Lesson 7:
Mirror and Lens Equations


HW #7 Optics due tomorrow
Exam repair on Friday
IV.C.2. Mirrors
Lesson 7:
Mirror and Lens Equations

Students should understand image formation by spherical mirrors, so they
can:
d) Use the mirror equation to relate the object distance, image distance,
and focal length for a lens, and determine the image size in terms of the
object size.
IV.C.3. Lenses

Students should understand image formation by converging or diverging
lenses, so they can:
c) Use the thin lens equation to relate the object distance, image distance,
and focal length for a lens, and determine the image size in terms of the
object size.
Students will be able to
1.
Determine the location and magnification of an image using
the thin mirror and thin lens equations

1/si + 1/so = 1/f
◦ si: image distance
◦ so: object distance
◦ f: focal length

M = hi/ho = -si/so
◦
◦
◦
◦
◦
si: image distance
so: object distance
hi: image height
ho: object height
M: magnification

Focal length (f)

Magnification (M)

Image Distance
◦ Positive for CONCAVE mirrors
◦ Negative for CONVEX mirrors
◦
◦
◦
◦
Positive for UPRIGHT images
Negative for INVERTED images
ENLARGED when |M| > 1
REDUCED when |M| < 1
◦ si is POSITIVE for real images
◦ si is NEGATIVE for virtual images
A spherical concave mirror, focal length 20 cm, has a
5-cm high object placed 30 cm from it.
a) Use the mirror equations to calculate
i.
the position of image
ii.
the magnification
iii. the size of image
A spherical concave mirror, focal length 20 cm, has a
5-cm high object placed 30 cm from it.
b) Draw a ray diagram and construct the image.
c)
Name the image
A spherical convex mirror, focal length -15 cm, has a
4 cm high object placed 10 cm from it.
a) Use the mirror equations to calculate
i.
the position of image
ii.
the magnification
iii. the size of image
b) Name the image
 We
use the same equations we
used for mirrors
◦ 1/si + 1/so = 1/f
◦ M = hi/ho = -si/so
A converging lens, focal length 30 cm, has a 10-cm high object
placed 70 cm from it.
a) Use the lens equations to calculate
i. the position of image
ii. the magnification
iii. the size of image
b) Name the image
A converging lens, focal length 10 cm, has a 2-cm high object
placed 5 cm from it.
a) Use the lens equations to calculate
i. the position of image
ii. the magnification
iii. the size of image
b) Name the image

A diverging lens, focal length -10 cm, has a 8-cm
high object placed 12 cm from it.
a)
b)
Use the lens equations to calculate
i. the position of image
ii. the magnification
iii. the size of image
Name the image.