A concave mirror gives an image three times as large as its object placed at a distance of 20 ~cm from it. For the image to be real, the focal length should be

Step-by-step solution

Given, size of image formed by concave mirror, h₂=3 h₁ where, h₁ is the size of object. m= h₂ h₁ = 3 h₁ h₁ =3 But for real image, magnification m will be negative. m=-3 - v u =-3 v=3 u Here, u=-20 ~cm , v=3(-20)=-60 ~cm By using mirror formula, 1 f = 1 u + 1 v = 1 -20 + 1 -60 =- 1 15 f=-15 ~cm