The magnitude of the focal length of the equiconcave lens with one surface silvered is D. 5 cm.
How to find the magnitude of the focal length?
The focal length (f_single) of a single concave surface of a lens can be calculated using the formula:
f_single = 1 / (2 * (n - 1) x (1 / r))
Plugging in the values, we get:
f_single = 1 / (2 * (1.5 - 1) x (1 / 20))
= 1 / (2 * 0.5 * (1 / 20))
= 10 c
Therefore, the effective focal length (f_effective) of the lens becomes:
f_effective = f_single / 2 = 10 cm / 2
= 5 cm