ĐỀ THI OLYMPIC QUỐC TẾ MÔN VẬT LÝ

Theory Question 1
Page 1 of 5


Theory Question 1: Gravity in a Neutron Interferometer

Enter all your answers into the Answer Script.

a a
a a
2
θ

2
θ

BS
BS
M
M
BS - Beam Splitters
M - Mirror
IN
OUT2
OUT1

Figure 1a



φ

OUT1
IN
OUT2






Figure 1b

Physical situation We consider the situation of the famous neutron-interferometer
experiment by Collela, Overhauser and Werner, but idealize the set-up inasmuch as we
shall assume perfect beam splitters and mirrors within the interferometer. The experiment
studies the effect of the gravitational pull on the de Broglie waves of neutrons.
The symbolic representation of this interferometer in analogy to an optical
interferometer is shown in Figure 1a. The neutrons enter the interferometer through the
IN port and follow the two paths shown. The neutrons are detected at either one of the
two output ports, OUT1 or OUT2. The two paths enclose a diamond-shaped area, which
is typically a few cm
2
in size.
The neutron de Broglie waves (of typical wavelength of 10
−10
m) interfere such
that all neutrons emerge from the output port OUT1 if the interferometer plane is
horizontal. But when the interferometer is tilted around the axis of the incoming neutron
beam by angle
φ

(Figure 1b), one observes a
φ
dependent redistribution of the neutrons
between the two output ports OUT1 and OUT2.
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Theory Question 1
Page 2 of 5


Geometry For °= 0
φ
the interferometer plane is horizontal; for °= 90
φ
the plane is
vertical with the output ports above the tilt axis.

1.1
(1.0) How large is the diamond-shaped area
A
enclosed by the two paths of the
interferometer?

1.2
(1.0) What is the height
H
of output port OUT1 above the horizontal plane of the
tilt axis?

Express

A
and
H
in terms of , a
θ
, and
φ
.


Optical path length The optical path length N
opt
(a number) is the ratio of the
geometrical path length (a distance) and the wavelength
λ
. If
λ
changes along the path,
N
opt
is obtained by integrating along the path.
1−
λ

1.3
(3.0) What is the difference
ΔN
opt
in the optical path lengths of the two paths
when the interferometer has been tilted by angle

φ
? Express your answer in terms
of ,
a
θ
, and
φ
as well as the neutron mass M, the de Broglie wavelength
0
λ
of
the incoming neutrons, the gravitational acceleration
g, and Planck’s constant . h

1.4


(1.0) Introduce the volume parameter
2
2
gM
h
V
=
and express
ΔN
opt
solely in terms of
A
, V ,

0
λ
, and
φ
. State the value of V for
M
= 1.675×10
−27
kg,
g
= 9.800 m s
−2
, and
h
= 6.626 × 10
−34
J s.

1.5
(2.0) How many cycles — from high intensity to low intensity and back to high
intensity — are completed by output port OUT1 when
φ
is increased from
°−= 90
φ
to
°= 90
φ
?

Experimental data The interferometer of an actual experiment was characterized by
a = 3.600 cm and
°= 10.22
θ
, and 19.00 full cycles were observed.

1.6
(1.0) How large was
0
λ
in this experiment?

1.7
(1.0) If one observed 30.00 full cycles in another experiment of the same kind that
uses neutrons with
0
λ
= 0.2000 nm, how large would be the area
A
?

Hint:
If 1<<x
α
, it is permissible to replace by
()
α
x+1 x
α
+

1 .



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Answer Script Theory Question 1
Page 3 of 5


Country Code Student Code Question Number





1


For
Examiners
Use
Only

1.0

1.0

Answer Script

Geometry



1.1
The area is


=A

1.2

The height is


=
H

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Answer Script Theory Question 1
Page 4 of 5


Country Code Student Code Question Number





1

Optical path length



1.3
In terms of ,
a
θ
,
φ
,
M
,
0
λ
,

g
, and :
h


ΔN
opt
=






1.4



In terms of
A
,
V
,
0
λ
, and
φ
:

ΔN
opt
=






The numerical value of
V
is


=V







1.5

The number of cycles is

# of cycles =

For
Examiners
Use
Only

3.0








0.8








0.2

2.0
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Answer Script Theory Question 1
Page 5 of 5


