Physics 2. MS Teams Lecture Phy 2 Bespalova

1. Physics 2

BESPALOVA IRINA VALER'EVNA

2.

MS Teams Lecture Phy 2 Bespalova

3. Physics 2

1
Waves and
01
Oscillations
02
Optics
03
Quantum
Mechanics
04
Nuclear
Physics
Physics 2

4.

What is light?

5.

1) Light is electromagnetic waves.
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2) Experiments performed early in the twentieth century
showed that light has corpuscular, or particle-like, properties.
The idea that light can display both wave and particle
characteristics is called wave-particle duality

6.

The Propagation of Light
The Speed of Light:
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Еhe speed of light in a vacuum c is one of the basic physical quantities
Speed of Light in Matter
The speed of light depends strongly on the type of material, since its interaction
varies with different atoms, crystal lattices, and other substructures. We can define
a constant of a material that describes the speed of light in it, called
the index of refraction n:

7.

The Ray Model of Light
In different cases, we can model the path of light as a straight line
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called a ray.
Experiments show that when light interacts with an object several times larger than
its wavelength, it travels in straight lines and acts like a ray. Its wave characteristics
are not pronounced in such situations. Since the wavelength of visible light is less
than a micron (a thousandth of a millimeter), it acts like a ray in the many common
situations in which it encounters objects larger than a micron. For example, when
visible light encounters anything large enough that we can observe it with unaided
eyes, such as a coin, it acts like a ray, with generally negligible wave characteristics.

8.

The Law of Reflection
The law of reflection states that the angle of reflection
equals the angle of incidence
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The law of reflection:
the angle of reflection
equals the angle of
incidence:
The incident ray, the
reflected ray, and the
normal all lie in the
same plane.

9.

6

10.

Refraction
The changing of a light ray’s direction when it passes through
substances of different refractive indices is called refraction and is
related to changes in the speed of light
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The Snell’s law of refraction:

11.

When the light beam moves
from air into glass, the light
slows down on entering the
glass and its path is bent
toward the normal.
When the beam moves from
glass into air, the light speeds up
on entering the air and its path is
bent away from the normal.

12.

From the previous slide it follows, that as light travels from one medium to
another, its frequency does not change but its wavelength does:
So if
l is the wavelength in vacuum,
l1 is the wavelength in a media,
n1 is the refraction index, then we have:
l1 = l/n1
Thus: the wavelength in any media is less then in vacuum.
Snell’s law
of refraction.

13.

Total Internal Reflection
If the index of refraction for the second medium is less than for the
first, the ray bends away from the perpendicular
What happens as the incident angle increases?
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14.

Total internal re ection can occur when light is directed
from a medium having a given index of refraction toward
the medium having a lower index of refraction.
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Rays travel from a medium of index
of refraction n1 into a medium of
index of refraction n2, where n2 < n1.
As the angle of incidence Q1
increases, the angle of refraction Q2
increases until Q2 = 90° (ray 4). For
larger angles of incidence, total
internal re ection occurs (ray 5).

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16.

Concave Mirror
Concave mirror reflects light from the inner, concave surface.
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17.

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the focal length f of a concave
spherical mirror is half of its radius
of curvature, R

18.

concave mirror
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convex mirror

19.

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mirror equation

20.

21.

Magnification of Images by Mirrors
The lateral magnification of the image is
h’ is the height of the image
h is the height of the object
When M is positive, the image is upright
and on the same side of the lens as the object.
When M is negative, the image is inverted
and on the side of the lens opposite the object.
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22.

Thin Lenses
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Converging lens is a biconvex
lens (it
has two convex surfaces)
Converging lens collects parallel rays
into its real focus.
Diverging lens is a biconcave
lens (it
has two concave surfaces)
Diverging lens disperses light from the
virtual focus.

23.

24. Ray Diagrams for Lens

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25.

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Thin-Lens Equation

26.

Magnification
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29. Huygens’s Principle

All points on a given wave front are taken as point
sources for the production of spherical secondary
waves, called wavelets, which propagate outward
through a medium with speeds characteristic of waves in
that medium. After some time interval has passed, the
new position of the wave front is the surface tangent to the
wavelets.

30. Huygen’s Principle for Reflection

At the instant that ray 1 22
strikes
the surface, it sends out a
Huygens wavelet from A and
ray 2 sends out a Huygens
wavelet from B. We choose a
radius of the wavelet to be c Dt,
where Dt is the time interval for
ray 2 to travel from B to C.
The line AB represents a wave front of the incident light just
as ray 1 strikes the surface. The line DC represents a wave
front of the reflected ray after Dt time interval.

31.

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Because both rays 1 and 2 move with the same speed, we
must have AD = BC = cDt. The triangles ABC and ADC are
congruent because they have the same hypotenuse AC
and because AD = BC.
Because AD = BC, we have g = g´.

32.

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g + Q1 = g´ + Q1´ = p/2
from it follows:
Q1 = Q1 ´
So the law of reflection is proved by the Huygen’s
principle.

33. Huygen’s Principle for Refraction

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Dt is the time interval from the
instant ray 1 strikes the surface to
the instant ray 2 strikes the
surface: cDt=BC. During this time
interval, the wave at A
sends out a Huygens wavelet (the
arc centered on A) toward D. In
the same time interval, the wave at
B sends out a Huygens wavelet (the
arc centered on B) toward C.
Because these two wavelets travel through different media, the radii
of the wavelets are different. The radius of the wavelet from A is AD =
v2Dt, where v2 is the wave speed in the second medium. The radius
of the wavelet from B is BC = v1 Dt, where v1 is the wave speed in
the original medium.

34.

From triangles ABC and ADC, we nd that
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and
Dividing the first equation to the second:
Using that v=c/n:
From this equation we get The Snell’s law of refraction:

35.

Fermat’s Principle
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Fermat’s principle states that when a light ray
travels between any two points, its path is the one
that requires the smallest time interval.
An obvious consequence of this principle is that the
paths of light rays traveling in a homogeneous
medium are straight lines.
The Fermat’s principle can be used for deriving the
Snell’s law of refraction.

36.

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Suppose that a light ray is to travel
from point P in medium 1 to point
Q in medium 2 where P and Q are
at perpendicular distances a and b,
respectively, from the interface. At
t=0 light leaves point P. Time at
which light arrives at Q is:

37.

To obtain the value of x for which t has its minimum value, we take
the derivative of t with respect to x and set the derivative equal to zero:
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38.

From the picture we can derive:
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And finally, we get the
Snell’s law of refraction:

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