Learn some Physics Formulas And Definitions In Simple Way About Light Chapter
Learn some Physics Formulas And Definitions In Simple Way About Light Chapter
Chapter - Light
Topic - 1 Electromagnetic Spectrum
Introduction
Visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight Visible light has a wavelength in the range of about 380 nanometres to about 740 nm.
Primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, polarisation, and speed.
Visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality.
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation.
Equations:
f= c/λ
f = E / h
E = hc / λ
where,
f = frequency
λ = wavelength
E = photon energy
h = Planck's constant
c = speed of light
Regions of the spectrum
The types of electromagnetic radiation are broadly classified into the following classes:
1. Gamma radiation
2. X-ray radiation
3. Ultraviolet radiation
4. Visible radiation
5. Infrared radiation
6. Terahertz radiation
7. Microwave radiation
8. Radio wave
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Topic - 2. Reflection And Refraction
Reflection and refraction
When a ray of light hits the boundary between two transparent materials, it is divided into a reflected and a refracted ray.
Reflection
The law of reflection says that the reflected ray lies in the plane of incidence, and the angle of reflection equals the angle of incidence.
Refraction
Refraction is the bending of light rays when passing through a surface between one transparent material and another.
It is described by Snell's Law:
n1 sinθ1 = n2 sinθ2
where
θ1 is the angle between the ray and the surface normal in the first medium (the angle of refraction)
θ2 is the angle between the ray and the surface normal in the second medium (the angle of incidence)
n1 and n2 are the indices of refraction, n = 1 in a vacuum and n > 1 in a transparent substance.
This phenomenon is also associated with a changing speed of light as seen from the definition of index of refraction provided above which implies:
v1 sinθ2 = v2 sinθ1
where v1 and v2 are the wave velocities through the respective media.
Lens : A device which produces converging or diverging light rays due to refraction is known as a lens.
The equation that determines the location of the images given a particular focal length (f) and object distance (S1):
1/S1 + 1/S2 = 1/f
where S2 is the distance associated with the image and is considered by convention to be negative if on the same side of the lens as the object and positive if on the opposite side of the lens.
The focal length f is considered negative for concave lenses.
The magnification of a lens is given by
M = - S2 / S1 = f / (f-S1)
lenses are placed in contact: if the lenses of focal lengths f1 and f2 are "thin", the combined focal length f of the lenses is given by
1/f = 1/f1 +1/f2
If two thin lenses are separated in air by some distance d, the focal length for the combined system is given by
1/f = 1/f1 +1/f2 – d / f1f2
Index of Refraction:
Materials of greater density have a higher index of refraction.
n = c / v
where
n = index of refraction
c = speed of light in a vacuum
v = speed of light in the material
n=λ0 / λm
where
λ0 = wavelength of the light in a vacuum
λn = its wavelength in the material
Critical Angle:
The maximum angle of incidence for which light can move from n1 to n2
sinθc = n2 / n1 for n1 > n2
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Of D - block elements
Topic - 3.Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle.
In classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings.
Similar effects occur when a light wave travels through a medium with a varying refractive index.
Single-slit diffraction
The path difference is given by
d sinθ / 2
so that the minimum intensity occurs at an angle θmingiven by
d sinθmin = λ
where
d is the width of the slit,
θmin is the angle of incidence at which the minimum intensity occurs
λ is the wavelength of the light
A similar argument can be used to show that if we imagine the slit to be divided into four, six, eight parts, etc., minima are obtained at angles θn given by
d sinθn = n λ
where
n is an integer other than zero.
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Diffraction grating
It splits and diffracts light into several beams travelling in different directions.
The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element.
The form of the light diffracted by a grating depends on the structure of the elements and the number of elements present, but all gratings have intensity maxima at angles θm which are given by the grating equation
D( sinθm + sinθi ) = m λ
where
θi is the angle at which the light is incident
d is the separation of grating elements
m is an integer which can be positive or negative.
Diffraction-limited imaging
The ability of an imaging system to resolve detail is ultimately limited by diffraction. This is because a plane wave incident on a circular lens or mirror is diffracted as described above. The light is not focused to a point but forms an Airy disk having a central spot in the focal plane with radius to first null of
d = 1.22 λ N
where
λ is the wavelength of the light
N is the f-number (focal length divided by diameter) of the imaging optics.
In object space, the corresponding angular resolution is
sinθ = 1.22 λ / D
where D is the diameter of the entrance pupil of the imaging lens (e.g., of a telescope's main mirror).
Particle diffraction
Quantum theory tells us that every particle exhibits wave properties. In particular, massive particles can interfere and therefore diffract.
The wavelength associated with a particle is the de Broglie wavelength
λ = h / p
where h is Planck's constant and p is the momentum of the particle (mass × velocity for slow-moving particles).
Bragg diffraction
Diffraction from a three dimensional periodic structure such as atoms in a crystal is called Bragg diffraction. It is similar to what occurs when waves are scattered from a diffraction grating. Bragg diffraction is a consequence of interference between waves reflecting from different crystal planes. The condition of constructive interference is given by Bragg's law:
m λ = 2 d sinθ
where
λ is the wavelength,
d is the distance between crystal planes,
θ is the angle of the diffracted wave.
m is an integer known as the order of the diffracted beam.
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