Light
Light
Light is a form of energy which is propagated as electromagnetic waves. In the spectrum of electromagnetic waves it lies between ultra-violet and infra-red region and has wavelength between 3900 A° to 7800 A°.
» Electromagnetic waves are transverse, hence light is transverse wave.
» Wave nature of light explains rectilinear propagation, reflection, refraction, interference, diffraction and polarisation of light.
» The phenomena like photoelectric effect, compton effect are not explained on the basis of wave nature of light. These phenomena are explained on the basis of quantum theory of light as proposed by Einstein.
» In quantum theory, light is regarded as a packet or bundle of energy called photon. Photon is associated with it an energy E , where E = hv.
» Clearly light behaves as wave and particle both. Thus light has dual nature.
» Speed of light was first measured by Roemer (1678 AD).
» Speed of light is maximum in vacuum and air (3 x 108 m/s)
» Speed of light is different in different media. Velocity of light is large in a medium which has small refractive index.
» Light takes 8 minute 19 second (499 second) to reach from sun to earth.
» The light reflected from moon takes 1.28 second to reach earth.
Luminous bodies
Those object which emit light by themselves are called luminous bodies. e.g.-sun, stars, electric bulb etc.
Non-luminous bodies
Those objects which do not emit light by themselves but are visible by the light falling on them emitted by self luminous bodies are called non-luminous bodies.
A material can be classified as :
1. Transparent : The substances which allow most of the incident light to pass through them are called transparent, e.g. glass, water.
2. Translucent : The substances which allow a part of incident light to pass through them are called translucent bodies e.g. oiled paper.
3. Opaque : The substances which do not allow the incident light to pass through them are called opaque bodies, e.g., mirror, metal, wood etc.
Shadow
shadow is the dark region on a screen when an opaque object is placed m the path of light. If is due to rectilinear propagation of light.
» In case of point source, the shadow has shapely defined outline.
» In case of extended source of light, the shadow has two distinct regions
Umbra : The region in which no light reaches, (region of complete darkness)
Penumbra : The region in which partial light reaches, ie. colour of body (star)