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 :
- Light is form of energy which causes in us the sensation of sight.
- Light is needed to see things around us.
- We detect light with ours eyes.
- Light travels in a straight line.
- Light is electromagnetic in nature.
- The visible light spectrum is the portion of the Electromagnetic spectrum visible to Human eyes.
- Light wave is transverse in nature and does not need any material to travel. Its Speed in vacuum is 3×10 ^8 m/s.
- The wavelength of visible light waves is very small being only about 4×10-7 m to 8×10-7m.
- The velocity and wavelength of light changes when it travels from one medium to another.
- The frequency of the light wave remains the same when it travels from one medium to another.
Type of Light Source (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 :
- Luminous Source : Object that emits light by itself is known as a luminous Source. Example : sun , stars , candles , tube-light , torch , fire etc.
- 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.
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 .
- 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.
- 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.