There is a range of distances over which objects will be in focus; this is called the depth of field of the lens. Objects closer or farther are

Transcription

1 Chapter 25 Optical Instruments

2 Some Topics in Chapter 25 Cameras The Human Eye; Corrective Lenses Magnifying Glass Telescopes Compound Microscope Aberrations of Lenses and Mirrors Limits of Resolution

3 25.1 Cameras, Film, and Digital There is a range of distances over which objects will be in focus; this is called the depth of field of the lens. Objects closer or farther are blurred.

4 25.2 The Human Eye; Corrective Lenses The human eye resembles a camera in its basic functioning, with an adjustable lens, the iris, and the retina.

5 The Human Eye Most of the refraction is done at the surface of the cornea; the lens makes small adjustments to focus at different distances.

6 The Human Eye; Corrective Lenses Near point, N: closest distance at which eye can focus clearly. Normal is about N = 25 cm. Far point: farthest distance at which an object can be seen clearly. Normal is at infinity. Nearsightedness: far point is too close. Farsightedness: near point is too far away.

7 Corrective Lenses Nearsightedness corrected with a diverging lens. Farsightedness corrected with a converging lens.

8 Magnifying Glass A simple magnifying glass 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. Image is virtual. Relaxed eye focus at infinity (Near point focus shown)

9 Magnifying Glass The power of a magnifying glass is described by its angular magnification: If the eye is relaxed focus at infinity If the eye is focused at the near point, M increases

10 Telescopes A refracting telescope consists of two lenses at ends of a long tube. The objective lens (fo) is closest to the object, and the eyepiece (fe) is closest to the eye. The magnification is given by:

11 25.4 Telescopes The eyepiece functions like a magnifying glass acting on the image from the objective lens

12 Telescopes Astronomical telescopes need to gather as much light as possible, meaning that the objective must be as large as possible. So mirrors are used instead of lenses, because they can be made much larger and cheaper with more precision.

13 Telescopes A terrestrial telescope, used for viewing objects on Earth, should produce an upright image. Here are two models, a Galilean type and a spyglass:

14 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. Their designs have some differences, but the basic optical function of the two lenses are the same for a microscope and telescope

15 Compound Microscope The magnification is given by:

16 Limits of Resolution; Circular Apertures Resolution is the distance at which a lens can barely distinguish two separate objects. For a well designed optical the resolution is limited by diffraction, which is unavoidable; it is due to the size of the lens compared to the wavelength of the light. This limit is often called the wavelength limit.

17 Limits of Resolution; Circular Apertures For a circular aperture of diameter D, the central maximum has an angular width:

18 Limits of Resolution; Circular Apertures The Rayleigh criterion states that two images are just resolvable when the center of one diffraction peak in a is over the first minimum of the other.

19 Resolution of Telescopes and Microscopes; the λ Limit For microscopes, assuming the object is at the focal point, the resolving power (RP) is given by: Typically, the focal length of a microscope lens is half its diameter, which shows that it is not possible to resolve details smaller than the wavelength being used.

20 Resolution of the Human Eye Useful Magnification The human eye can resolve objects that are about 1 cm apart at a distance of 20 m, or 0.1 mm apart at the near point. This limits the useful magnification of a light microscope to about 500x 1000x.