Country Code Student Code Question Number





1

Experimental data

For
Examiners
Use
Only

1.0

1.6
The de Broglie wavelength was


=
0
λ

1.7

The area is

=A











1.0

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SOLUTIONS to Theory Question 1
Geometry Each side of the diamond has length L =
a
cos θ
and the dis-
tance between parallel sides is D =
a
cos θ
sin(2θ) = 2a sin θ. The area is the
product thereof, A = LD, giving
1.1
A = 2a
2
tan θ .
The height H by which a tilt of φ lifts OUT1 above IN is H = D sin φ or
1.2
H = 2a sin θ sin φ .
Optical path length Only the two parallel lines for IN and OUT1 matter,
each having length L. With the de Broglie wavelength λ
0
on the IN side and
λ
1
on the OUT1 side, we have
∆N
opt
=
L

λ
0
−
L
λ
1
=
a
λ
0
cos θ

1 −
λ
0
λ
1

.
The momentum is h/λ
0
or h/λ
1
, respectively, and the statement of energy
conservation reads
1
2M

h
λ

0

2
=
1
2M

h
λ
1

2
+ MgH ,
which implies
λ
0
λ
1
=

1 − 2
gM
2
h
2
λ
2
0
H .
Upon recognizing that (gM

2
/h
2
)λ
2
0
H is of the order of 10
−7
, this simplifies
to
λ
0
λ
1
= 1 −
gM
2
h
2
λ
2
0
H ,
and we get
∆N
opt
=
a
λ
0

cos θ
gM
2
h
2
λ
2
0
H
or
1
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1.3
∆N
opt
= 2
gM
2
h
2
a
2
λ
0
tan θ sin φ .
A more compact way of writing this is
1.4
∆N
opt

=
λ
0
A
V
sin φ ,
where
1.4
V = 0.1597 × 10
−13
m
3
= 0.1597 nm cm
2
is the numerical value for the volume parameter V .
There is constructive interference (high intensity in OUT1) when the optical
path lengths of the two paths differ by an integer, ∆N
opt
= 0, ±1, ±2, . . ., and
we have destructive interference (low intensity in OUT1) when they diff er by
an integer plus half, ∆N
opt
= ±
1
2
, ±
3
2
, ±
5

2
, . . . . Changing φ from φ = −90
◦
to φ = 90
◦
gives
∆N
opt




φ=90
◦
φ=−90
◦
=
2λ
0
A
V
,
which tell us that
1.5
 of cycles =
2λ
0
A
V
.

Experimental data For a = 3.6 cm and θ = 22.1
◦
we have A = 10.53 cm
2
,
so that
1.6
λ
0
=
19 × 0.1597
2 × 10.53
nm = 0.1441 nm .
And 30 full cycles for λ
0
= 0.2 nm correspond to an area
1.7
A =
30 × 0.1597
2 × 0.2
cm
2
= 11.98 cm
2
.
2
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Theory Question 2
Page 1 of 5

Theory Question 2: Watching a Rod in Motion

Enter all your answers into the Answer Script.
pinhole camera
D
ro
d
x
v
0

Physical situation A pinhole camera, with the pinhole at
0
=
x
and at distance from
the
D
x
axis, takes pictures of a rod, by opening the pinhole for a very short time. There are
equidistant marks along the x axis by which the apparent length of the rod, as it is seen
on the picture, can be determined from the pictures taken by the pinhole camera. On a
picture of the rod at rest, its length is . However, the rod is not at rest, but is moving
with constant velocity
L
υ
along the
x

axis.

Basic relations A picture taken by the pinhole camera shows a tiny segment of the rod
at position .
x
~

2.1 (0.6) What is the actual position
x
of this segment at the time when the picture is
taken? State your answer in terms of
x
~
, , , D L
υ
, and the speed of light
=3.00×10c
8
m s
-1
. Employ the quantities
c
υ
β
= and
2
1
1
β
γ

−
=

if they help to simplify your result.

2.2 (0.9) Find also the corresponding inverse relation, that is: express x
~
in terms of
x
,
,
D
L
,
υ
, and . c
Note: The actual position is the position in the frame in which the camera is at rest

Apparent length of the rod
The pinhole camera takes a picture at the instant when the
actual position of the center of the rod is at some point .
0
x

2.3
(1.5) In terms of the given variables, determine the apparent length of the rod on
this picture.

2.4
(1.5) Check one of the boxes in the

Answer Script to indicate how the apparent
length changes with time.
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Theory Question 2
Page 2 of 5


Symmetric picture One pinhole-camera picture shows both ends of the rod at the same
distance from the pinhole.

2.5
(0.8) Determine the apparent length of the rod on this picture.

