The focal point is located 20.0 cm from a double concave lens. An object is placed 12 cm from the lens. Determine the image distance.

Answer: **d**_{i} = -7.5 cm

Given: f = -20.0 cm and d_{o} = +12.0 cm

(Careful: double concave lens have focal lengths which are negative.)

Use the equation 1 / f = 1 / d_{o} + 1 / d_{i}

Substitute and solve for d_{i}.

1 / (-20.0) = 1 / (12.0) + 1 / d_{i}

1 / (-20.0) - 1 / (12.0) = 1 / d_{i}

-0.1333 cm^{-1} = 1 / d_{i}

d_{i} = 1 / (-0.1233 cm^{-1})

**d**_{i} = -7.5 cm