Lecture 25-1 Aberrations
Chromatic aberration Cameras, …
correct
nblue > nred
Spherical aberration
Parabolic mirrror
Large telescopes, … Lecture 25-2 The Human Eye (1) Refraction at the cornea and lens surfaces produces an inverted real image on the retina of the eye (the brain interprets it upright !) For an object to be seen clearly, the image must be formed at the location of the retina The shape of the lens controls the distance of the image
≈ 2.5 cm Lecture 25 - 3 The Human Eye (2) • The lens is held in place by ligamen that connect it to the ciliary muscle that allows the lens to change shape and thus change the focus of the lens • The index of refraction of the two fluids in the eye are close to that of water with a value of 1.34; the index of refraction of the material making up the lens is 1.40 • Thus most of the refraction occurs at the air/cornea boundary. ≈ 2.5 cm Lecture 25-4 The Human Eye (3)
≈ 2.5 cm
111 dd0 i f
f depends on d0
d0 ↓ => f↓ to keep di at ≈ 2.5 cm Lecture 25-6 READING QUIZ 1
Which of the following statements is incorrect ? The statements describe the optical characteristics of the human eye.
A| A near sighted eye focuses in front of the retina when observing an object located at infinity. B| A far sighted eye focuses behind the retina when observing an object located at infinity. C| A converging lens is used to correct the vision of a near sighted eye. D| Astigmatism is a condition where the eye does not have cylindrical symmetry about the central optical axis. E| A cataract operation of the second lens inside the eye removes the clouded lens and replaces it with a plastic implant. Lecture 25-5 Example: Corrective Lenses (1) Question: A hyperopic (far-sighted) person whose uncorrected near point is 75 cm wishes to read a newspaper at a distance of 25 cm. What is the power of the corrective lens needed for this person? Answer: • The corrective lens must produce a virtual, upright image of the newspaper at the near point of the person’s vision as shown below
• The object and image are on the same side so the image distance is negative Lecture 25-6 Example: Corrective Lenses (2)
• Thus the object distance is 25 cm and image distance is -75 cm
111 111 2.66 D 0.25 m 0.75 m f dd0 i f • The required lens is a converging lens with a power of +2.66 diopters or +2.66 D (focal length of +0.376 m = 1/ 2.66 D) Lecture 25-21 Compound Microscope
A microscope consists of two converging lenses: an objective (the front lens) and an eyepiece.
An object is placed near the first focal point of the yL' objective. The separation of the lenses is adjusted so mo that the image produced by the objective is formed yfo just inside the first focal point of the eyepiece. The lateral magnification of the objective is xnp The eyepiece angular magnification (eye near point = xnp ) M e f e
The overall magnifying power is defined as
L xnp MmM oe foef Lecture 25-14 The Telescope • Like the microscope, telescopes come in many forms • First we will discuss (1) the refracting telescope and then (2) reflecting telescopes • The refracting telescope consists of two lenses – The objective lens and the eyepiece • In our example we represent the telescope using two thin lenses • However, an actual refracting telescope will use more sophisticated lenses
Lecture 25-22 Astronomical Telescopes
Refractor Telescope