not concave mirror?
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Answer 1

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(a) If image is real, it will be inverted which means height of image will be taken as negative.

Also, the height of object is always positive.

Now as magnification = Height of image / Height of object.

Therefore, in case of real image,

Magnification = Height of image (-) / height of object (+)=Negative.

So, Magnification will be negative.

Similarly, if image is virtual, it will be erect.

Therefore, in case of virtual image,

Magnification = Height of image (+) / height of object (+)= Positive

So, Magnification will be positive.

(b) Convex mirror is commonly used in rear view mirrors in vehicles because it gives an erect, virtual, full size diminished image of distant objects with a wider field of view. Thus, convex mirrors enable the driver to view much larger area which is not possible with a concave mirror.

Answer 2

(a) Magnification is the ratio between the height of the object and the height of the image.

In the case of a real image, the height of the object carries a positive sign while the height of the image carries a negative one.

Hence, the magnification is negative.

In case of a virtual image, the height of the image, as well as the height of the object, carries a positive sign.

Therefore, the magnification is positive in this case.

(b) As convex mirror produces virtual and erect image and has a wider field of view whereas, concave mirror produces real and inverted image which makes difficult for the driver to see. Also concave mirror produces enlarged image. That's why convex mirror is used as rear view mirror.

Hope it helps.

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