What is light | Characteristics | Type of Source | Wave , Particle and Quantum theory - Param Himalaya

What is light | Characteristics | Type of Source | Wave , Particle and Quantum theory - Param Himalaya 

Definition of light  : 

the brightness that comes from the sun, fire, electrical devices etc that allows things to be seen. 

Characteristics of light :

  1. Light is form of energy which causes in us the sensation of sight.
  2. Light is needed to see things around us. 
  3. We detect light with ours eyes. 
  4. Light travels in a straight line.
  5. Light is electromagnetic in nature.
  6. The visible light spectrum is the portion of the Electromagnetic spectrum visible to Human eyes.
  7. Light wave is transverse in nature and does not need any material to travel. Its Speed in vacuum is 3×10 ^8 m/s.
  8. The wavelength of visible light waves is very small being only about 4×10-7 m to 8×10-7m.
  9. The velocity and wavelength of light changes when it travels from one medium to another.
  10. The frequency of the light wave remains the same when it travels from one medium to another. 

Type of Light Source (objects) :

Luminous and non Luminous objects

A light source is a source that emits light in all directions. The source can be a point source or an extended source. 

There are two types of source :

  1. Luminous Source : Object that emits light by itself is known as a luminous Source. Example : sun , stars , candles , tube-light , torch , fire etc.
  2. Non - Luminous Source : Objects that donot emit light of their own but become visible only when light from luminous objects falls on them are called non-luminous sources. Example - Moon , planets , chair , human being.
Reflection of Light - Param Himalaya

Type of Image : Real and Virtual 

An image is formed when the light rays coming from an object meet ( or appear to meet ) at a point , after reflection from a mirror ( or Refraction through a lens ).

The images are of two types .

  1. Real image : A real image is formed when two or more than two reflected rays meet at a point in front of the mirror. A real image can be obtained on a screen. A real image is inverted with respect to the objects. 
  2. Virtual image : A virtual image is formed when two or more rays appear to be coming from a point behind the mirror ( or lens) . A virtual image cannot be obtained on a screen. A virtual image is Erect with respect to the object.

Different theories : Nature of Light 

There are two theories about the nature of light - wave theory and Particle theory of light.

According to wave theory : 

  • Light consists of EMW( Electromagnetic waves ) which donot require a material medium ( solid , Liquid or gas ) for the Propagation.
  • The Wavelength of visible light waves is very small ( about 4×10^-7 m to 8×10-7 m).
  • The speed of light waves is very high ( 3×10^8) metres per second in vaccum.
  • The phenomena of Diffraction ( bending of light around the corners of tiny particles) , interference and polarization of light can only be explained if light is considered to be of wave nature.
  • Phenomenon of Reflection , Refraction and casting of shadows of light cannot be explain by wave theory.
  • Photoelectric effect could not be explained with the help of wave theory of light.

According to Particle theory :

  • Light is composed of particles which travel in a straight line at very high speed.
  • The elementary particle that defines light is the ' Photon'.
  • Phenomenon of Reflection , Refraction and casting of shadows of light can be explain by Particle theory.

According to Quantum theory of light :

  • Light has a dual nature (double nature) , light exhibits the properties of both waves and particle.
  • The modern theory of light called 'Quantum theory of light combines both the wave and particle models of light.
  • Photoelectric effect was successfully explained with the help of Quantum theory of light.

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