Numerical aperture

Definition

Numerical aperture (NA) is a dimensionless number that characterizes the range of angles over which a microscope objective can accept light. Higher NA values correspond to better resolution and brightness in microscopy.

5 Must Know Facts For Your Next Test

1. Numerical aperture is calculated using the formula NA = n * sin(θ), where n is the refractive index of the medium between the lens and the specimen, and θ is half the angle of the maximum cone of light that can enter or exit the lens.
2. A higher numerical aperture indicates better resolving power, allowing finer details to be observed.
3. Oil immersion lenses have higher numerical apertures than dry lenses because oil increases the refractive index.
4. The numerical aperture directly affects both resolution and brightness in microscopy; higher NA objectives gather more light and provide clearer images.
5. Numerical aperture plays a crucial role in determining the depth of field, with higher NAs leading to shallower depths of field.

Review Questions

• How does numerical aperture affect the resolving power of a microscope objective?
• What is the formula for calculating numerical aperture?
• Why do oil immersion lenses have higher numerical apertures compared to dry lenses?

Related terms

Refractive Index: A measure of how much light bends when it enters a medium.

Resolution: The ability to distinguish two points as separate entities in an image.

Depth of Field: The distance between the nearest and farthest objects in focus within an image.