2. OPTICS
• Optics is the branch of physics that studies the behavior and
properties of light, including its interactions with matter and
the construction of instruments that use or detect it.
• More specifically, optics is a branch of physics describing
how light behaves and interacts with matter.
3. Many phenomena in our everyday life depend on the properties of
light. When you watch an screen or view photos on a cellphone, you
are seeing millions of colors formed from combinations of only
three colors that are physically on the screen: red, blue, and green.
• The blue color of the daytime sky is a result of the optical
phenomenon of scattering of light by air molecules, as are the red
and orange colors of sunrises and sunsets.
• You see images of other cars in your rearview mirror when you are
driving. These images result from reflection of light.
• If you wear glasses or contact lenses, you are depending on
refraction of light for clear vision.
4. Nature of light
• Most things we see are due to reflections, since most objects
don’t produce their own visible light. Much of the light
incident on an object is absorbed but some is reflected.
• A ray of light heading towards an object is called an incident
ray. If it reflects off the object, it is called a reflected ray.
• A perpendicular line drawn at any point on a surface is called
a normal (just like with normal force).
•
5. • The angle between the incident ray and normal is called the
angle of incidence, i, and the angle between the reflected
ray and the normal ray is called the angle of reflection, r.
• The law of reflection states that the angle of incidence is
always equal to the angle of reflection.
7. • Light is extremely fast, about 3 108 m/s in a vacuum.
• Light, however, is slowed down by the presence of matter. The
extent to which this occurs depends on what the light is
traveling through.
• Light travels at about 3/4 of its vacuum speed (0.75 c ) in
water and about 2/3 its vacuum speed (0.67 c ) in glass.
• If light travels from one medium to another, and if the speeds
in these media differ, then light is subject to refraction (a
changing of direction at the interface).
8. • At an interface between two media, both reflection and
refraction can occur. The angles of incidence, reflection, and
refraction are all measured with respect to the normal. The
angles of incidence and reflection are always the same. If light
speeds up upon entering a new medium, the angle of
refraction, r , will be greater than the angle of incidence, as
depicted on the left. If the light slows down in the new
medium, r will be less than the angle of incidence, as shown
on the right.
9. • The bending of light as it passes from one medium to another
is called refraction. The angle and wavelength at which
the light enters a substance and the density of that substance
determine how much the light is refracted.
The bending occurs because light travels more slowly in a
denser medium and vice versa.
• The refraction occurs at the boundary and is caused by
a change in the speed of the light wave upon crossing the
boundary. The tendency of a ray of light to bend in one
direction or another is dependent upon whether
the light wave speeds up or slows down upon crossing the
boundary.
10. • Index of Refraction, n
The index of refraction of a substance is the ratio of the speed in
light in a vacuum to the speed of light in that substance:
n = Index of Refraction
c = Speed of light in vacuum
v = Speed of light in medium
n = c
v
Note that a large index of
refraction corresponds to a
relatively slow light speed in that
medium.
Medium
Vacuum
Air (STP)
Water (20º C)
Ethanol
Glass
Diamond
n
1
1.00029
1.33
1.36
~1.5
2.42
11. Snell’s Law
Snell’s law states that a ray of light bends in
such a way that the ratio of the sine of the
angle of incidence to the sine of the angle of
refraction is constant. Mathematically,
ni sin i = nr sinr
Here ni is the index of refraction in the original
medium and nr is the index in the medium the
light enters. i and r are the angles of
incidence and refraction, respectively.
i
r
ni
nr
Willebrord
Snell
12.
13. • A light ray of wavelength 589 nm traveling
through air is incident on a smooth, flat slab
of crown glass at an angle of 30.0° to the
normal.
• (A) Find the angle of refraction.
• (B) Find the speed of this light once it enters
the glass. ( find % reduction)
• (C) What is the wavelength of this light in the
glass?
14. Total Internal Reflection
• An interesting effect called total internal reflection can occur
when light is directed from a medium having a given index of
refraction toward one having a lower index of refraction.
• The refracted rays are bent away from the normal because n1
is greater than n2. At some particular angle of incidence ,
called the critical angle, the refracted light ray moves
parallel to the boundary so that Ɵ2 is 90°.
• For angles of incidence greater than Ɵc , the ray is entirely
reflected at the boundary
16. •
This equation can be used only when n1 is greater than n2.
That is, total internal reflection occurs only when light is directed
from a medium of a higher index of refraction toward a medium of
lower index of refraction.
If n1 were less than n2, equation would give an answer greater 1,
which is a meaningless result because the sineof an angle can never
be greater than unity.
17. • The critical angle for total internal reflection is small when n1
is considerably greater than n2.
• For example, the critical angle for a diamond in air is 24°.
• Any ray inside the diamond that approaches the surface at an
angle greater than 24° is completely reflected back into the
crystal.
