Question

19. An object 4 cm in height, is placed at
15 cm in front of a concave mirror of focal length
10 cm. At what distance from the mirror should
a screen be placed to obtain a sharp image of the
object. Calculate the height of the image.
​

Answer 1

Answer:

The "u" distance of the image is given as -15 cm while the focal length of the object is given is -10 CM.

Now putting these values in the lens formula we can get the distance of the image formed

\frac{1}{v} + \frac{1}{u} = \frac{1}{f} \rightarrow \frac{1}{v} = \frac{1}{15} - \frac{1}{10}

v

1

+

u

1

=

f

1

→

v

1

=

15

1

−

10

1

⇒ v = -30 cm

After knowing the distance of object to be placed let us see the formula for magnification given by -\frac{v}{u}−

u

v

.

On applying the value we get,

m =\frac{h'}{h}

h

h

′

= - \frac{-30}{-15}−

−15

−30

⇒ h’ = -30 \times \frac{4}{15}−30×

15

4

= -8cm

for the purpose of getting an image that is sharp it should be placed at 30 cm.