^What is optics?
^What is optics?
The branch of Physics which deals with light, it’s nature, cause, source, properties & it’s effects is called Optics.
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^What is light?
^What is light?
In general light means following three parts
- Infra red radiation or thermal radiations
- visible light
- ultraviolet light,
As these are parts of EM spectrum, thus light is electromagnetic, thus properties of light are same as that of any EM wave. Light can behave like a ray, a wave & as a particle, depending on its interaction with the objects it encounters on its way.
^Electromagnetic Force
^Electromagnetic Force
EM – force acts between charged particles. It binds electrons with nuclei to form atoms & binds atoms together to form molecules. It is responsible for the properties of solids, liquids, & gases. Force of attraction/repulsion between two static charges is described by the Coulomb’s law i.e 
Electric forces and magnetic forces were considered to be separate forces until the 1860s when James Clerk Maxwell was able to demonstrate that they were different manifestations of the same force – the electromagnetic force. Infact, all contact forces such as: Frictional force (f), Viscous force (Fv), Air drag (fd) Spring force (Fspring or Fr), Normal reaction (N), Tension force in ropes, strings & muscles (T), Buoyant force (B or U), Surface tension force (FS) are actually due to the electromagnetic force.
^Maxwell equations
^Maxwell equations
Following five equations give a complete description of all em– interactions and are called Maxwell eq:
^ γ-rays
^γ – rays
(a) Produced in nuclear reactions and are also emitted by radioactive nuclei (such as Co 60 & Cs 137).
(b) Properties: Effect on photographic plate, fluorescence, ionization, diffraction, high penetrating power & can cause serious damage if absorbed by living tissues.
(c) Used
- in radio therapy to treat certain cancers and tumors
- to produce nuclear reactions
- to study the atomic nuclei.
^Ultraviolet light
^Ultraviolet light
(a) Produced by special lamps like mercury and iron are lamps and by very hot bodies like sun.
(b) Properties: Effect on photographic plate, fluorescence, ionization, highly energetic, tanning of the human skin.
(c) Used
- To destroy bacteria and for sterilizing the surgical instruments
- In the detection of forged documents finger prints
- In burglar alarms etc as they can cause photoelectric effect.
- For studying fluorescence
- In the study of molecular structure and arrangement of electrons
- To prevent the food stuff.
^Visible light
^Visible light
- Produced by hot bodies like sun, electric bulbs and tubes etc. as a result of electronic transitions. Our eyes are sensitive to visible light which helps us to collect the information about our surrounding.
- Properties: Reflection, refraction, interference, diffraction, polarization, photoelectric effect, photographic action, sensation of sight.
^Infra red waves
^Infra red waves
(a) Also called heat waves & are produced by hot bodies and molecules.
(b) Properties: Heating effect, reflection, refraction, diffraction & propagation through fog.
(c) Used
- In taking photographs during the conditions of fog, smoke etc. as these waves are scattered less than visible rays and hence can travel longer distances through atmosphere under fog, smoke etc.
- In weather forecasting
- In checking the purity of chemicals
- In the study of molecular structures
- For producing dehydrated fruits
- In green houses to keep the plants warm
- To provide electrical energy to satellites using solar cells.
- To treat muscular strain.
- In solar water heaters and cookers.
^Permanent magnets
Permanent magnets
The materials used in making permanent magnets should have high retentivity, high coercivity & high permeability. High retentivity makes the magnet is strong and high coercivity ensures the magnetisation is not erased by stray magnetic fields, temperature fluctuations or minor mechanical damage.
Suitable materials for permanent magnets are steel, alnico, cobalt steel and ticonal. Steel is one-favoured choice. Coercivity of steel is very large as compared to that of the soft iron but its retentivity is slightly smaller retentivity. Permanent magnetism is used in making Electric meters, Loudspeekers etc.. e.g. Steel, Alnico, Alcomax & Ticonal.
^Corecivity (oc):
Corecivity (oc):
The value of –H which has to be applied to a magnetic material so as to reduce residual magnetism to zero. The area of hysteresis loop is proportional to the thermal energy loss per unit volume for one hysteresis cycle of magnetization & demagnetization. Thus in order to minimize the energy losses that material must be selected which have minimum area of the hysteresis loop. e.g. Soft iron is preferred over steel.
The hysteresis curve allows us to select suitable materials for permanent magnets.
