Resolving Power of a Microscope
Formula and factors affecting the resolving power of a microscope, including the role of numerical aperture and wavelength.
What is Resolving Power of a Microscope?
The ability of an optical instrument to distinguish between two closely spaced objects or points.
Key formula / rule: Minimum Resolvable Distance (d)
Key points
- Define resolving power of a microscope.
- State the factors affecting the resolving power.
- Explain the relationship between resolving power, wavelength, and numerical aperture.
- Calculate the minimum resolvable distance or resolving power given the necessary parameters.
Common exam trap
Confusing resolving power with magnification.
Definitions
- Term
Resolving Power
- Meaning
The ability of an optical instrument to distinguish between two closely spaced objects or points.
- Term
Numerical Aperture (NA)
- Meaning
A dimensionless number that characterizes the range of angles over which the instrument can accept or emit light. It is a measure of the light-gathering ability of an objective lens.
Learning objectives
Define resolving power of a microscope.
State the factors affecting the resolving power.
Explain the relationship between resolving power, wavelength, and numerical aperture.
Calculate the minimum resolvable distance or resolving power given the necessary parameters.
Formulae
- Name
Minimum Resolvable Distance (d)
- Note
λ is the wavelength of light, NA is the numerical aperture of the objective lens.
- Expression
d = λ / (2 NA)
- Name
Numerical Aperture (NA)
- Note
n is the refractive index of the medium, θ is the half-angle of the cone of light collected by the lens.
- Expression
NA = n sin θ
Prerequisites
Understanding of light waves and diffraction.
Basic knowledge of lenses and magnification.
Concept of numerical aperture.
Common mistakes
Confusing resolving power with magnification.
Assuming higher magnification always means better resolution.
Incorrectly applying the formula for resolving power.
Forgetting that NA depends on the refractive index of the medium.
Keywords
Resolving Power
Microscope
Numerical Aperture
Wavelength
Diffraction
Abbe's Criterion
Resolution Limit
Practice preview
Which of the following factors does NOT affect the resolving power of a microscope?…
medium
The resolving power of a microscope is inversely proportional to:…
easy
If a microscope uses blue light (λ ≈ 450 nm) and another uses red light (λ ≈ 700 nm), and both have the same numerical aperture, which microscope will have better resolving power?…
easy