LAB 6
Mirror Images
OBJECTIVES
1. Analyze and measure the behavior of images formed by plane, concave, and convex
mirrors.
2. Experimentally identify whether an image is real or virtual, upright or inverted, as well as
enlarged or reduced.
3. Observe how an image changes as the object is moved from “infinity” to a distance
within the focal length of the mirror.
4. Experimentally measure the image distance and confirm the mirror equation.
EQUIPMENT
Plane mirror, optical bench, light source, concave and convex mirrors, and meter sticks.
THEORY
Spherical mirrors can form images of a source of light (the object) by reflecting rays
emerging from the source. The image occurs where the reflected rays cross (forming a real
image) or where backward extensions of those rays cross (forming a virtual image). The
mirror equation is a relation between the object distance p (which is positive) and the image
distance i (which is positive for real images and negative for virtual images):
1 1 1
 
f p i
(Real images form on the side of a mirror where the object is located, and virtual images
form on the opposite side.) The lateral magnification m produced by a spherical mirror is
image height h'
i
m


object height h
p
where h´ and h are the heights of the image and object, respectively.
PROCEDURE
Part 1: Virtual Images in a Plane Mirror
a. Stand and look at your image in the plane mirror. Now 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.
b. 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 three measured values (iexp). Is your data
consistent with the prediction ithy that the object and image distances are equal?
c. How does the height of your image himage compare with your actual height hobject (use
your partner's height to estimate your image height)?
d. Describe the image characteristics: real or virtual? Erect or inverted? Smaller, larger or
the same size?
Part 2: Focal Point of a Concave Mirror
Estimate the focal length of the converging mirror using two methods:
a. Using a concave mirror, get as far as possible from the “OPEN” neon light, and focus it
onto a sheet of paper. Measure the image location from the center of the concave mirror
surface; call this “fa”.
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 concave mirror. 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 Concave Mirror
a. Mount a light source, the mirror from part (2) and a half screen onto an optical bench. On
the optical bench, center the mirror and tape off the two focal points (f-point) and the two
twice-the-focal distances (2f-points).
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(cm)
2f(cm)
p(cm)
p > 2f
p = 2f
i(cm)
R/V
I/E
|m|
2f > p > f
p<f
Mirror-Lens Equation
c. Now mount the light source (the object) at one end of the optical bench and place the
concave mirror at distances indicated in the table below.
d. Using the mirror-lens and magnification equations, complete the table below by
predicting and measuring the image distances and image height. How do they compare?
f(cm)
p(cm)
Challenge
1.5f
2.5f
Inside f
ithy (cm)
iexp (cm)
%
hthy (cm)
hexp (cm)
%
Hint on challenge: use your hand as a “close distant” object to estimate the image
distance as you did in part (1) with the plane mirror. That is, the distance that a virtual
image of your hand is produced behind the mirror
e. Use ray tracing (use applet to guide you) to draw an accurate scaled ray diagram for (i) p
= 1.5f and (ii) p < f.
Part 4: Convex (Diverging) Mirror
a. Roughly estimate the focal length of the diverging mirror by estimating its radius of
curvature.
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(cm)
2f(cm)
p(cm)
p = 2f
2f > p > f
i(cm)
R/V
I/E
|m|
p<f
d. Use ray tracing (use applet to guide you) to draw an accurate scaled ray diagram for p =
2f.