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Download the collection of questions asked in previous CBSE board exams from the chapter Wave Optics.

Practising the previously asked questions is a good idea to prepare for the exams.

Download (PDF, 607KB)

Newton second law and pendulum

A pendulum is hanged in the car.
Car starts to move by constant acceleration
How to measure acceleration by angle the pendulum?

Asked Afshin



If the pendulum makes an angle θ with the vertical, then a = g tan θ

This is based on the idea of pseudo force experienced by a body in an accelerated frame of reference.

When a body is in an accelerated frame of reference, it feels as if it is acted upon by a force equal to its mass multiplied by the acceleration of the system. Please have a look at the diagram below.


Please feel free to ask further doubts on the topic via comments

reference books for the preparation of NGPE (national graduate physics examination)

Name a few reference books for the preparation of NGPE (national graduate physics examination) at B.Sc. level

Asked Abhivyakti

Click on the links below to downloadngpe

NGPE Question Paper 2014

NGPE Solutions 2014

NGPE-2013 Question Paper: Download

Click here to see NGPE 2012 Question Paper

Click here to see Solutions NGPE 2012

Click here to see NGPE 2011 Question Paper

Click here to see NGPE 2011 Section A answers

Click here to see Solutions NGPE2011


The following Books are recommended

Other than your regular BSc textbooks on,

You can take these …

  1. HC Verma’s Concepts of Physics Volume – 1
  2. HC Verma’s Concepts of Physics Volume – 2
  3. Resnick Halliday Fundamentals of Physics
  4. Irodov’s problems in physics
  5. University Physics
  6. Physics Formulae

And there are many … Try these first

Work Energy Problem

A 2kg hammer with a speed of 40m/s strikes a 100g nail driving it into a wooden block. If there is no dissipation of energy in this process and the frictional force applied by the wooden block is 8000N, how deep would be nail penetrate into the block?

Asked Jaisy

Physics Video Project

The video project was given in 2008 for XI class students of Meghalaya. The best video among them is posted here. Though there are some errors, the video is inspiring to note the work done entirely from scratch by the XI Class students.

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)

Magnetism of Earth – What’s the cause?

Selwyn posted this Message: I was recently showing my grandchildren the effect on a compass needle by the magnetic field surrounding a magnet placed in opposition to the earth’s magnetic field – Magnetism 1.01 no? Their mother then asked me how is it that the earth has a magnetic field. I confidently answered that it was due to the central molten iron core of the earth acting as a magnet, and of course as we learned in Magnetism 1.1 68 years ago on of the ways of creating a magnet is a) by striking it several blows with a hammer while holding it in alignment with the earth’s N/S axis or b) heating it. In both cases this allows the atoms to move more freely and align themselves similarly with the axis. But that started me thinking: We know that heating ANYTHING applies energy to the atoms/molecules and this then causes them to display greater and more violent movement within the body of the material. If these molecules are so agitated, how can this identical phenomenon allow the molecule/atoms of the earth’s core to “settle down quietly” into a N/S configuration and remain so?


The answer is not so simple. Nobody has actually drilled into the centre of earth. What we know is by analysing the seismic waves and the shockwaves.

The Earth’s magnetic field is believed to be generated by electric currents in the conductive material of its core, created by convection currents due to heat escaping from the core. However the process is complex, and computer models that reproduce some of its features have only been developed in the last few decades. (Wikipedia)

The Earth and most of the planets in the Solar System, as well as the Sun and other stars, all generate magnetic fields through the motion of highly conductive fluids. The Earth’s field originates in its core. This is a region of iron alloys extending to about 3400 km (the radius of the Earth is 6370 km). It is divided into a solid inner core, with a radius of 1220 km, and a liquid outer core. The motion of the liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 K (5,730 °C; 10,340 °F), to the core-mantle boundary, which is about 3,800 K (3,530 °C; 6,380 °F). The pattern of flow is organized by the rotation of the Earth and the presence of the solid inner core.

The mechanism by which the Earth generates a magnetic field is known as a dynamo.


The Dynamo Effect

The simple question “how does the Earth get its magnetic field?” does not have a simple answer. It does seem clear that the generation of the magnetic field is linked to the rotation of the earth, since Venus with a similar iron-core composition but a 243 Earth-day rotation period does not have a measurable magnetic field. It certainly seems plausible that it depends upon the rotation of the fluid metallic iron which makes up a large portion of the interior, and the rotating conductor model leads to the term “dynamo effect” or “geodynamo”, evoking the image of an electric generator.

Convection drives the outer-core fluid and it circulates relative to the earth. This means the electrically conducting material moves relative to the earth’s magnetic field. If it can obtain a charge by some interaction like friction between layers, an effective current loop could be produced. The magnetic field of a current loop could sustain the magnetic dipole type magnetic field of the earth. Large-scale computer models are approaching a realistic simulation of such a geodynamo.


Refer to this Wikipedia Page and this Hyperphysics page for more details

The GeoMag page is also very informative and useful(Natural Resources Canada)

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