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Chapter 25: Optics

25.C: Optical Instruments

Optical Instruments
Devices that use lenses or mirrors to form images of objects.
Microscope
An instrument that uses lenses to produce a magnified image of a small object.
Telescope
An instrument that uses lenses or mirrors to observe distant objects.
Magnification
The process of enlarging the appearance of an object using an optical instrument.
Resolution
The ability of an optical instrument to distinguish between two closely spaced objects.

Microscopes

  • A compound microscope uses two lenses to achieve high magnification.
  • The objective lens forms a real, inverted image of the object.
  • The eyepiece lens magnifies this image to produce a virtual, upright image.
  • Magnification is the product of the magnifications of the objective and eyepiece lenses.

Telescopes

  • Refracting telescopes use lenses to gather and focus light.
  • Reflecting telescopes use mirrors to gather and focus light.
  • The primary mirror or lens collects light and forms a real image.
  • The eyepiece lens magnifies this image to produce a virtual image.
  • Telescopes are used for astronomical observations and terrestrial viewing.

Magnification and Resolution

  • Magnification is given by the ratio of the image size to the object size.
  • Resolution is limited by the wavelength of light and the quality of the optical components.
  • Higher magnification does not always mean better resolution.

Example: Calculating Magnification

Calculate the magnification of a microscope with an objective lens magnification of 40x and an eyepiece lens magnification of 10x.

Magnification = 40 × 10 = 400x

Example: Calculating Resolution

Calculate the resolution of a microscope using light with a wavelength of 550 nm and an objective lens with a numerical aperture of 1.25.

Resolution = 0.61 × λ / NA

Resolution = 0.61 × 550 nm / 1.25

Resolution = 268 nm

Questions for Students

  1. Define optical instruments and give examples.
  2. Explain the working principle of a compound microscope.
  3. Describe the difference between refracting and reflecting telescopes.
  4. Calculate the magnification of an optical instrument given the magnifications of its components.
  5. Explain the concept of resolution and how it affects the performance of optical instruments.