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23 Nov 2019

Problem 6.
A) For a fixed focus camera to take clear pictures, the objectdistance must satisfy a certain inequality. What is this
inequality?
B) Why do older people need reading glasses or bifocals?

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Bunny Greenfelder
Bunny GreenfelderLv2
15 Jun 2019

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Related textbook solutions

Physics: Principles with Applications
7th Edition, 2013
Giancoli
ISBN: 9780321625922

Related questions

Problem 4. A frame of the film in a certain camera is 3 cm by 3 cmand the focal length of the lens in the camera is 5 cm.
A) If the camera is to take pictures of objects that are from 20 cmto far away from the camera, through what range of
lens to film distances must the camera be able to adjust?
B) What is the largest object that is 2 m from the camera thatcould entirely be imaged onto a film frame?

Background information
A person with normal vision can focus on objects as close as a fewcentimeters from the eye up to objects infinitely far away. Thereexist, however, certain conditions under which the range of visionis not so extended. For example, a nearsighted person cannot focuson objects farther than a certain point (the far point), while afarsighted person cannot focus on objects closer than a certainpoint (the near point). Note that even though the presence of anear point is common to everyone, a farsighted person has a nearpoint that is much farther from the eye than the near point of aperson with normal vision.

Both nearsightedness and farsightedness can be corrected with theuse of glasses or contact lenses. In this case, the eye convergesthe light coming from the image formed by the corrective lensrather than from the object itself.

Part A
When glasses (or contact lenses) are used to correctnearsightedness, where should the corrective lens form an image ofan object located at infinity in order for the eye to form a clearimage of that object?

The lens should form the image at the far point.


Correct
PArt B If a nearsighted person has a far point df thatis 3.50 m from the eye, what is the focal length f1 thecontact lenses that the person would need to see an object atinfinity clearly?

f1 = ? m

A 60 year old person has a threshold of hearing of 95.0 dB for asound with frequency f=10,000 Hz. By what factor must the intensityof a sound wave of that frequency, audible to a typical youngadult, (sound level=43.0 dB) be increased so that it is heard bythe older person.

If you know the sound level in dB, then you can compute the soundintensity and take the ratio.


Helpful Hint*
In the problem, they gave the intensity as decibels but want toknow by what factor to increase the intensity (I) for the youngperson to get a sound that the older person can hear.

Ignore f completely.

Decibels (dB) need to be converted to I using this equation:
I=(10^(dB/10))*10^-12
Do this for each dB and divide the older person's dB by theyounger's.


Thank you for all your help.

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