A farsighted person has a nearpoint of 54 cm from her eyes. She wants glasses th

Question

A farsighted person has a nearpoint of 54 cm from her eyes. She wants glasses that will let her see objects at a distance of only 25 cm from her eyes. Determine the focal length of the glasses needed if the glasses are 2 cm and 3 cm from her eyes. (Remember, a converging lens has a positive focal length and a diverging lens has a negative focal length.) fright =

A compound microscope consists of a 10X eyepiece and a 60X objective, and has a standard tube length L = 160 mm. The microscope is designed for a user with normal vision, i.e. with a near point of 25 cm.

The microscope is used to image a cell of size 4 µm. What is the angular size of the cell as viewed through the microscope aided, in radians?

Explanation / Answer

a) A farsighted person has a nearpoint of 54 cm from her eyes. She wants glasses that will let her see objects at a distance of only 25 cm from her eyes. Determine the focal length of the glasses needed if the glasses are 2 cm and 3 cm from her eyes.

To find F2, you need to subtract 2 cm from nearpoint and 25 cm.
1/f = -1/52 + 1/23
1/f= 29/1196
f = 41.24
:. F2 cm = 41.24cm

To find F3, you need to subtract 3 cm from nearpoint and 25 cm.
1/f = -1/51 + 1/22
1/f= 29/1122
f = 38.69
;. F3 cm = 38.69 cm

2. angular magnification is given by
M =  25x10^-2/4x10^-6 =  62500

angluar size = tan-1(60/10) = 80.53 deg = 1.40 radian

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