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Optics Notes
Topics in these notes are
- wave properties
- Polarization
- focal lengths
- Geometrical optics
I. WAVE PROPERTIES
In general there are two types of waves. They are transverse and longitudal waves. Longitudal waves are those in which the displacement of the medium(or field) is in the direction of the wave. Examples of longitudal waves include sound waves and pressure waves. Transverse waves are waves in which the direction is perpendicular to the direction of motion. Examples of transverse waves include light and vibrations of a string. The general equation for a wave is given by
y(x, t) = ym sin kx − ωt (1)
Where ym is the maximum amplitude of oscillation, k is the wave number, and ω is the angular frequency. The relations of k and ω to physical quantities are given by.
k =
2 π λ
ω = 2πf (3)
Where λ is the wavelength and f is the frequency given by.
f =
number of occurences of repeating event time
T
Where T is the period of the wave. Another usefull equation is:
ω =
2 π T
From these we can obtain the speed v of a wave. Namely,
v =
distance the wave travels in one period period
λ T
= λf =
ω k
The formulas for the speeds of different waves are given below.
- Waves on a string
v =
τ μ τ = tension μ = mas per unit length
- E-M waves
v =
c n c = speed of light n = index of refraction (7)
- Sound waves
v =
B
ρ B = bulk modulus ρ = density (8)
A. Reflections from a boundary
If a string wave is incident on a hard wall then the reflected wave is inverted. If the wave is free to move at the boundary then it is not inverted. The follwing url depicts the wave reflections well. http://paws.kettering.edu/
~drussell/Demos/reflect/reflect.html
B. Standing waves
Standig waves do not propogate but just fluctuate in place. If a wave is confined to a cartain region of length L, then only certain frequencies are allowed to exist within it(see figure 1). The allowed frequencies depend on the boundary conditions. In general the two boundary conditions are fixed and free. For fixed boundary conditions we have y(0, t) = y(L, t) = 0. The free boundary conditions are y′(0, t) = y′(L, t) = 0, where y(x, t) is the displacement at a time t and position x. When is comes to sound waves in a pipe fixed end = closed end, free end = open end. By drawing allowed waves we can figure out the allowed wavelengths. For sound the allowed wavelengths for various conditions can be found by the following equations:
- Both ends open
L =
nλ 2
where n = 1, 2 , 3 ...
- One open end
L =
nλ 4
where n = 1, 3 , 5 ...
C. sound waves
The intensity I of a sound wave is given by;
I =
ρvω^2 A^2 (11)
Where ρ is the density of the medium, v is the velocity, ω is the angular frequency, and A is the amplitude of the wave. The decibel level(or sound level) denoted by β is given by
FIG. 2: Depiction of a standard polarizer problem. NOTE: θ = θ 1 − θ 0
θB
Incident ray
(unpolarised)
Reflected ray
(polarised)
Refracted ray
(slightly polarised)
FIG. 3: Depiction of polarization using brewster angle
B. brewsters angle
Polarizers are not the only way one can polarize light. Another method is to have the beam incident on a surface at the brewster angle(see fig 3). If an unpolarized beam is incident at the brewster angle then the reflected beam is plane polarized (the electric field is out of the page). The brewster angle is given by
tan θB =
n 2 n 1
Where n 2 is the bottom medium.
III. FOCAL LENGTHS
The importance of focal lengths is their use in geometrical optics which will be disussed in the next section. One will probably not need to calculate the focal length of a lens but if one does it will probably be a spherical mirror whose focal length is given by.
f =
R
Where R is the sphere radius
IV. GEOMETRICAL OPTICS
A. Snell’s Law
One of the fundamental equations in geometric optics is snell’s law. It is given by
n 1 sin θ 1 = n 2 sin θ 2 (18)
Where n 1 is the index of refraction of the 1st medium(where the incident light ray starts), n 2 is the index of refraction of the second medium, θ 1 is the angle of incidence, and θ 2 is the angle of refraction. NOTE: the angle of reflection is equal to the angle of incidence.
B. Plane Mirrors
For a plane mirror the object distance is equal to the image distance. NOTE: for a mirror the image distance is taken to be negative, namely;
q = −p (19)
Furthermore if we define magnification as.
M =
Image height object height
For a plane mirror M = 1.
C. spherical mirrors
Definition: A virtual image is an image behind a mirror which usually has the same orientation as the object. Definition: A real image is in front of a mirror and is upside down. NOTE: The focal length for a concave mirror is positive and the focal length for a convex mirror is negative. With these sign conventions the image distance can be obtained with
1 p
i
f
Where p is the object distance, i is the image distance, and f is the focal length. NOTE: the previous formula is the main formula in geometrical optics. Furthermore the magnification is given by,
M =
−i p
A plus sign in the magnification means the images have the same orientation. We now lists the properties of these mirrors and their images.
- A ray that passes through the focal point will reflect parallel to the central axis
- A ray that passes through the center of curvature will reclect back on itself.
- A ray that reflects off the intersection of the mirror with the central axis will reflect symmetrically(Not a very good ray) The intersection of any of the two lines will give you the location of the image.
V. THIN LENSES
There are two types of lenses, converging, and diverging. These are also called convex and concave respectively. Just as before i > 0 if the image is real and i < 0 if the image is virtual. However, for lenses we must pay carefull attention to the meaning of real and virtual image. In the case of lenses the meaning is the reverse of that of mirrors. DEFINITION: For a lens an image is virtual if it is formed on the side where the object is. DEFINITION: For a lens an image is real if it is formed on the opposite side of the lens. With these sign conventions the same equations that we had for a mirror hold for a lens. We now list properties of the images formed by the two lenses Convex(converging) Image location = behind lens (if p > f ) in front of lens (if p < f ) Image type = real (if p > f ) virual (if p < f ) orientation = inverted (if p > f ) same (if p < f ) sign of f = + Concave(diverging) Image location = in front of lens Image type = virtual orientation = same as object sign of f = -
A. Ray Tracing
The three rays one can draw to find the object(remember one needs only two) are:
- A ray that is initially parallel to the central axis of the lens will pass through the focal point behind the lens.
- A ray that passes through the focal point in fron of the lens wil emerge from the lens parallel to the central axis.
- A ray that is initially derected toward the center of the lens wil emerge from the lens with no change in its direction. NOTE: For ray tracing there is no substitute for seeing lots of ray tracing diagrams.
B. Two lens systems
When looking for an image of an object that is in front of two lenses we must simply follow two steps. The steps are as follows
- Ignore the second(farther) lens and find its image using Eq. (21)
- Now ingoring the presence of lens 1 we treat the image found in step one as the object. NOTE: if the image found is one goes beyond the second lens then the object distance, p 2 is negative. The magnification for a two lens system is
M = m 1 m 2 (23)
Where m 1 , 2 are the magnifications produced by lens 1,2.
C. Sign Conventions
Because sign conventions are the trickiest part of geometrical optics I summarize them here. (f > 0) Concave mirror
Converging lens (convex lens) (f < 0) Convex mirror Diverging Lens (concave lens) (i > 0) (REAL IMAGE, Inverted) Image in front of mirror Image behind lens (i < 0) (VIRTUAL IMAGE, Same orientation) Image behind mirror Image in front of lens
