Final answer:
Two identical lenses with a refractive index of 1.5 and a focal length of 12 cm, when kept in contact and submerged in a liquid with a refractive index of 1.35, behave as a single lens with an increased combined focal length compared to their focal length in air.
Step-by-step explanation:
The student is asking about the behavior of two identical lenses with a focal length of 12 cm when they are kept in contact and immersed in a liquid with a refractive index of 1.35. The lenses originally have a refractive index of 1.5. According to the lens maker's equation, the focal length of a lens depends on the refractive indices of the lens and the medium in which it is placed. When the lenses are placed in a liquid with a refractive index of 1.35, the effective refractive index contrast between the lenses and the surrounding medium decreases. This will lead to an increase in the effective focal length of the lens combination compared to their focal length in air.
Since the two lenses are identical and in contact, they can be considered a single lens system with a combined focal length. The combined focal length, when lenses are in contact, can be found using the formula:
1/F = 1/f1 + 1/f2
Because f1 = f2, and both are 12 cm, the combined focal length F in air would be 12 cm / 2 = 6 cm. However, when submerged in the liquid, the effective focal lengths of these lenses will change due to the reduced refractive index contrast. This results in a combined focal length that is greater than 6 cm.