Phys 375 – Wednesday/Thursday sections – Prof. Fuhrer
Homework #3, due in class week of October 1-2, 2008
1) The normal human eye can focus on objects at distances ranging from 25 cm to
infinity. The distance from lens to retina is about 1.7 cm. What is the range of focal
lengths of the lens of the eye?
2) Use a ruler and pencil and paper to find the image distances and magnifications for the
following two situations. You must draw these by hand, and use the measured distances
to find the image distances and magnifications! You may check your results with the
thin-lens equation.
A) An object of height 2 cm at a distance 12 cm from a converging lens of focal
length 8 cm.
B) An object of height 2 cm at a distance 12 cm from a diverging lens of focal
length 8 cm.
3) A lightbulb is placed at x = 0 cm on an optical bench. A diverging lens of unknown
focal length is placed at x = 15 cm. A converging lens of focal length 10 cm is placed at
x = 25 cm. A screen is placed at x = 55 cm and shows a well-focused image of the light
bulb filament.
A) What is the focal length of the diverging lens?
B) What is the lateral magnification of the image?
C) Is the image upright or inverted?
4) You construct an astronomical telescope with using a converging lens of focal length
30 cm to form an image of an object at infinity, and a second converging lens (eyepiece)
of focal length 5 cm placed beyond the image to magnify that image. You wish to place
the second lens such that the image is at the near point of the eye (25 cm) when the eye is
close to the lens.
A) Where should you place the second lens?
B) What is the angular magnification of the telescope?
C) Is the image upright or inverted?