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Polarisation – Malus Law

Two polariods P1 and P2 are placed with their pass axes perpendicular to each other. Unpolarised light of intensity Io is incident on P1. A third polariod P3 is kept between P1 and P2 such that its pass axis makes an angle of 30 with that of P1. Determine the intensity of light transmitted through P1,P2 and P3.


Asked Rashmi



Appply Malus’ Law

The intensity of light from the first polaroid P1 is Io/2

From P3, the intensity is (Io/2) cos230 =(Io/2)(3/4)=3Io/8



polarisationFrom P2, the intensity is (3Io/8)cos260= (3Io/8)(1/4)= 3Io/32

Why dispersion does not occur in a glass slab?

Why dispersion does not occur in a glass slab?
I know the reason that the opp. sides are parallel but dispersion occurs when light enters from one medium to another.. so what is the reason?prism

Asked Karan


glass slabThough there is slight dispersion when light enters glass slab at the first phase, all the colours are rendered parallel at the second phase and the different colours enter our eye without any angular separation. (Angular dispersion is the angle between the component colours and if the angle between the colours is zero, we will not see the colours)

Also read http://www.askphysics.com/why-dispersion-does-not-occur-in-a-glass-slab/


thermal properties of matter

In an experiment on the specific heat of a metal, a 0.02kg block of the metal at 150 degree celcius is dropped in a copper calorimeter(of water equivalent 0.025kg) containing 150 cubic centimetre of water at 27 degree celcius. the final temp. is 40 degree celcius.Compute the specific heat of the metal.


Asked Abel

motion in plane

The sum of magnitude of two forces acting at a point is 18N an the magnitude of their resultant is 12N.If the resultant makes an angle of 90 degree with the force of smaller magnitude what are magnitude of the two forces?

Asked Abel

Magnetic fields and how magnetism is caused

How is magnetism caused?

Electric fields are caused by more or less electrons, but how is magnetism caused? Is it something similar?’

Asked Rose

Lightwaves, color, scattering

Message Body:
If you put a red dot and violet dot on a wall and move far away from them, after certain period/distance  you will only be able to see the red light, then my teacher explained why/how this happens , which i forgot

I know the W. L. of red is more than violet, hence speed is also more and frequency is less, but how is all that relevant to visibility?
Asked Nafiz

The teacher might have tried to explain the dependence of scattering on wavelength.
According to Raliegh’s law of scattering, the amount of scattering is inversely proportional to the fourth power of wavelength. This means that the amount of scattering for violet is about 16 times that of red since the wavelength of violet is appropriately half that of red. So even after the light from violet source has completely scattered, the light from red source will be Reaching our eyes.
So, scattering is the concept involved here.

Speed of Light – a thought experiment

Suppose that you are a person 1 standing on a planet.You could see a person 2 moving in a space craft.2 has a mirror(on the surface of planet) exactly at is down which is moving exactly with the same speed that space craft is moving(and also,the line of translatory motion of both the craft and mirror are parallel to each other).If 2 has shot a beam of light from the bottom of space ship,as the mirror is moving exactly with the space craft;for 2,the path of light is straight line and gets reflected back along the same path in time t.If you are observing the whole thing from the surface of planet,for spaceyou,the path of light would obviously be ‘V’ shaped(let the time taken be t’).As the ‘V’ shaped path is longer than straight path and speed of light is same for observers,the time measured by 2 is obviously not the same as you measure.If you are considered to be reference frame,will the clock of 2 appear to be moving slower than yours? (Asked Charan)

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