Table of Contents
Numerical aperture (NA) is an important parameter in microscopy that determines the resolving power and brightness of the image. Calculating the NA of a microscope objective helps in selecting the appropriate lens for specific applications. This article provides a practical approach to understanding and calculating the NA.
Understanding Numerical Aperture
The NA of a microscope objective quantifies its ability to gather light and resolve fine details. It is defined by the equation:
NA = n × sin(θ)
where n is the refractive index of the medium between the lens and the specimen, and θ is the half-angle of the maximum cone of light entering the lens.
Calculating Numerical Aperture
To calculate the NA, measure or obtain the following parameters:
- The refractive index of the immersion medium (n)
- The half-angle of the light cone (θ)
For example, if using immersion oil with n = 1.515 and a measured θ of 30°, the NA is calculated as:
NA = 1.515 × sin(30°) = 1.515 × 0.5 = 0.7575
Practical Tips
Ensure accurate measurement of the half-angle θ using appropriate tools. When using different immersion media, adjust the refractive index accordingly. Higher NA values generally indicate better resolution and brightness.