Question

Q38. Draw a diagram and apply (the new Cartesian sign) conventions for calculating focal length and
nature of a spherical
mirror which forms a 1/3 times magnified virtual image of an object placed 18 cm, in front of it.
20 nettbacian conventione for reflection of lihthonborilmirrarna Jim
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Answer 1

Explanation:

The following sign convention is used for measuring various distances in the ray diagrams of spherical mirrors:

Object is always placed to the left of mirror

All distances are measured from the pole of the mirror.

Distances measured in the direction of the incident ray are positive and the distances measured in the direction opposite to that of the incident rays are negative.

Distances measured along y-axis above the principal axis are positive and that measured along y-axis below the principal axis are negative.

Since the image formed is magnified and virtual, the mirror is convex.

According to the convention

u=−18cm

m=1/3=−

u

v

⟹v=6cm

From mirror equation,

v

1

+

u

1

=

ff

1

⟹f=

u+v

uu

⟹f=9cm

Answer 2

Explanation:

To draw a Ray diagram we need v and f in this case.

so to calculate v we have the formula M=-v/u

here m is given to be 1/3 and u is 18cm.

1/3=v/18

=6cm....

And about focal length. when we have v and u it is very to find f i.e., 1/f=1/v+1/u

1/f=1/6cm(as we calculated above)+1/18

=9cm...