Compound Microscope and Resolution
For normal adjustment in the standard long-tube approximation, M = (L/f_o)(D/f_e) in magnitude. Resolution improves with shorter wavelength and larger numerical aperture.
Why this shows up in the exam
Biological microscopy · Metallurgical inspection · Electron-microscope comparison
Learn the idea
A compound microscope multiplies the objective's linear magnification by the eyepiece's angular magnification. The short-focus objective first makes a large real intermediate image, then the eyepiece acts as a magnifier.
🧠 Memory hook: Objective enlarges first; eyepiece enlarges the angle.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- |M| approximately (L/f_o)(D/f_e) — normal adjustment and L much larger than f_o
- d_min approximately 0.61 lambda/NA — diffraction-limited lateral resolution
How to approach it
- 1Find the objective image or use the stated approximation
- 2Compute eyepiece angular magnification
- 3Multiply magnitudes and treat resolution separately
Common slip-ups that cost marks
- •Adding component magnifications
- •Using tube length as exact objective image distance without approximation
- •Equating magnification with resolving power
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A real object is placed 30 cm from a converging lens of focal length 10 cm. Find the real image distance.
More from Optics
Interference of light
Examine the principle of superposition, Young's double slit experiment, fringe width, intensity distribution, and the conditions for constructive and destructive interference.
Diffraction of light
Understand the bending of light around obstacles, single slit diffraction patterns, their width, and the effect of wavelength on diffraction.
Lenses and mirrors
Explore the image formation, ray diagrams, lens and mirror formulas, and the behavior of light with concave/convex lenses and mirrors, including combinations and virtual objects.
Optical instruments
Understand the working principles, magnification, resolving power, and design of devices like microscopes and telescopes, including their adjustments and measurement techniques.
Polarization of light
Learn about the polarization of light, Brewster's law, Malus' law, and the use and function of polaroids.
Dispersion and rainbow formation
Study how light splits into its constituent colors through dispersion in prisms and natural phenomena like rainbows, including minimum deviation and dispersive power.