• This property, combined with proper faceting, causes
diamonds to sparkle.
• The angles of the facets are cut so that light is “caught” inside
the crystal through multiple internal reflections.
19. • Find the critical angle for an air–water
boundary. (Assume the index of refraction of
water is 1.33.)
20. • A ray of light traveling through glass with an
index of refraction n=1.5 strikes the interface
of the glass-water. Let the index of refraction
of water be n=1.33. At what angle, the ray
does not enter the water?
21. • Another interesting application of total internal reflection is
the use of glass or transparent plastic rods to “pipe” light from
one place to another.
• Light is confined to traveling within a rod, even around curves,
as the result of successive total internal reflections.
• Such a light pipe is flexible if thin rather than thick. A flexible
light pipe is known as optical fiber.
22. The 2009 Nobel Prize in Physics was awarded to Charles K. Kao (b. 1933)
for his discovery of how to transmit light signals over long distances
through thin glass fibers.
This discovery has led to the development of a sizable industry known as
fiber optics.
• Practical optical fiber consists of a transparent core surrounded by a
cladding, a material that has a lower index of refraction than the
core.
• The combination may be surrounded by a plastic jacket to prevent
mechanical damage. Figure 35.30 shows a cutaway view of this
construction.
23. • Because the index of refraction of the cladding is less than that
of the core, light traveling in the core experiences total internal
reflection if it arrives at the interface between the core and the
cladding at an angle of incidence that exceeds the critical angle.
• In this case, light “bounces” along the core of the optical fiber,
losing very little of its intensity as it travels.
• Any loss in intensity in an optical fiber is essentially due to
reflections from the two ends and absorption by the fiber
material.
• Optical fiber devices are particularly useful for viewing an object
at an inaccessible location.
• For example, physicians often use such devices to examine
internal organs of the body or to perform surgery without
making large incisions.
24. Find the index of refraction for medium 2, assuming medium 1 is
glass with an index of 1.54 and given that the incident
angle is 30.0° and the angle of refraction is 22.0° .
25. Suppose that in a situation like that light goes from air to
diamond with an index of 3.9 and that the incident angle
is 30.0° . Calculate the angle of refraction θ2 in the diamond.
26. What is the critical angle for light traveling in a polystyrene (a
type of plastic) pipe surrounded by air? The index
of refraction for polystyrene is 1.49.
27.
28. What is Laser?
Light Amplification by Stimulated
Emission of Radiation
• A device produces a coherent beam of optical
radiation by stimulating electronic, ionic, or
molecular transitions to higher energy levels
29. 29
Properties of Laser
The light emitted from a laser is monochromatic, that is, it is
of one color/wavelength. In contrast, ordinary white light is a
combination of many colors (or wavelengths) of light.
Lasers emit light that is highly directional, that is, laser light
is emitted as a relatively narrow beam in a specific direction.
Ordinary light, such as from a light bulb, is emitted in many
directions away from the source.
These properties of laser light are what can make it more
hazardous than ordinary light. Laser light can deposit a lot
of energy within a small area.
30.
31.
32.
33.
34.
35. The elecron maintains its higher energy level for time in
nanoseconds. It comes back to its original state and loses energy in
photon.(the energy is same that was absorbed earlier)
36. The electron is in high energy level (excited state) and a photon (of
same energy as the electron has) interacts with it at that time, the
electron comes in its ground state (before excitation) while emitting a
photon. Both photons have same energy, direction and phase.
38. The property of Gain Medium is that its atoms are made
to achieve excited state.
Gain Medium is a cavity and it can contain solid, liquid or gaseous
depending on kind of Laser.
It has two mirrors attached to ends, and a light source (energy source).
39. The function of light source is to excite the atoms (maximum possible)
Optical pumping : to excite atoms at ground state to a desired energy
Level by using an external energy source.
This phenomenon is also known as population inversion.
A population inversion occurs while a system exists in a state in
which more members of the system are in higher, excited states than in
lower, unexcited energy states
40. The material used in Gain medium is selected in a way, that it can have
a suitable stability (stay more time) at Excited state.
Co2, neon, metal oxides.
41. When a pair of photons starts by stimulated emmission.
The beam gets reflected by mirror at end of gain medium cavity.
And the chain reaction starts.
43. The intensity of beam increases to a level that at one time it is unable
to be bounced back by the partial reflective mirror.
Laser beam is monochromatic (one color).
Different colors of laser beams depend on the materials used in their
Respective Gain Mediums.
44.
45.
46.
47. Diffraction..
What happens when light shining through an open door enters
into a dark room?
For light, we observe a sharp shadow of the doorway on the
floor of the room, and no visible light bends around corners into
other parts of the room.
When sound passes through a door, we hear it everywhere in
the room and thus observe that sound spreads out when passing
through such an opening.
