Lecture 11 Optical Instruments Overview

Transcription

1 Lecture 11 Optical Instruments Overview Lenses were discovered about 700 years ago. They were mostly used as magnifying glasses for visual aids. It took humanity roughly another 300 years to figure out that if you combine two lenses in a particular manner, you would obtain a telescope. Galileo is often credited with inventing the telescope in the early 1600 s, but this is false. Galileo was the first scientist to point his telescope toward the heavens. He did not discover the telescope. An unknown Dutch lens maker invented the telescope in 1608 in Holland. The inventor of an opera-glass like telescope was called Lipperhey. He was unable to get a patent, however, because his invention was deemed too easy to reproduce. Perhaps the reason it had not been done before was that to get magnification, one needs a concave lens stronger than the convex lens being used with it, and commonly the lenses in wide use were the other way around. Galileo found out about this invention in the spring of 1609, and immediately set about improving it. He saw it as a possible way out of his financial difficulties. He was an oldest son, and so was responsible for his younger sisters dowries. He also had three children of his own, by his mistress. At the time, he was a Professor of Mathematics in the University of Padua, in the Venetian Republic. He soon put together a spyglass with a magnification of three, which many other people had already done. Galileo was an excellent experimentalist, and working with different lenses, he realized that the magnification was proportional to the ratio of the power of the concave (eyepiece) lens to the convex (more distant) lens. In other words, to get high magnification he needed a weak convex lens and a strong concave lens. the problem was that the opticians only made glasses in a narrow range of strengths, and three or so was the best magnification available with off the shelf lenses. Galileo therefore learned to grind his own lenses, and by August, he had achieved about ninefold linear magnification. This was an enormous improvement over everything else on the market. Galileo therefore approached the Senate of Venice to demonstrate his instrument. Many senators climbed the highest bell towers in Venice to look through the glass at ships far out at sea, and were impressed by the obvious military potential of the invention.

2 Magnifying Glass A magnifying glass (simple magnifier) is a converging lens. It allows us to focus on objects closer than the near point, so that they make a larger, and therefore clearer, image on the retina. The power of a magnifying glass is described by its angular magnification: If the eye is relaxed (N is the near point distance and f the focal length): If the eye is focused at the near point: Since N=25 and 25/f is much greater than 1, we can neglect the last term and we get M! 25 f

3 Keplerian Telescope A Keplerian telescope consists of two converging lenses at opposite ends of a long tube. The objective lens is closest to the object, and the eyepiece is closest to the eye. The distance between the lenses is f o +f e.. The angular magnification of the Keplerian telescope is given by

4 Galileo Telescope A Galileo telescope consists of a converging lenses and a diverging lens at opposite ends of a long tube. The objective lens is closest to the object, and the eyepiece is closest to the eye. It produces an upright image and sometimes is called a terrestrial telescope. The distance between the lenses is f o -f e.. M =! '! =! h / 2 f e h / 2 f o = f o f e

5 Compound Microscope A compound microscope also has an objective and an eyepiece; it is different from a telescope in that the object is placed very close to the eyepiece. The magnification is given by

6 Aberrations of Lenses and Mirrors Spherical aberration: rays far from the lens axis do not focus at the focal point. Solutions: compound-lens systems; use only central part of lens. Aberrations of Lenses and Mirrors Distortion: caused by variation in magnification with distance from the lens. Barrel and pincushion distortion:

7 Aberrations of Lenses and Mirrors Chromatic aberration: light of different wavelengths has different indices of refraction and focuses at different points. Solution: Achromatic doublet, made of lenses of two different materials