Question

an object of 4 centimetre in size is placed 20cm away from cancave mirror if the size of image is 6 CM then find out the focal length of the middle​

Answer 1

ANSWER : -6 CM (-STATES REAL SIDE)

3/2 = magnification

u= -20 (REAL SIDE SO NEGATIVE)

we know,

$m = \frac{ - v}{u}$

so,

$\frac{v}{ - 20} = \frac{ - 3}{2}$

${v} = { 30}$

we know,

$\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$

so,

$f = \frac{uv}{u + v}$

$f = \frac{ - 20 \times 30}{10}$

$f = - 6$

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