^Paraxial & marginal rays
^Paraxial & marginal rays
In a wide beam of incident rays the rays travelling close to principal axis are called paraxial & the rays away from principal axis are called marginal.
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^Law of reflection
^Law of reflection
- The Incident ray, Reflected ray & normal to the reflecting surface at the point of incidence all are coplanar i.e. lie in the same plane
- Angle of incidence = Angle of reflection Maximum value of angle of incidence & angle of reflection is 900 & minimum is 00.
- Normal lies between Incident ray & reflected ray.
Using law of reflection we can say
(a) For normal incidence (i.e. i = 0) rays of light reflect back & retrace the path.
(b) Light ray passing through pole along principal axis of a mirror of any kind retraces its path i.e. suffers a deviation of 1800.
(c) Light rays parallel to principal axis passes through the focus after reflection.
(d) Light rays through focus become parallel to the principal axis after reflection.
(e) Light ray incident at some angle to principal axis emerges at equal angle with the principal axis after the reflection.
^Reflection of light
^Reflection of light
- Most of the light on striking a denser boundary comes back in same medium.
- There is a phase change of π if reflection takes place from a denser medium.
- Frequency, wavelength & speed of the light all remains the same.
- Intensity and hence, amplitude (I ∝ A2) usually ↓ es due to absorption at the boundary.
^Visible light
^Visible light
Visible light is a mixture of photons λ = 380 nm (violet) to 780 nm (violet) in appropriate proportion. Colour of light is determined by its frequency & not wavelength.
^Energy of a photon
^Energy of a photon
Energy ‘E’ of a photon depends upon its frequency ‘f’ or wavelength ‘λ’ & is described by Max Planck quantum radian law
[called energy condition
Energy of visible light is greater than that of infra red but smaller than ultraviolet. Red colour photons have maximum wavelength thus are least energetic. Also violet colour photons have minimum wavelength thus are most energetic.
^Optical frequencies
^Optical frequencies
Using the relation c = f λ, with c = 3 x 108 m /s & λ = 380 nm to 780 nm we can check that the optical frequencies are of the order of 1015 Hz.
^What is visible light
^What is visible light
EM-radiation that can be detected by the human eye is called white light or simply visible light. It is mainly through visible light and the sense of vision that we know and interpret the world around us. Sensation of colour of light perceived by the human eye is determined by its frequency of the wave falling on ratina cells and not on its wavelength.
Visible light is a mixture of photons of wavelengths from 380 nm to 780 nm in appropriate proportion. Following table displays the wavelength range for colours.
In terms of micrometer (μm) & angstroms (A0) we can write 570 nm = 0.57 μm = 5700 A0
^Quantum optics
^Quantum optics
Quantum optics treats light as a particle (localised energy packet) called photon’ to deal with photoelectric effect, Raman effect & LASER, etc.
^Wave optics
^Wave optics
Light behaves as a wave when the size of obstacle is comparable with the wavelength of light. Wave optics branch explains the following phenomena
- Interference (i.e. non uniform distribution of energy when two or more light waves superpose)
- Diffraction (i.e. bending & spreading of light wavefronts on encountering obstructions of size comparable or smaller than its wavelength)
- Polarisation (i.e. restricting the vibrations of light to one plane)
^Geometrical optics
^Geometrical optics
Geometrical optics is also called ray optics. It treats propagation of light in terms of rays and is valid only if wavelength of light is much lesser than the size of obstacles. It deals with the following phenomena
- rectilinear propagation (i.e. light propagates in a straight line, due to its small wavelength)
- reflection (i.e. coming back in same medium on striking a shining surface)
- refraction (i.e. change of speed on changing transparent medium)
- dispersion (i.e. light splits up in to its constituent colours on changing transparent medium)
- image formation ( i.e. intersection of two or light rays after undergoing reflection or refraction).
