Question

What is the position of the image when an object is placed at distance of 20 cm from a concave mirror of
focal length 20 cm ?​

Answer 1

Question :-

What is the position of the image when an object is placed at distance of 20 cm from a concave mirror of focal length 20 cm ?

Answer:-

$\green{ \bf{position \: of \: imag e \: (v)} = \red{\infty (not \: defined)}}$

Explanation:-

Formula used :-

Mirror formula →

$\implies \: \red{\boxed{\bf{\frac{1}{f} = \frac{1}{u} + \frac{1}{v} }}}$

Given that :-

• Distance of object (u) = -20 cm
• focal length (f) = -20 cm
• position of image (v) =?

Negative sign in u due to position of object opposite to the direction of incident ray .

Negative sign of f because focal length of concave mirror is always negative.

Solution :-

Using the given mirror formula

$\implies \: \bf{ \frac{1}{ - 20} = \frac{1}{ - 20} + \frac{1}{v}} \\ \\ \implies \: \bf{ \frac{1}{v} = \frac{1}{20} - \frac{1}{20} } \\ \\ \implies \: \bf{ \frac{1}{v} = 0 }\\ \\ \\ \implies \: \pink{ \boxed{ \bf{ v = \infty }}} \: (not \: defined \: )$

Image is formed at infinity.

Hope it helps you.