48. What is the difference between the behavior of sound
waves and light waves in this case?
The answer is that light has very short wavelengths and acts like
a ray. Sound has wavelengths on the order of the size of the door
and bends around corners (for frequency of 1000 Hz,
λ = v/f = 330 m/s 1000 s−1 = 0.33 m,
about three times smaller than the width of the doorway).
49. a) Light passing through a doorway makes a sharp outline on the floor.
Since light’s wavelength is very small compared with the size of the
door.
(b) Sound waves bend into all parts of the room, a wave effect, because
their wavelength is similar to the size of the door.
50. If we pass light through smaller openings such as slits, we can see that
light bends as sound does.
The bending of a wave around the edges of an opening or an obstacle
is called diffraction.
51. If there are two sources of waves, the waves could be made to
interfere, as in the case of waves on water
The points where the water is calm (corresponding to
destructive interference) are clearly visible.
52. Water Waves
A wave generator sends periodic water waves into a barrier with a small gap, as
shown below.
A new set of waves is observed emerging from the gap to the
wall.
53. Interference of Water Waves
An interference pattern is set up by water waves leaving two slits at the same
instant.
54. In constructive interference, the amplitude of the resultant wave is
greater than that of either individual wave, whereas in destructive
interference, the resultant amplitude is less than that of the larger wave.
Light waves also interfere with one another.
If light is an electromagnetic wave, it must therefore exhibit interference
effects under appropriate circumstances.
In Young’s double slit experiment, light was passed through a pinhole on
a board. The emerging beam fell on two pinholes on a second board.
The light emanating from the two pinholes then fell on a screen where a
pattern of bright and dark spots was observed. This pattern, called
fringes, can only be explained through interference, a wave
phenomenon.
55. We can analyze double-slit interference with the help of Figure,
which depicts an apparatus analogous to Young’s.
Light from a monochromatic source falls on a slit So .
The light emanating from So is incident on two other slits S1 and
S2 that are equidistant from So .
A pattern of interference fringes on the screen is then produced by
the light emanating from S1 and S2 .
Slits S1 and S2 are a distance d apart ( d ≤ 1 mm ), and the
distance between the screen and the slits is D( ≈ 1 m) , which is
much greater than d.
56.
57. When the light from S1 and that from S2
both arrive at a point on the screen such that constructive
interference occurs at that location.
When the light from the two slits combines
destructively at any location on the screen,
a dark fringe results.
58. Consider how two waves travel from the slits to the screen (Figure
).
Each slit is a different distance from a given point on the screen.
Thus, different numbers of wavelengths fit into each path.
Waves start out from the slits in phase (crest to crest), but they
may end up out of phase (crest to trough) at the screen if
the paths differ in length by half a wavelength, interfering
destructively.
If the paths differ by a whole wavelength, then the
waves arrive in phase (crest to crest) at the screen, interfering
constructively.
59. More generally, if the path length difference
Δl between the two waves is any half-integral number of
wavelengths [(1 / 2) λ , (3 / 2) λ , (5 / 2) λ , etc.], then destructive
interference occurs.
Similarly, if the path length difference is any integral number of
wavelengths ( λ , 2 λ , 3 λ , etc.), then constructive interference
occurs.
Δl = mλ, for m = 0, 1, 2, 3 … (constructive interference)
Δl = (m +1/2)λ, for m = 0, 1, 2, 3 … (destructive interference)
60. Analytical Methods for Fringes
x
y
d sin
s1
s2
d
l1
l2
Bright fringes: d sin = nl, n = 0, 1, 2, 3, . . .
Dark fringes: d sin = (n+1/2) l , n = 1, 3, 5, . . .
DL = l1 – l2
DL = d sin
Path difference determines
light and dark pattern.
61. The lengths of r1 and r2 differ by Δl , as indicated by the two dashed
lines in the figure. Simple trigonometry shows
where d is the distance between the slits.
d sin θ = mλ, for m = 0, ±1, ±2, ±3,…(constructive interference).
d sin θ = (m +12)λ, for m = 0, ±1, ±2, ±3,… (destructive interference).
The closer the slits are, the more the bright fringes spread apart.
We can see this by examining the equation
d sin θ = mλ, for m = 0, ±1, ±2, ±3… . For fixed λ and m, the smaller d is,
the larger θ must be, since sin θ = mλ/d .
Δl = d sin θ
62. Finding a Wavelength from an Interference Pattern
Suppose you pass light from a laser source through two slits
separated by 0.100 mm and find that the 4th dark
line on a screen is formed at an angle of 15° relative to the incident
beam.
What is the wavelength of the light?
63. Interference patterns do not have an infinite number of lines,
since there is a limit to how big m can be.
What is the slit distance with the system having light with a
wavelength 0.3 micrometer, and the 3rd dark fringe appears at
an angle of 20o?
