Name __________________________________
Physics 208
Section _____________
Group Problem Week 4
You look at letters on this page with your eye, holding the page about 25 cm in front of
your eye lens as in the figure below. This is far enough away for most people that they
can adjust their eye lens to form on their retina a sharp, in-focus, image of the letters.
Eye
Lens
Page
Eye
Retina
25 cm
1.7cm
a) A single letter on this page has a height of 0.3cm.
How tall is the image of this letter on your retina?
The magnification is
1.7cm
= 0.068 , so the image on your retina is 0.02cm tall.
25cm
€
b) Is the image on your retina upright or inverted with respect to the letters on the page?
Now you use a magnifying glass of focal length 5 cm as shown below.
Page
Magnifying
lens
Eye
Eye
Lens
Retina
4.2 cm
2cm
1.7cm
c) Is the image produced by the magnifying lens alone real or virtual? Explain
The image is virtual, because the object is closer to the lens than the lens focal length.
Name __________________________________
Section _____________
d) How far from the lens, and on what side of the lens, is this image?
Since the image is virtual, it appears on the same side of the lens as the object. Hence the
image distance will be negative number. It is determined from the lens equation
1
1
1
1 1 1
+ = ⇒
+ =
⇒ s′ = −26.25cm
4.2cm s′ 5cm
s s′ f
€
e) Your eye lens uses the image produced by the magnifying lens as an object to form
a sharp image on your retina, 1.7 cm behind your eye lens.
Is this image on your retina inverted or upright relative to letters on the page? Explain.
Since the image formed by the magnifying lens is virtual, it is upright relative to the letter on
the page. Your eye lens uses this virtual image as an object, producing a real image on your
retina. This real image is inverted with respect to the virtual image produced by the
magnifying lens, hence it is also inverted with respect to the letters on the page.
f) A letter on this page is 0.3cm tall. With paper, magnifying lens, and eye arranged as
in the figure, how tall is the image of that letter on the retina?
26.25cm
= 6.25 times bigger than the
4.2cm
1.7cm
= 0.062 times bigger than the
letters on the page. The image on your retina is
26.25cm + 2cm
virtual image produced by the magnifying glass. Hence the real image on your retina is
€
0.11cm tall.
The virtual image produced by the magnifying lens is
€
g) How much bigger are the letters with the magnifying glass than without?
This is the ratio of the two image sizes:
things about five times bigger.
€
0.11cm
= 5.4 . Using this magnifying glass makes
0.0209cm