Lecture%2039%202220%20Spring%202009 - Reminders and...

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1 Reminders and Announcements • Exam 3 average before curve: 55 HW 22 is due Friday ; HW 23 is due next Tuesday; HW 24 will be optional (extra Tuesday; HW 24 will be optional (extra credit) and will be due a week from Friday • Help Room Hours (Physics 117): – M 4-8 pm, T 3-6:30, W 3-6:30, R 4:30-8, F 2-6 The Camera • The photographic camera is a simple optical instrument • Components – Light-tight chamber – Converging lens • Produces a real image – Film or CCD behind the lens • Receives the image Camera, f -numbers • The ƒ-number of a camera lens is the ratio of the focal length of the lens to its diameter ƒ-number =ƒ/ D ƒ number ƒ / The ƒ-number is often given as a description of the lens “speed” • A lens with a low f -number is a “fast” lens • The intensity of light incident on the film is related to the ƒ-number: I 1/(ƒ-number) 2 Camera, Depth of Field • A high value for the ƒ-number allows for a large depth of field – This means that objects at a wide range of This means that objects at a wide range of distances from the lens form reasonably sharp images on the film – The camera would not have to be focused for various objects The Eye • The normal eye focuses light and produces a sharp image • Essential parts of the eye: – Cornea – light passes through hi this transparent structure – Aqueous Humor – clear liquid behind the cornea The Eye – Close-up of the Cornea
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2 The Eye – Near and Far Points • The near point is the closest distance for which the lens can accommodate to focus light on the retina Typically at age 10 this is about 18 cm – Typically at age 10, this is about 18 cm – The average value is about 25 cm – It increases with age • Up to 500 cm or greater at age 60 • The far point of the eye represents the largest distance for which the lens of the relaxed eye can focus light on the retina – Normal vision has a far point of infinity The Eye – Seeing Colors • Only three types of color-sensitive cells are present in the retin retina – They are called red, green and blue cones • What color is seen depends on which cones are stimulated Conditions of the Eye • Eyes may suffer a mismatch between the focusing power of the lens-cornea system and the length of the eye • Eyes may be: – Farsighted • Light rays reach the retina before they converge to form an image – Nearsighted • Person can focus on nearby objects but not those far away Farsightedness • Also called hyperopia • The near point of the farsighted person is much farther away than that of the normal eye • The image focuses behind the retina • Can usually see far away objects clearly, but not nearby objects Correcting Farsightedness • A converging lens placed in front of the eye can correct the condition
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Lecture%2039%202220%20Spring%202009 - Reminders and...

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