When an object is placed at a distance of 15cm from a concave mirror, its image is formed at 10cm in front of the mirror. Calculate the focal length of the mirror.
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u = -15cm, v = -10cm f = ? We know that$\frac{1}{\text{v}}+\frac{1}{\text{u}}=\frac{1}{\text{f}}$
$\Rightarrow\frac{1}{(-10)}+\frac{1}{(-15)}=\frac{1}{\text{f}}$
$\Rightarrow\frac{1}{\text{f}}=\frac{1}{10}-\frac{1}{15}= \frac{-3-2}{30}=-\frac{5}{30}=-\frac{1}{6}$
$\therefore \text{f}=-6\text{cm}$
$\therefore $ The focal length of the concave mirror is 6cm.
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Identify the device used as a spherical mirror or lens in following cases, when the image formed is virtual and erect in each case.
Object is placed between device and its focus, image formed is enlarged and behind it.