<< Chapter < Page Chapter >> Page >

Summary

  • Many optical devices contain more than a single lens or mirror. These are analyzed by considering each element sequentially. The image formed by the first is the object for the second, and so on. The same ray-tracing and thin-lens techniques developed in the previous sections apply to each lens element.
  • The overall magnification of a multiple-element system is the product of the linear magnifications of its individual elements times the angular magnification of the eyepiece. For a two-element system with an objective and an eyepiece, this is
    M = m obj M eye .

    where m obj is the linear magnification of the objective and M eye is the angular magnification of the eyepiece.
  • The microscope is a multiple-element system that contains more than a single lens or mirror. It allows us to see detail that we could not to see with the unaided eye. Both the eyepiece and objective contribute to the magnification. The magnification of a compound microscope with the image at infinity is
    M net = ( 16 cm ) ( 25 cm ) f obj f eye .

    In this equation, 16 cm is the standardized distance between the image-side focal point of the objective lens and the object-side focal point of the eyepiece, 25 cm is the normal near point distance, f obj and f eye are the focal distances for the objective lens and the eyepiece, respectively.
  • Simple telescopes can be made with two lenses. They are used for viewing objects at large distances.
  • The angular magnification M for a telescope is given by
    M = f obj f eye ,

    where f obj and f eye are the focal lengths of the objective lens and the eyepiece, respectively.

Key equations

Image distance in a plane mirror d o = d i
Focal length for a spherical mirror f = R 2
Mirror equation 1 d o + 1 d i = 1 f
Magnification of a spherical mirror m = h i h o = d i d o
Sign convention for mirrors
Focal length f + for concave mirror for concave mirror
Object distance d o + for real object for virtual object
Image distance d i + for real image for virtual image
Magnification m + for upright image for inverted image
Apparent depth equation h i = ( n 2 n 1 ) h o
Spherical interface equation n 1 d o + n 2 d i = n 2 n 1 R
The thin-lens equation 1 d o + 1 d i = 1 f
The lens maker’s equation 1 f = ( n 2 n 1 1 ) ( 1 R 1 1 R 2 )
The magnification m of an object m h i h o = d i d o
Optical power P = 1 f
Optical power of thin, closely spaced lenses P total = P lens 1 + P lens 2 + P lens 3 +
Angular magnification M of a simple magnifier M = θ image θ object
Angular magnification of an object a distance
L from the eye for a convex lens of focal length
f held a distance from the eye
M = ( 25 cm L ) ( 1 + L f )
Range of angular magnification for a given
lens for a person with a near point of 25 cm
25 cm f M 1 + 25 cm f
Net magnification of compound microscope M net = m obj M eye = d i obj ( f eye + 25 cm ) f obj f eye

Conceptual questions

Geometric optics describes the interaction of light with macroscopic objects. Why, then, is it correct to use geometric optics to analyze a microscope’s image?

Microscopes create images of macroscopic size, so geometric optics applies.

Got questions? Get instant answers now!

The image produced by the microscope in [link] cannot be projected. Could extra lenses or mirrors project it? Explain.

Got questions? Get instant answers now!

If you want your microscope or telescope to project a real image onto a screen, how would you change the placement of the eyepiece relative to the objective?

The eyepiece would be moved slightly farther from the objective so that the image formed by the objective falls just beyond the focal length of the eyepiece.

Got questions? Get instant answers now!
Practice Key Terms 6

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'University physics volume 3' conversation and receive update notifications?

Ask