Final answer:
To find the focal length of the biconcave lens, we can use the thin lens formula and the magnification formula. The focal length is approximately -10.27 cm.
Step-by-step explanation:
The focal length of the biconcave lens can be found using the thin lens formula, which states that 1/f = 1/v - 1/u, where f is the focal length, v is the image distance, and u is the object distance. In this case, the lens forms a virtual image that is 0.8 times the size of the object, so the magnification (m) is -0.8. Using the magnification formula, m = v/u, we can rearrange it to find v/u = -0.8.
Since the image formed is virtual, the image distance (v) is negative. Substituting the given values, we have -0.8 = v/23.1. Solving for v, we find that v = -18.48 cm. Now, substituting the values of v and u into the thin lens formula, we have 1/f = 1/-18.48 - 1/23.1. Simplifying this equation, we get 1/f = -0.0541 - 0.0433, which gives us 1/f = -0.0974. Solving for f, we find that f = -10.27 cm.