Mirrors
Mirrors are used in many different optical devices. Cameras, telescopes, flashlights, searchlights and recently,
solar power generation all incorporate mirrors. The Hubble Telescope orbiting the Earth uses mirrors to capture
images from other areas of the universe.
The three main types are converging, diverging and plane mirrors.
Figure 1: Draw a converging, diverging and plane mirror.
Converging Mirrors
This mirror causes light to converge to a common point, hence is called a converging mirror. The common
point where the rays intersect is called the _________ ________.
Figure 2: Draw the reflected rays. Label the principal axis and the focal point (focal point is 4 cm from center
of mirror)
Converging Mirrors and Image Formation
Mirrors can be used to create real images of objects. The image formed by this mirror can be projected on a
screen or a sheet of paper. The real image can also be seen in the mirror if viewed from head on.
Distances are used to learn more about the
properties of the images. The ________ is the
point on the mirror that is intersected by the
principal axis. The focal point is labeled f. 2f is
twice the distance of the focal length and is labeled
c (center of curvature). The distance between the
object and the vertex is called the _________
____________ (do). The distance between the
image and the vertex is called the ____________
_______________ (di).
Label do, di, f and c. Label the image, the principal
axis and the vertex.
Figure 3:
.
.
Image Distance and Ray Diagrams
To determine the location of an image,
Step 1. Place the object so that its bottom is on the ___________ _______.
Step 2. Draw each of the three rays for the ______ _________ of the object only. If the 3 rays do not intersect,
re-draw until they do.
For the ray that is incident on the vertex of the mirror, the angle of incidence must equal the angle of reflection.
This ray serves as a check to determine if the first two rays were drawn correctly, and technically does not need
to be drawn to locate the image.
Step 3. Draw the image, so that its _________ is on the ____________ _________ and its top is where the 3
rays ____________.
Example 1: Object height is 1 cm, object distance is 11 cm and focal length is 4 cm. Draw the ray diagram and
measure the image distance.
Image distance =
Image height =
.
Image Characteristics: Real, __________, _____________
Example 2: Object height is 1 cm, object distance is 8 cm and focal length is 4 cm. Draw the ray diagram and
measure the image distance.
Image distance =
Image height =
Image Characteristics: __________, __________, _____________
Example 3: Object height is 1 cm, object distance is 6 cm and focal length is 4 cm. Draw the ray diagram and
measure the image distance.
Image distance =
Image height =
Image Characteristics: __________, __________, _____________
1. How does image distance and image size vary with object distance?
2. In general, where does the object have to be in relation to the focal length to get a larger image?
3. In general, where does the object have to be in relation to the focal length to get a smaller image?
4. In general, where does the object have to be in relation to the focal length to get an image with the same
size?
Example 4: Object height is 1 cm, object distance is 6 cm and focal length is 2 cm. Draw the ray diagram and
measure the image distance.
Image distance =
Image height =
Image Characteristics: __________, __________, _____________
Example 5: Object height is 1 cm, object distance is 6 cm and focal length is 1 cm. Draw the ray diagram and
measure the image distance.
Image distance =
Image height =
Image Characteristics: __________, __________, _____________
5. Compare example 3, example 4 and example 5. For the same object distance, how does image distance and
size vary with focal length?
Plane Mirrors and Image Formation
To locate the image of an object for a plane mirror, at least 3 rays need to be drawn. Begin by choosing to find
the location of one end of the object. For this example in figure 4, we will start with the top end of the pencil.
In figure 4, ray 1 is drawn so that the incident angle is 0 degrees. Ray 2 and ray 3 are drawn such that the
incident angle is not 0 degrees and can be any angle, so long as the ray is incident on the mirror. For ray 2 and
3, the angle of reflection must equal the angle of incidence. The reflected rays are the lines that are gray.
Part a:
i) Draw the normal for ray 2 and ray 3.
ii) Measure the angle of incidence and the angle of reflection for each ray.
iii) Draw dashed lines projected backwards for each reflection (the lighter lines) and where they intersect, draw
the top part of the object (pencil).
Part b:
i) Repeat the same steps for the bottom part of the object (pencil) to locate the image in the mirror.
Figure 4:
3
1
2
object
Object distance =
Object size =
Image distance =
Image size =
(measure from front of mirror)
Exercise: On a separate sheet of paper, draw a mirror (10 cm length) and place an object (4 cm in height) 3 cm
from the mirror. Using 3 rays for each end of the object, locate the image and measure image distance and
image height. What can you conclude about the characteristics of the image?
Virtual Images
As seen in figure 4, for plane mirrors the reflected rays do not converge to a common point. Instead, the
reflected rays diverge (spread apart). When these reflected rays are projected backwards, they appear to come
from a common point behind the mirror. When rays do not converge, the image is said to be virtual. If a screen
were placed behind the mirror where the dashed lines meet, nothing would appear on it as rays cannot pass
through the mirror.
The images formed by the converging mirror and the converging lens were called real, as the rays converged to
a common point and the image could be seen when a screen is placed at that point. Nevertheless, virtual images
can also be formed by a converging mirror or lens.
Investigate where an object would have to be placed relative to the focal point to form a virtual image in a
converging mirror or lens.