Physics 1 – Concave Lenses and Mirrors
Concave LENSES AND MIRRORS
Aims
 To correctly setup an optics track.
 To study how images are formed by simple concave mirrors and lenses.
 To make measurements of object distances, image distances and focal lengths of
concave mirrors and lenses.
 To draw ray diagrams, with principle rays, for both experimental situations.
In this experimental tutorial you will first attempt a question which tests your
knowledge and ability to draw principle ray diagrams concerning concave mirrors.
Then you will perform the necessary experimental procedures to attain quality
measurements, verifying theoretical prediction.
Part 1: Tutorial Question (10 mins)
1.1
A real image is produced by a concave mirror
on the same plane as the object as shown
opposite. The size of a real image, i, is trebled
if the object, o, distance is decreased from
0.75 m to 0.50 m.
o
i
(a) Describe the corresponding movement
of the image.
(b) Calculate the radius of curvature of the mirror.
(c) Draw the principle ray diagram.
0.75 m
Figure 1
Part 2: Using a concave mirror on the optical bench (15 mins)
In this part of the experimental tutorial you will set up the necessary equipment on the
optical bench and determine the focal length of a concave mirror via two different
methods and therefore verify the answer.
Calculating f through the measurement of the radius of curvature of a concave
mirror, R.
 Using the illuminated triangle in the lamp screen as an object, form a reflected
image in the object plane using a concave mirror.
2.1
Draw a ray diagram of this arrangement, showing the principle rays for an
extended object transverse to the principle axis.
The distance from the mirror to the image is the radius of curvature of the mirror, R.
 Use the vertex pointer to find R.
For a mirror, the radius of curvature is related to the focal length of the mirror, f, by
the following equation:
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Physics 1 – Concave Lenses and Mirrors
R = 2f
2.2
What is the focal length of your mirror?
Calculating f through the use of the mirror equation.
 Using the same object as before, use a screen with a large aperture and form an
image on this screen by reflection from the mirror.
The mirror equation relates the focal length of a mirror, or lens, to the object and
image distances as follows:
1 1 1
 
f s s'
2.3
Measure the object and image distances and hence calculate the mirrors focal
length.
 Comment on your two results for focal length i.e. do they compare well.
Part 3: Tutorial Question (10mins)
3.1
A thin diverging lens is placed on
an optical bench. Parallel beams of
light pass through the lens. On
viewing the lens they appear as if
they came from a point 20cm before
the centre of the lens, as shown in
figure 2. You want to use this lens
to form a vertical virtual image that
is a third of the height of the object.
.
0.20
m
(a) Where should the object be placed?
(b) Draw a principle ray diagram.
Figure 2
Part 4: Using a concave lens on the optical bench (15 mins)
In this part of the experimental tutorial you will determine the focal length of a
converging lens then observe the change when the same lens is used in conjunction
with a diverging lens. Measuring this change accurately will enable you to calculate
the focal length of the diverging lens.
 Select a converging lens and place it into the holder on the optical track.
 Use the illuminated triangle in the lamp screen as an object.
 Form a bright image on the object plane by reflection from a plane mirror on the
opposite side of the lens from the object, as shown in Figure 4.
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Physics 1 – Concave Lenses and Mirrors
4.1
What is the focal length of the convex lens?
The power, P, of a lens is related to its focal length by the expression: P 
1
,
f
where f is measured in metres, and the units of P are dioptres.
4.2
Calculate the power of this lens.
 Select the diverging lens and put it into contact with the converging lens.
 Place the plane mirror on the opposite side of the object as before and form an
image in the object plane. This image may be fainter so it’ll be harder to spot, and
the object/image distance will be larger.
Beware of images formed by reflection, there will be two images formed by
reflection from the lenses which you want to ignore.
 Use the method for combined lenses to obtain the power of the lenses in contact.
 Use PC = P1+P2 algebraically to calculate the power of the diverging lens.
4.3
Using your result for the power of the diverging lens calculate its focal
length.
Part 5: Diverging lens and concave mirror (Optional)
Virtual Image
Real Object
O
M
I
C
L
The virtual image I formed by the lens L is
thus at the centre of curvature of the mirror
Figure 3
5.1
On the optical bench set up the arrangement shown in figure 3; use a concave
mirror of known radius of curvature. Hence take measurements to obtain the
focal length of the diverging lens.
Further work
The following questions are related to the topic covered by this experimental tutorial.
 Exercise book questions L1- L16.
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Physics 1 – Concave Lenses and Mirrors
 Mastering Physics: Optics 1 (parts 5 and 6); Optics 2
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Physics 1 – Concave Lenses and Mirrors
Demonstrators' Answers, Hints, Marking Scheme and Equipment List
Marking Scheme
Section
1.1a
1.1b
1.1c
2.1
2.2
2.3
3.1a
3.1b
4.1
4.2
4.3
Discretionary mark
TOTAL
Mark
0.5
1
0.5
1
1
1
0.5
0.5
1
1
1
1
10
Answers
1.1(a) The image moves away from the mirror.
1 1 2
 
1.1(b)
s s' R
1 1 1
1 
1
  


R 2  0.75 0.75  0.75
R  0.75 m
1.1(c)
o
i
0.50 m
2.1
See Figure 1.
2.2/2.3 They should be very similar; differences tend to arise due to rounding of
numbers.
3.1(a) From the information given: m 
1
 s' 1
s
 , i.e. s ' 
, therefore
3
s
3
3
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Physics 1 – Concave Lenses and Mirrors
so substituting into
and s' 0.133 m
1 1
1
1 1 1
gives  
so s  0.40 m
 
s s' f
s s  0.20
3
3.1(b)
.
.
0.20
m
Part 4: The power obtained for the concave lens should be a negative value.
Equipment List
Concave mirror
Concave lens
Convex lens
Plane mirror
Light source
Screen with triangular hole
Pointer
3 x saddles
3 x holders
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Physics 1 – Concave Lenses and Mirrors
Example Numbers
Part 2:
Quantity
r
Pointer
Measurement
1 (m)
Pointer
Measurement
2 (m)
0.571
0.734
Length
of
pointer
(m)
0.2
f
Length
(m)
0.363
0.1815
s
s'
0.571
0.866
0.934
0.934
0.2
0.2
f
0.563
0.268
0.1816
Part 3:
Pointer
Measurement
1 (m)
Pointer
Measurement
2 (m)
Length
of
pointer
(m)
s1
0.571
0.698
0.2
0.327
s1'
0.571
0.698
0.2
0.327
Quantity
f1
Length
(m)
0.1635
s2
0.578
1.246
0.2
0.8680
s2'
0.578
1.246
0.2
0.8680
fc
0.4340
P1
(dioptre)
Pc
(dioptre)
P2
(dioptre)
6.116208
2.304147
-3.81206
f2
-0.26233
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