LAB 7
Mirrors
OBJECTIVES
1. Use the image-object equation to determine the image characteristic and location of an
image.
EQUIPMENT
Plane and concave mirrors and optical system.
THEORY
A concave mirror will focus parallel rays of light at the focal point. When a light source is shined
off of a concave mirror, it produces images given by the image-object and the magnification
equations:
1

1

1
and
|m|  
di
h
 i
d0 h0
f d0 di
where d0 (h0) is the object location (object height) and di (hi) is the image location (image height).
PROCEDURE
Part 1: Plane
Stand with the toes of your shoes even with tape on the floor and look at your image in the
plane mirror, then have your partner stand behind and to the side of the mirror so that their toes
are right next to the image of your toes.
a. Measure the object distance (from the mirror to your toes) and the image distance (from the
mirror to your partner’s toes). Swap places with your partner and repeat the image distance
measurements so you have at least two measured values. Is your data consistent with the
prediction that the object and image distances are equal?
b. Describe the image characteristics: real or virtual, erect or inverted, smaller, same or larger?
c. How does the height of your image compare with your actual height (use your partner's
height to estimate your image height)?
Part 2: Converging (Concave) Mirror Focal Length
Estimate the focal length of the concave mirror using two methods:
a. Hold the concave mirror and focus the overheads lights onto a sheet of paper by focusing
the image. Measure the focal length fa from the center of the concave mirror surface to the
focal point.
b. Mount the same concave mirror and a half screen on an optical bench. Using a very
distance object, accurately measure the focal length fb of the converging lens. Repeat this
as many times as there people in your group and obtain an average value.
c. Compare the focal length measures from part (2a) and (2b) using a percent difference to
make your comparison. How do they compare? Explain any discrepancies.
Part 3: Image Characteristics of a Converging Lens
a. Mount a light source and the lens from part (2). On the optical bench, center the lens and
tape off the focal point (f-point) and twice-the-focal distances (2f-points) locations.
b. Without doing any calculations, move the light source to the values indicated in the table
below and describe the image characteristics using the table below.
f
2f
d0
Real/Virtual
Inverted/Erect
Size
d0 > 2f
d0 = 2f
2f > d0 > f
d0 < f
d. Now mount the light source at one end of the optical bench and place the concave mirror a
distance outside the 2f-point from the light source.
e. Using the image-object equation, predict the image distance di,thy and the image height hi,thy.
f. Use a screen to find the image location di,exp and image height hi,exp. Compare di,thy & di,exp and
hi,thy & hi,exp using a percent difference. How do they compare?
g. Keeping the light source at one end of the optical bench and adjusting the mirror distance
from the light source, repeat steps (3c – 3e) for object distances of 40 cm, 2f & 20 cm.
f(cm)
d0(cm)
di,thy (cm)
di,exp (cm)
% hi,thy (cm)
hi,exp (cm) %
40 cm
2f
20
Part 4: Telescope
Explain how the telescope works by explaining the role of the mirror and the magnifier.