2.6
(1.0) What is the actual position of the middle of the rod at the time when this
picture is taken?

2.7
(1.2) Where does the picture show the image of the middle of the rod?

Very early and very late pictures The pinhole camera took one picture very early,
when the rod was very far away and approaching, and takes another picture very late,
when the rod is very far away and receding. On one of the pictures the apparent length is
1.00
m, on the other picture it is 3.00 m.

2.8
(0.5) Check the box in the
Answer Script to indicate which length is seen on

which picture.

2.9 (1.0) Determine the velocity
υ
.

2.10
(0.6) Determine the length
L
of the rod at rest.

2.11
(0.4) Infer the apparent length on the symmetric picture.

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Answer Script Theory Question 2
Page 3 of 5


Country Code Student Code Question Number





2


For
Examiners
Use
Only


0.6

0.9













1.5

1.5
Answer Script

Basic Relations

2.1

x
value for given value:
x
~


x
=




2.2

x
~
value for given
x
value:


x

~
=








Apparent length of the rod


2.3

The apparent length is


)(
~
0
xL =







2.4

Check one: The apparent length
 increases first, reaches a maximum value, then decreases.
 decreases first, reaches a minimum value, then increases.
 decreases all the time.
 increases all the time.


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Answer Script Theory Question 2
Page 4 of 5


Country Code Student Code Question Number





2


For
Examiners
Use
Only

0.8

1.0













1.2

Symmetric picture


2.5

The apparent length is


L
~
=










2.6

The actual position of the middle of the rod is

=
0
x










2.7

The picture shows the middle of the rod at a distance

=
l









from the image of the front end of the rod.

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Answer Script Theory Question 2
Page 5 of 5


Country Code Student Code Question Number





2


For
Examiners
Use
Only

0.5





1.0

0.6













0.4

Very early and very late pictures


2.8

Check one:
 The apparent length is 1 m on the early picture and 3 m on the
late picture.
 The apparent length is 3 m on the early picture and 1 m on the
late picture.

2.9

The velocity is


υ
=









2.10

The rod has length


L
=

at rest.

2.11


The apparent length on the symmetric picture is

L
~
=







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SOLUTIONS to Theory Question 2
Basic relations Position ˜x shows up on the picture if light was emitted
from there at an instant that is earlier than the instant of the picture taking
by the light travel time T that is given by
T =

√
D
2
+ ˜x
2

c .
During the lapse of T the respective segment of the rod has moved the dis-
tance vT , so that its actual position x at the time of the picture taking
is
2.1
x = ˜x + β
√
D
2
+ ˜x
2
.
Upon solving for ˜x we find
2.2
˜x = γ
2
x −βγ

D
2
+ (γx)
2
.
Apparent length of the rod Owing to the Lorentz contraction, the

actual length of the moving rod is L/γ, so that the actual positions of the
two ends of the rod are
x
±
= x
0
±
L
2γ
for the

front end
rear end

of the rod.
The picture ta ken by the pinhole camera shows the images of the rod ends
at
˜x
±
= γ

γx
0
±
L
2

− βγ

D

2
+

γx
0
±
L
2

2
.
The apparent length
˜
L(x
0
) = ˜x
+
− ˜x
−
is therefore
2.3
˜
L(x
0
) = γL + βγ

D
2
+


γx
0
−
L
2

2
− βγ

D
2
+

γx
0
+
L
2

2
.
Since the rod moves with the constant speed v, we have
dx
0
dt
= v and therefore
the question is whether
˜
L(x
0

) increases or decreases when x
0
increases. We
sketch the two square root terms:
1
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
D
2
+


γx
0
± L/2

2
0
.
.
.
.
.
.
.
.
.
.
L/2
.
.
.
.
.
.
.
.
.
.
−L/2
“−”

“+”
“+”
“−”
The difference of the square roots with “−” and “+” appears in the expression
for
˜
L(x
0
), and this difference clearly decreases when x
0
increases.
2.4 The apparent length decreases all the time.
Symmetric picture For symmetry reasons, the apparent length on the
symmetric picture is the actual length of the moving rod, because the light
from the two ends was emitted simultaneously to reach the pinhole at the
same time, that is:
2.5
˜
L =
L
γ
.
The apparent endpoint positions are such that ˜x
−
= −˜x
+
, or
0 = ˜x
+
+ ˜x

