^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.
^Micro waves
^Micro waves
(a) Produced by special vacuum tubes like klystrons & magnetrons. Due to their shorter wavelengths the microwaves don’t spread or bend around the corners of any obstacle coming in their way, hence can be transmitted as beam signals in a particular direction.
(b) Properties: Reflection, refraction, diffraction and polarization.
(c) Used in
- Radar systems used in aircraft navigation.
- In communication via satellites.
- Speed guns to determine speed of moving automobiles, cricket balls etc.
- Ovens for cooking
- In the study of atomic and molecular structure.
^Neutral points
^Neutral points
Neutral points are the positions where the magnetic field of a magnet is completely neutralised by the horizontal component of earth’s magnetic field. The position & number of neutral points depends upon the position of magnet.
If a magnet is placed vertically as shown then we get only one neutral point situated at point L at left side of magnet if magnet’s north is down. On reversing the magnet neutral point will be at right side of magnet at point R.
Consider a magnet of dipole moment M, length 2L placed horizontally. Following drawings show the top view of the situation
Horizontal component of earth’s field is towards 
. When the magnet’s north (n) is towards earth’s south (S), then magnetic field of the magnet will be towards opposite to that of BH & both get cancelled at a point somewhere on the axial line of magnet & thus two neutral points (shown by cross marks) are obtained on the axial line (i.e. towards geographic north south line). If neutral points are situated at distance r from the centre of the magnet then at these points we can write 
On reversing the magnet i.e. when magnet’s north (n) is towards earth’s north (N), two neutral points are obtained on the equatorial line (i.e. towards east – west line). At these points we can write 
