7. Optical instruments 1) Cameras There is a certain range of distances over which objects will be in focus; this is called the depth of field of the lens.
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7. Optical instruments1) Cameras
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There is a certain range of distances over which objects will be in focus; this is called the depth of field of the lens. Objects closer or farther will be blurred.
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1a) Basic parts of a camera. Camera adjustments.
•Focusing: adjusting the position of the lens so that the image is positioned on the film•Shutter speed: controls the amount of time light enters the camera•f-stop: controls the maximum opening of the shutter. This allows the right amount of
light to enter to properly expose the film, and must be adjusted for external light conditions.
A digital camera uses CCD sensors instead of film. The digitized image is sent to a processor for storage and later retrieval.
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Example: Suppose that a correct exposure is 1/250 s at f/11. Under the same conditions, what exposure time would be needed for a pinhole camera if the pinhole diameter is 1.0 mm and the film is 7.0 cm from the hole? The pinhole camera uses a tiny pinhole instead of a lens.
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Example: What is the focal length of the eye-lens system when viewing an object (a) at infinity, (b) 33 cm from the eye? Assume that the lens-retina distance is 2.0 cm.
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2) The Human Eye
The human eye resembles a camera in its basic functioning, with an adjustable lens, the iris, and the retina.
•Most of the refraction is done at the surface of the cornea.•The lens makes small adjustments to focus at different distances.
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3) Corrective Lenses
a) Nearsightedness can be corrected with a diverging lens
b) And farsightedness with a diverging lens
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Far point: Farthest distance at which object can be seen clearly. Normal FP is at infinity.
Nearsightedness: far point is too close.
Near point: Closest distance at which eye can focus clearly. Normal NP is about 25 cm.
Farsightedness: Near point is too far away.
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To calculate lens power we use lens equation:
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Correction for the distance between glass and eye:
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Example: A person cannot see clearly objects more than 70.0 cm away. What power of lens should be prescribed if the glass is to be worn 1.00 cm in front of the eye?
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Example: A nearsighted person wears glasses whose lenses have power of -0.15D. What is the person's far point if the glasses are very close to the eyes?
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Example: What power lens is needed to correct for farsightedness where the uncorrected near point is 75 cm?
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Example: A farsighted person can read a newspaper held 25 cm from his eyes, if he wears glasses of +3.33 diopters. What is this person's near point?
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Vision is blurry underwater because light rays are bent much less than they would be if entering the eye from air.
This can be avoided by wearing goggles.
Vision underwater
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4) Aberrations of Lenses and Mirrors
a) 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
b) Distortion: caused by variation in magnification with distance from the lens axis. Barrel and pincushion distortion:
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c) 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
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5) Magnifying glass (Simple magnifier)A converging lens that allows one to focus on objects closer than the near point, so that it makes a larger, and therefore clearer, image on the retina.
Note!
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The power of a magnifying glass is described by its angular magnification:
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Example: A magnifying glass with a focal length of 8.5 cm is used to read print placed at a distance 7.5 cm. Calculate (a) the position of the image; (b) the angular magnification
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Example: A person uses a converging lens of focal length 5.0 cm as a magnifying glass. What is the maximum possible magnification?
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6)Telescopes
A refracting telescope consists of two 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 magnification is given by:
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Example: A170x astronomical telescope is adjusted for a relaxed eye when the two lenses are 1.25 m apart. What is the focal length of each lens?
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Example: A student constructs an astronomical telescope with a magnification of 10. If the telescope has a converging lens of focal length 50 cm, what is the focal length of the eyepiece? What is the resulting length of the telescope?
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6a) Different types of telescopes
Astronomical telescopes need to gather as much light as possible, meaning that the objective must be as large as possible. Hence, mirrors are used instead of lenses, as they can be made much larger and with more precision.
A terrestrial telescope, used for viewing objects on Earth, should produce an upright image. Here are two models, a Galilean type and a spyglass:
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7) 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:
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Example: A microscope has an eyepiece with a focal length of 3.5 cm. An object is placed 1.5 cm in front of the objective lens and its image appears 21.0 cm behind the objective lens. The microscope is adjusted for a relaxed normal eye. What is the overall magnification of the microscope after the adjustment?
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