Question

You switch from a 1.44NA 60× oil immersion objective to a 0.67NA 20× air immersion objective. What are the acceptance angles for each (in degrees)? (Assume the index of refraction of the oil is n = 1.51.) θ1.44NA 60× = ?(degrees) θ0.67NA 20× =?(degrees) Compare and comment on the values. Which would you use first to locate the target area on your specimen?

Answer 1

Numerical Aperture (NA) is the light gathering capacity of an optical element. It is given by:

where n_i is the refractive index of the medium and is the acceptance angle.

For Numerical Aperture of 1.44 with oil as the medium,

=>

for NA = 0.67,

So, a numerical aperture of 1.44 gathers more light than a numerical aperture of 0.67. To locate the target area, a higher NA must be used to have a larger field of view. Once the target is located, a specific low value NA can be used for studying a targeted area.