−
= 2γ
2
x
0
− βγ

D
2
+

γx
0
+
L
2

2
− βγ

D
2
+

γx
0
−
L
2


2
.
2
#14 _IPO_2006, Sưu tầm và đóng gói bởi, Gv. Phan Hồ Nghĩa, THPT Chuyên Hùng Vương, PleiKu, Gia Lai
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In conjunction with
L
γ
= ˜x
+
− ˜x
−
= γL −βγ

D
2
+

γx
0
+
L
2

2
+ βγ

D
2
+


γx
0
−
L
2

2
this tells us that

D
2
+

γx
0
±
L
2

2
=
2γ
2
x
0
± (γL − L/γ)
2βγ
=
γx

0
β
±
βL
2
.
As they should, both the version with the upper signs and the version with
the lower signs give the same answer for x
0
, namely
2.6
x
0
= β

D
2
+

L
2γ

2
.
The image of the middle of the rod on the symmetric picture is, therefore,
located at
˜x
0
= γ
2

x
0
− βγ

D
2
+ (γx
0
)
2
= βγ



(γD)
2
+

L
2

2
−

(γD)
2
+

βL
2


2


,
which is at a distance ℓ = ˜x
+
− ˜x
0
=
L
2γ
− ˜x
0
from the image of the front
end, that is
2.7
ℓ =
L
2γ
− βγ

(γD)
2
+

L
2

2

+ βγ

(γD)
2
+

βL
2

2
or
ℓ =
L
2γ






1 −
βL
2

(γD)
2
+

L
2


2
+

(γD)
2
+

βL
2

2






.
Very early and very late pictures At the very early time, we have a
very large negative value for x
0
, so that the apparent length on the very early
picture is
˜
L
early
=
˜
L(x

0
→ −∞) = (1 + β)γL =

1 + β
1 − β
L .
3
#15 _IPO_2006, Sưu tầm và đóng gói bởi, Gv. Phan Hồ Nghĩa, THPT Chuyên Hùng Vương, PleiKu, Gia Lai
Website: wWw.HoNghia.Net E-mail: Mobile: +84914 171 299
Likewise, at the very late time, we have a very large positive value for x
0
, so
that the apparent length on the very late picture is
˜
L
late
=
˜
L(x
0
→ ∞) = (1 −β)γL =

1 −β
1 + β
L .
It follows that
˜
L
early
>

˜
L
late
, that is:
2.8
The apparent length is 3 m on the early picture
and 1 m on the late picture.
Further, we have
β =
˜
L
early
−
˜
L
late
˜
L
early
+
˜
L
late
,
so that β =
1
2
and the velocity is
2.9
v =

c
2
.
It follows that γ =
˜
L
early
+
˜
L
late
2

˜
L
early
˜
L
late
=
2
√
3
= 1 .1 547. Combined with
2.10
L =

˜
L
early

˜
L
late
= 1 .7 3 m ,
this gives the length on the symmetric picture as
2.11
˜
L =
2
˜
L
early
˜
L
late
˜
L
early
+
˜
L
late
= 1.50 m .
4
#16 _IPO_2006, Sưu tầm và đóng gói bởi, Gv. Phan Hồ Nghĩa, THPT Chuyên Hùng Vương, PleiKu, Gia Lai
Website: wWw.HoNghia.Net E-mail: Mobile: +84914 171 299
Theory Question 3
Page 1 of 8

Theory Question 3

This question consists of five independent parts. Each of them asks for an estimate of an
order of magnitude only, not for a precise answer. Enter all your answers into the
Answer Script.

Digital Camera Consider a digital camera with a square CCD chip with linear
dimension
L
= 35 mm having N
p
= 5 Mpix (1 Mpix = 10
6
pixels). The lens of this
camera has a focal length of = 38 mm. The well known sequence of numbers (2, 2.8, 4,
5.6, 8, 11, 16, 22) that appear on the lens refer to the so called F-number, which is
denoted by and defined as the ratio of the focal length and the diameter
D
of the
aperture of the lens, .
f
#F
DfF /# =

3.1
(1.0) Find the best possible spatial resolution
min
x
Δ
, at the chip, of the camera as

limited by the lens. Express your result in terms of the wavelength
λ
and the F-
number and give the numerical value for #F
λ
= 500 nm.

3.2
(0.5) Find the necessary number
N
of Mpix that the CCD chip should possess in
order to match this optimal resolution.

