Question

A convex mirror forms an image of half the size of the object when it is moved 15 cm away from the object the size of the image becomes 2/5 times that of the object.find the focal length of mirror??

Answer 1

Explanation:

The organisations of cricket reflect the nature of society in England:

(i) The rich who could afford to play cricket for pleasure were called amateurs

(ii) Amateurs were called Gentlemen.

(iii) The poor who played for living were called professionals.

(iv) The professionals were described as players.

(v) Most professionals worked as minors or in other forms of working class

employment

Answer 2

Answer:

-20 cm

Explanation:

Given,

1st case,

u/2 = v ---- equation 1

2nd case,

2/5(u+15) = v

(2u + 30)/5= v ----- equation 2

From equation 1 and 2 ,

u/2 = (2u + 30)/5

or, 5u = 2(2u + 30)

or, 5u = 4u + 60

or, 5u - 4u = 60

or, u= 60cm

Putting value of u in equation 1, we get,

u/2 = v

or, 60/2 = v

or, v = 30cm

Now,

By mirror formula,

1/f = 1/u + 1/v

or, 1/f = 1/60 + 1/30

or, 1/f = (1+2)/60

or, 1/f = 3/60

or, f = 60/3

or, f = 20

Therefore, f = -20 cm (focal length of convex mirror is always negative)