Optical Instruments

Focus: Simple microscope, compound microscope, astronomical telescope, terrestrial telescope and Galilean telescope (JEE Main level).

1) Simple Microscope (Magnifying Glass)

• Single convex lens of short focal length.
• Magnifying power is the ratio of angle subtended by image to object at the eye.
• Least distance of distinct vision: D = 25 cm

Note: Smaller focal length → greater magnifying power.

2) Compound Microscope

Total magnification is the product of objective and eyepiece magnifications.

Where:
f_o = focal length of objective
f_e = focal length of eyepiece
L = length of microscope tube

Normal adjustment (final image at infinity):

3) Astronomical Telescope

(a) Final image at infinity (normal adjustment)

(b) Final image at least distance of distinct vision

4) Terrestrial Telescope

• Uses an additional inverting lens.
• Produces erect image.
• Inverting lens increases length but does not affect magnification.

5) Galilean Telescope

• Eyepiece is a concave lens.
• Produces erect image.

Note: Field of view is smaller compared to astronomical and terrestrial telescopes.