Lens Practice Problems:Answers
1. A person is using a converging lens (f=35.0 cm) to focus an
image of the moon onto a piece of paper. The diameter of the
moon is 3.48 x 106m, and its mean distance from the earth is
3.85 x 108m. What is the diameter of the moon’s image on the
paper?
Answer= 1/0.35=1/3.85 x 108m + 1/di
1099999999/3.85 x 108m =1/di
di=0.35m or 35 cm
M=-di/do=-0.35/3.85x108m=9.09 x 10-10m
So multiply this by the diameter of the moon to get the
diameter of the image
9.09 x 10-10m x 3.48 x 106m=0.003 m or 0.3 cm or 3 mm
2. A converging lens (f=12.0 cm) is held 8.00 cm in front of a
newspaper. Find (a) the image distance and (b) the
magnification
Answer=
a) -24 cm
B) 3.0
3. An object 0.50 cm high is placed 8.6 cm in front of a diverging
lens whose focal length is -7.5cm. Find the height of the image.
Answer= 0.23 cm
4. A diverging lens has a focal length of -38 cm. An object is placed
28 cm in front of this lens. Calculate (a) the image distance and
(b) the magnification. Is the image (c) real or virtual, (d) upright
or inverted, and (e) enlarged or reduced in size?
Answer=
a)-16 cm
b) 0.57
c) virtual
d) upright
e) reduced (b/c less than 1 magnification)
5. An object is located 30.0 cm to the left of a converging lens
whose focal length is 50.0 cm (a) draw a ray diagram to scale
and from it determine the image distance and the magnification.
(b) Use the lens and magnification equations to verify your
answers in part (a)
Answer=
di=-75.2 cm
M=75.2/30=2.5
6. A magnifying glass uses a converging lens whose focal length is
15 cm. The magnifying glass produces a virtual and upright
image that is 3.0 times larger than the object (a) How far is the
object from the lens? (b) What is the image distance?
Answer= we know that the object is in front of the focal length
because a virtual image is produced. So do is less than 15. Lets say
10,
Then 1/f=1/di+1/do = 1/15=1/di + 1/10 = 1/di=-5/150 di=-30
(virtual)
Then to confirm the relationship is correct lets look at the
Magnification to see if it matches
M=-di/do  M=-(-30)/10  M=3 which matches the question.
7. On a roll of movie film, each picture has a width of 70.0 mm.
The projector lens has a focal length of 305 mm. If the screen in
a theatre is 60.0 m from the projector lens, what is the width of
the image projected on the screen?
Answer= the width of the image on the screen is 13.68 m across.
tricky because you have to convert a lot of the mm to m or vice
versa.