3.3
(0.5) Sometimes, photographers try to use a camera at the smallest practical
aperture. Suppose that we now have a camera of = 16 Mpix, with the chip size
and focal length as given above. Which value is to be chosen for such that the
image quality is not limited by the optics?
0
N
#F

3.4
(0.5) Knowing that the human eye has an approximate angular resolution of
φ
= 2 arcmin and that a typical photo printer will print a minimum of 300 dpi
(dots per inch), at what minimal distance should you hold the printed page from
your eyes so that you do not see the individual dots?
z

Data 1 inch = 25.4 mm
1 arcmin = 2.91 × 10
−4
rad












#17 _IPO_2006, Sưu tầm và đóng gói bởi, Gv. Phan Hồ Nghĩa, THPT Chuyên Hùng Vương, PleiKu, Gia Lai
Website: wWw.HoNghia.Net E-mail: Mobile: +84914 171 299
Theory Question 3
Page 2 of 8


Hard-boiled egg An egg, taken directly from the fridge at temperature = 4°C, is
dropped into a pot with water that is kept boiling at temperature .
0
T
1
T

3.5

(0.5) How large is the amount of energy
U
that is needed to get the egg
coagulated?

3.6
(0.5) How large is the heat flow that is flowing into the egg?
J

3.7
(0.5) How large is the heat power transferred to the egg?
P

3.8
(0.5) For how long do you need to cook the egg so that it is hard-boiled?

Hint
You may use the simplified form of Fourier’s Law
rTJ
Δ
Δ
=
/
κ
, where is the
temperature difference associated with
TΔ
r
Δ
, the typical length scale of the problem.

The heat flow is in units of W
m
J
−2
.

Data Mass density of the egg: μ = 10
3
kg m
−3
Specific heat capacity of the egg: C = 4.2 J K
−1
g
−1
Radius of the egg: R = 2.5 cm
Coagulation temperature of albumen (egg protein): = 65°C
c
T
Heat transport coefficient:
κ
= 0.64 W K
−1
m
−1
(assumed to be the same for liquid
and solid albumen)

Lightning An oversimplified model of lightning is presented. Lightning is caused by
the build-up of electrostatic charge in clouds. As a consequence, the bottom of the cloud
usually gets positively charged and the top gets negatively charged, and the ground below

the cloud gets negatively charged. When the corresponding electric field exceeds the
breakdown strength value of air, a disruptive discharge occurs: this is lightning.

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I

max
= 100 kA
0
τ =0.1ms


Idealized current pulse flowing between the cloud and the ground during
lightning.

#18 _IPO_2006, Sưu tầm và đóng gói bởi, Gv. Phan Hồ Nghĩa, THPT Chuyên Hùng Vương, PleiKu, Gia Lai
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Theory Question 3
Page 3 of 8


Answer the following questions with the aid of this simplified curve for the current as a
function of time and these data:
Distance between the bottom of the cloud and the ground: h = 1 km;
Breakdown electric field of humid air: = 300 kV m
0
E
-1
;
Total number of lightning striking Earth per year: 32 × 10
6
;
Total human population: 6.5 × 10
9
people.

3.9
(0.5) What is the total charge
Q
released by lightning?

3.10
(0.5) What is the average current
I
flowing between the bottom of the cloud and
the ground during lightning?

3.11
(1.0) Imagine that the energy of all storms of one year is collected and equally
shared among all people. For how long could you continuously light up a 100 W
light bulb for your share?



Capillary Vessels Let us regard blood as an incompressible viscous fluid with mass
density μ similar to that of water and dynamic viscosity
η
= 4.5 g m
−1

s
−1
. We model
blood vessels as circular straight pipes with radius r and length L and describe the blood
flow by Poiseuille’s law,

DRp
=
Δ
,

the Fluid Dynamics analog of Ohm’s law in Electricity. Here
p
Δ
is the pressure
difference between the entrance and the exit of the blood vessel,
υ
SD
=
is the volume
flow through the cross-sectional area S of the blood vessel and
υ
is the blood velocity.
The hydraulic resistance R is given by

4
8
r
L
R
π
η
=
.

For the systemic blood circulation (the one flowing from the left ventricle to the right

auricle of the heart), the blood flow is D
≈
100 cm
3
s
−1
for a man at rest. Answer the
following questions under the assumption that all capillary vessels are connected in
parallel and that each of them has radius r = 4
μm and length L = 1 mm and operates
under a pressure difference = 1
kPa.
pΔ

3.12
(1.0) How many capillary vessels are in the human body?

3.13 (0.5) How large is the velocity
υ
with which blood is flowing through a capillary
vessel?


