Pre-lab exercises: 1. What is the focal length of a magnifying glass of 3.0 X ma

Question

Pre-lab exercises: 1. What is the focal length of a magnifying glass of 3.0 X magnification for a relaxed normal eye? 2. What is the magnification of an astronomical telescope whose objective lens has a focal length of 70 cm and whose eyepiece has a focal length of 3.0 cm? What is the overall length of the telescope when adjusted for a relaxed eye? 3. A microscope uses an eyepiece with a focal length of 1.5 cm. Using a normal eye with a final image at infinity, the tube length is 17.5 cm and the focal length of the objective lens is 0.60 cm. What is the magnification of the microscope?

Explanation / Answer

1.

formula for the focal length of the magnifying lens is

f = N/M

where N is the near point , and M is magnification .

f = 25cm/M
f = (25cm)/(3.0 )

f = 8.33 cm

2.

magnification of telescope

M = fo /fe = 70/3 = 23.33

when an adjusted for relaxed eye,the overall length of the telescope is given by

L = fo +fe = 70 +3 = 73 cm

3.

given

length ( l ) = 17.5 cm

focal length of the objective lens ( fo ) = 0.60 cm.

focal length of the eyepiece lens (fe ) = 1.5 cm

The angular magnification will be

M = N* l / fo* fe

N is the near point .

M = ( 25 ) *( 17.5 ) /( 0.60) *( 1.5 )

M = 486.11

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