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

Answer:

your answer is here !

Explanation:

(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.

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Answer 2

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$magnification= \frac{height \: of \: the \: object}{height \: of \: the \: image}$

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as per the Cartesian convention, distanced above the optical axis are taken as positive and whereas the distances taken below the optical axis are take negative.

(1)

in case of real image ...the image is converted.so the height of the image has negative sign while the height of the image has positive sign

therefore..in this case magnification is➥ negative.

in case of virtual image ...the image is erect and so the height of the image has positive sign and the height of the image has positive sign also

therefore..in this case magnification is➥ positive.

(2)

According to the spherical mirror equation,

$\frac{1}{v} + \frac{1}{u} = \frac{2}{r}$

➽Since ... u ➜is positive and R➜ is negative,

v ➜will always be negative,

and smaller in magnitude than u.

Therefore ....

the image will always be upright (not upside-down), virtual and diminished.

....so...

➽ In the case of concave mirror,

images of objects further from the focal point ( R/2 from the mirror) will be inverted, which is no good to a driver.

As a result

(i)For objects nearer than the focal point, the image would be magnified.

(ii) the area of the rear is small so less images can fit in it.

But more importantly,

(iii) near objects may not be of little interest to drivers, and concave mirrors are not suitable to view far objects.

➽For convex mirrors,

(i)diminished images means that images appears smaller than the objects that means a larger field of vision could be obtained.

This is also why a convex mirror is preferred over a planar one.

➽conclusion

The driver needs a wide view of the road situation that are (relatively) far away, and convex mirrors are able to acheive that..

....XD......

❤❤$:\: \underline{\underline{\bf{\large\mathfrak{~hope ~this ~help~you~~}}}}$❤❤