Light
Light:
Light is electromagnetic radiation that moves in a straight line. It has a dual Nature particle nature as well as a wave nature. Light has tiny packets of energy called photons. Light does not require any physical medium to travel. Speed of light in vacuum =3*108 m/sec, faster than speed of sound.
Scattering of light:
Scattering of light is the phenomenon in which light deviates from its path on interacts with obstacles like dust particles, gas molecules or water vapours.
Raleigh-Scattering:
- It takes place when the particles are small in size.
- Scattering = 1/λ4, where λ is the wavelength of light.
Mie-Scattering:
- It takes place when particles are large.
- Here scattering is evenly distributed for all types of wavelength.
Spectacular phenomena of Scattering:
- Tyndall effect. (Scattering of light by colloidal particles)
- The blue colour of the sky in the daytime.
- The red cloud at the time of Sun set and Sun rise.
- The red colour of the emergency signal (as maximum wavelength).
- Cloud and fog are white.
- The blue colour of ocean water.
- The black colour of the moon sky.
Reflection of light:
The bounce back of light when it falls on a surface is called the reflection of light.
Law of reflection:
- The angle of incidence is equal to the angle of reflection are same.
- The incident ray, the reflected ray and the normal, all lie in the same plane.
Properties of reflection:
- Green plants look green as they reflect green light towards our eyes and absorb all the rest colours. Hence plants have a low rate of photosynthesis in green light.
- Red roses look black in green light.
Plane Mirror:
Plane mirror image formation
- The focal length of the plane Mirror is indefinite.
- The power of the plane Mirror is always Zero.
- The nature of images formed by the plane mirror are, Virtual, erect, lateral inverted, unmagnified, of the same size and at the same distance as the object.
- No screen is required to obtain the image.
Lateral Inverted - A mirror of insufficient size does not affect the image as even a small of mirror forms an image of half of the universe.
- A plane mirror is used for making periscopes.
- Numbers of images formed by two plane mirrors:
(i) θ=180•, n=1
(ii) θ=0•, n= infinite
(iii) θ=90•, n= 3 (changing room)
(iv) θ=10•, n= 35 (barber’s shop)
Refraction of light:
Refraction of light is a phenomenon in which light rays bend when they pass through from one medium to another medium. For the refraction of light two mediums are required i.e Rarer (air) and denser (water)
Law of reflection:
- The ratio of the sine of the angle of incidence and the sine of angle of refraction is constant [Snell’s Law]
- The incident ray, the reflected ray and the normal are all in the same plane.
Application of refraction of light
- The twinkling of stars.
- The Sun appears two minutes earlier and remains two minutes later than actual time.
- The pond seems less deep than it.
- The pencil seems bend in a water bottle.
When light transfers from a rarer medium to a denser medium
- The speed of light decreases.
- The wavelength of light decreases.
- Light bends towards normal (N).
- Refractive index (μ) > 1
- The angle of incidence (i) > the angle of refraction (r)
- The frequency and colour of light remain the same.
when light transfers from a rarer to a denser medium
- all points get reversed except frequency and colour as they remain the same.
Refractive Index (μ):
- It is the ratio of the Speed of light in vacuum to the speed of light in the medium.
- The value of μ depends upon Medium-I, Medium-II, Temperature and Colour of Light.
- The value of the refractive index does not depend upon the Incidence angle.
| Material | Refractive Index (μ) |
| Vacuum | 1 |
| Air | 1.0003 |
| Water | 1.33 |
| Diamond | 2.4 |
| Silicon | 3.9 |
| Germanium | 4 |
Dispersion of Light:
It is a phenomenon in which a white beam of light gets split into a spectrum of colours when it passes through a prism called dispersion. The power of dispersion depends upon the material of the prism.
Total Internal Reflection (TIR):
It is a phenomenon in which a ray of light come form one to another medium is not refracted into second medium but it get completely reflected back into the first medium.