Objective lens, optical system, and microscope

An objective lens consisting of, disposed in order from an object:

• a first lens group having positive refractive power;
• a second lens group having positive refractive power;
• and
• a third lens group having negative refractive power, wherein:
• the first lens group consists of, disposed in order from the object:
• a positive lens;
• a negative meniscus lens cemented to the positive lens and having a concave surface facing the object;
• and
• a positive meniscus lens having a concave surface facing the object;
• the second lens group has three cemented lenses;
• the objective lens is configured to vary a distance between the second lens group and the third lens group depending on a thickness of a cover glass, the cover glass being disposed on an object side of the first lens group;
• the third lens group comprises:
• a first opposing negative lens having a concave surface facing an image on an image side;
• a second opposing negative lens disposed on an image side of the first opposing negative lens and having a concave surface facing the object on an object side;
• and
• a second opposing positive lens cemented to the second opposing negative lens on the image side and having a concave surface facing the object on the image side;
• and
• the following conditional expressions are satisfied:
• 2.03≤n1m≤2.30;
• 20≤ν1m≤40;
• 1.70≤n3p≤2.00;
• and
• 25≤ν3p≤45,
• where:
• n1m is a refractive index of the negative meniscus lens with respect to a d-line;
• ν1m is an Abbe number of the negative meniscus lens;
• n3p is a refractive index of the second opposing positive lens with respect to the d-line;
• and
• ν3p is an Abbe number of the second opposing positive lens.