C
CraftExam
medium difficulty10 min

Optical Instruments: Microscope and Telescope

Optical instruments use lenses or mirrors to increase the apparent size or resolving usefulness of objects. A microscope magnifies nearby small objects, while a telescope increases the angular size of distant objects.

In one line: Microscope: small object, short objective. Telescope: distant object, long objective.

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Understand it

  • A simple microscope is a convex lens used so that the final image is formed at the near point or at infinity.
  • A compound microscope uses an objective of small focal length to form a real magnified intermediate image, which is further magnified by the eyepiece.
  • A refracting telescope uses an objective of large focal length and aperture to form an image of a distant object, and an eyepiece to view it at a larger angle.
  • The design goal differs: a microscope needs high magnification for nearby objects, while a telescope needs angular magnification and light-gathering ability for distant objects.

Example

A laboratory compound microscope uses a short focal length objective near the specimen and an eyepiece near the eye. An astronomical telescope uses a large focal length objective pointed at distant stars or planets.

Formula and rule

Simple microscope magnifying power for final image at near point: M = 1 + D/f; for relaxed eye: M = D/f.

Here D is least distance of distinct vision, usually 25 cm, and f is focal length of the convex lens in the same unit.

Telescope in normal adjustment: M = fo/fe, where fo is objective focal length and fe is eyepiece focal length.

These formulas assume thin lenses and small-angle viewing.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

A simple microscope has focal length f = 5 cm.

  1. 1Taking D = 25 cm, magnifying power for near-point viewing is M = 1 + D/f = 1 + 25/5 = 6.
  2. 2For relaxed viewing, M = D/f = 25/5 = 5.
  3. 3Near-point viewing gives greater magnification but is less comfortable for long observation.

Method you can reuse

Identify the instrument; note whether final image is at near point or infinity; choose the correct magnifying power formula; substitute focal lengths in the same unit; interpret whether magnification is linear or angular depending on the instrument.

Visual tip

Microscope diagram should label object, objective, intermediate image, eyepiece and final virtual image. Telescope diagram should label distant object rays, objective, focal plane, eyepiece, tube length and final rays for normal adjustment.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why does a telescope objective have a large focal length and large aperture?

Think first, then show answer
A telescope observes very distant objects, so a large focal length helps produce greater angular magnification, and a large aperture collects more light. This makes the distant image brighter and more useful for viewing.

Common wrong answer

A common wrong answer is using M = 1 + D/f for a telescope, which is a simple microscope formula and does not apply to distant-object angular magnification.

Don't mix this up

Students interchange the lens choices: microscope objective should have small focal length, while telescope objective should have large focal length and large aperture.

Exam tip

For instrument questions, identify final image position first: near point gives larger magnifying power but more eye strain; final image at infinity is comfortable viewing.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

It appears in derivations of magnifying power, comparison of microscope and telescope lens choices, labelled ray diagrams, and numerical questions using D = 25 cm.
  1. 1

    Enough for 1 mark

    Optical instruments use lenses or mirrors to increase the apparent size or resolving usefulness of objects. A microscope magnifies nearby small objects, while a telescope increases the angular size of distant objects.

  2. 2

    Add for 2 marks

    Example: A laboratory compound microscope uses a short focal length objective near the specimen and an eyepiece near the eye. An astronomical telescope uses a large focal length objective pointed at distant stars or planets.

  3. 3

    Add for 3 marks

    A simple microscope is a convex lens used so that the final image is formed at the near point or at infinity.

Practice

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