#19 _IPO_2006, Sưu tầm và đóng gói bởi, Gv. Phan Hồ Nghĩa, THPT Chuyên Hùng Vương, PleiKu, Gia Lai
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Theory Question 3
Page 4 of 8


Skyscraper At the bottom of a 1000 m high skyscraper, the outside temperature is

T
bot
= 30°C. The objective is to estimate the outside temperature T
top
at the top. Consider a
thin slab of air (ideal nitrogen gas with adiabatic coefficient
γ
= 7/5) rising slowly to
height z where the pressure is lower, and assume that this slab expands adiabatically so
that its temperature drops to the temperature of the surrounding air.

3.14
(0.5) How is the fractional change in temperature related to , the
fractional change in pressure?
TdT /
pdp /

3.15
(0.5) Express the pressure difference in terms of , the change in height.
dp
dz

3.16
(1.0) What is the resulting temperature at the top of the building?

Data Boltzmann constant: k = 1.38 × 10
−23
J K
−1
Mass of a nitrogen molecule: m = 4.65 × 10

−26
kg
Gravitational acceleration: g = 9.80 m s
−2

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Answer Script Theory Question 3
Page 5 of 8


Country Code Student Code Question Number





3


For
Examiners
Use
Only

0.7

0.3








0.5







0.5







0.5

Answer Script

Digital Camera


3.1

The best spatial resolution is

(formula:)
=Δ
min
x




which gives
(numerical:)
=Δ
min
x

for
λ
= 500 nm.

3.2

The number of Mpix is

N =





3.3

The best F-number value is

= #F






3.4

The minimal distance is

z =






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Answer Script Theory Question 3
Page 6 of 8


Country Code Student Code Question Number





3
Hard-boiled egg

For
Examiners
Use
Only

0.5

0.5









0.5










0.5


3.5

The required energy is


=U






3.6

The heat flow is


=J






3.7

The heat power transferred is


=P

3.8

The time needed to hard-boil the egg is


=
τ












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Answer Script Theory Question 3
Page 7 of 8


Country Code Student Code Question Number

3
Lightning

For
Examiners
Use
Only

0.5





0.5







1.0

1.0









0.5


3.9

The total charge is


=Q

3.10

The average current is

=
I






3.11

The light bulb would burn for the duration

=t





Capillary Vessels


3.12

There are

=N



capillary vessels in a human body.

3.13

The blood flows with velocity


=
υ





#23 _IPO_2006, Sưu tầm và đóng gói bởi, Gv. Phan Hồ Nghĩa, THPT Chuyên Hùng Vương, PleiKu, Gia Lai
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Answer Script Theory Question 3
Page 8 of 8


Country Code Student Code Question Number

3


For
Examiners
Use
Only

0.5










0.5

1.0









Skyscraper


3.14

The fractional change in temperature is


=
T
dT

3.15

The pressure difference is


dp
=








3.16

The temperature at the top is

T
top
=

#24 _IPO_2006, Sưu tầm và đóng gói bởi, Gv. Phan Hồ Nghĩa, THPT Chuyên Hùng Vương, PleiKu, Gia Lai
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SOLUTIONS to Theory Question 3
Digital Camera Two factors limit the resolution of the camera as a pho-
tographic tool: the diffraction by the aperture and the pixel size. For diffrac-
tion, the inherent angular resolution θ
R
is the ratio of the wavelength λ of
the light and the aperture D of the camera,
θ
R
= 1.22
λ
D
,
where the standard factor of 1.22 reflects the circular shape of the aperture.
When taking a picture, the object is generally sufficiently far away from the
photographer for the image to form in the focal plane of the camera where
the CCD chip should thus be placed. The Rayleigh diffraction criterion then
states that two image points can be resolved if they are separated by more
than
3.1

∆x = fθ
R
= 1.22λ F  ,
which gives
∆x = 1.22 µm
if we choose the largest possible aperture (or smallest value F  = 2) and
assume λ = 500 nm for the typical wavelength of daylight
The digital resolution is given by the distance  between the center of two
neighboring pixels. For our 5 Mpix camera this distance is roughly
 =
L

N
p
= 15.65 µm .
Ideally we should match the optical and the digital resolution so that neither
aspect is overspecified. Taking the given optical resolution in the expression
for the digital resolution, we obtain
3.2
N =

L
∆x

2
≈ 823 Mpix .
Now looking for the unknown optimal aperture, we note that we should
have  ≥ ∆x , that is: F  ≤ F
0
with

F
0
=
L
1.22λ
√
N
0
= 2

N
N
0
= 14.34 .
1
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