Introduction:
Optics is a branch of physics which deals with the study of light.
Optics is further classified into two types. They are
Optics is further classified into two types. They are
- Ray optics (Geometrical optics)
- Physical optics (Wave optics)
Ray Optics :
- When the wavelength of light is far less than the size of the object on which light incident then the study of light is called Ray optics
- In this branch of optics light is treated as a ray, to study the properties of light
Physical Optics :
- When the wavelength of light is comparable with the size of the object on which light incidents then the study of light is called Physical optics.
- In this branch of optics light is treated as a wave, to study the properties of light.
Light : Light is a form of energy which cause the sensation of sight. On striking the eye, it makes things visible.
Some important definitions in optics :
Optical medium : The substance through which the light can pass more or less is known as Optical medium.
They are
- Transparent
- Translucent
- Opaque
Transparent :
It is the medium through which light can pass easily. Here the light is not absorbed or very little absorbed.
Example : Air, thin glass, clear water of shallow depth etc. |
Translucent :
It is the medium thorough which the light can partially pass but things can not be clearly seen. Here the light is absorbed to a great extent.
Example : Ground glass, tracing paper etc. |
Opaque :
It is the medium through which light can not pass.
Example : Wood stone, stone etc. |
Luminous body :
A body which emits light s known as luminous body.
Example : Sun,burning lamp etc.
Example : Sun,burning lamp etc.
Non luminous body :
A body which does not emit light is known as Non-luminous body. They are visible by means of light received from self luminous bodies.
Example : Moon
Example : Moon
Homogeneous medium : When the physical properties of the medium are same every where, it is known as Homogeneous medium.
Heterogeneous medium : The medium in which the physical properties are different at different parts is known as Heterogeneous medium.
Ray of light : The path along which the light travels from one point to another point is a Ray of light
Beam of light :
Rays very near to each other and taken together constitute a Beam of light. There are three types of beams of light.
- Parallel
- Convergent
- Divergent
Parallel beam of light : In a parallel beam of light all the rays are parallel to one another.
Convergent beam of light : In a convergent beam of light all the rays proceed to converge or converge to a point.
Divergent beam of light : In a divergent beam of the light, the rays proceeding in a particular directions are gradually separated more and more form each other.
Source of light : The body which produces or appears to produce light is called a Source of light.
Point source : The source of light whose size can be considered as a point size is called Point source.
Properties of light : Some important properties of light are
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Rectilinear propagation of light : Light travels in straight line unless it is reflected by a polished surface or the medium of propagation is changed.
Reflection of light : Light comes back into the same medium from a boundary separating two media, is called Reflection.
- During reflection, some of the light energy is absorbed by the surface.
- During the Total internal reflection there is no loss of light energy.
- Reflection from smooth surface is called Regular reflection. If regular reflection takes place the surface is not visible.
- Reflection from rough surface is called Irregular reflection. It is possible to see objects because of irregular reflection.
- Refraction of light : When light enters a different medium, it deviates from its original path. This phenomenon is called Refraction of light.
Diffraction of light : When a light wave encounters an edge, or an obstacle, or an aperture the size of which is comparable to the wave length of the light wave, the light wave spread out and undergo interference is called Diffraction.
Interference of light : The redistribution of light emitted bye two point sources is called interference of light.
Polarization of light : When light passes through certain crystal, like Calcite, Tourmaline etc. the absorption of light changes on rotation of crystals. This phenomenon is called Polarization of light.
Photo electric effect : Emission of electrons when light of required frequency falls on certain metal surface. It is called Photo electric effect.
Theories
Many scientists studied light and proposed their theories on it. The following are the theories.
Assumptions of Newtons corpuscular theory :
- According to this theory light is combination of mass particles, called Corpuscles.
- Different colors are due to difference in the sizes of the corpuscles.
- Speed of light is more in denser medium than in rarer medium.
Merits :
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Demerits :
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Assumptions of Huygens wave theory :
- According this theory, light propagates as mechanical longitudinal waves.
- Propagation of mechanical waves require a material medium, Huygens assumed that the entire universe is pervaded with a medium called 'Ether'.
- Propagation of light takes place by the advancement of wave fronts.
- Wave front is the joining of terminal points of light rays after time 't' starting from the same source.
Merits :
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Demerits :
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Fresnel modified Huygens theory by considering light as transverse mechanical wave.
Maxwell's electromagnetic theory :
According to this theory, light propagates as an electromagnetic wave. This theory could explain all the properties of light ( except photo electric effect )
Planks Quantum theory :
According to this theory, electromagnetic radiations are emitted or absorbed by matter in small discrete energy packs called ' Quanta'.
The energy associated with quanta is give by
The energy associated with quanta is give by
Einstein's Quantum theory : According to this theory, electromagnetic radiations are not only emitted or absorbed by matter in small discrete energy packets but also propagate as 'Photons'.
Energy of the photon is given by
Energy of the photon is given by
He concluded that light has ' Dual nature '.
Dual nature : Dual nature of light is neither particle nature nor wave nature, but it is a new state which is both particle and wave nature.