NDA GK Questions 2026

Doping replaces a few host atoms with impurity atoms, and each impurity atom brings its own nucleus along with its electrons, so the total positive and negative charge in the crystal stays balanced.

John Bardeen, Walter Brattain and William Shockley built the first working transistor at Bell Laboratories in 1947 and shared the Nobel Prize in Physics in 1956 for the discovery of the transistor effect.

A full-wave rectifier conducts during both halves of the input cycle, since one pair of diodes handles the positive half and the other pair the negative half.

They are called universal gates because either one of them, used repeatedly, can build every other logic gate, including NOT, AND, OR, XOR and XNOR.

The base of a transistor is made very thin and only lightly doped so that most of the carriers injected by the emitter pass straight through it to the collector instead of recombining in it.

A Zener diode is heavily doped so that breakdown occurs at a low and sharply defined reverse voltage. Beyond that point the voltage across it stays almost constant even when the current through it changes a great deal, which is precisely the property a voltage regulator needs.

A p-n junction conducts freely in forward bias, where the potential barrier is reduced, and blocks current in reverse bias, where only a tiny leakage current flows.

Silicon is the most widely used semiconductor and its forbidden energy gap, the separation between the top of the valence band and the bottom of the conduction band, is about 1.1 electron volts at room temperature.

A p-type semiconductor is made by adding a trivalent impurity such as boron, aluminium, gallium or indium.

An n-type semiconductor is made by doping with a pentavalent element, that is one with five valence electrons, such as phosphorus, arsenic, antimony or bismuth.

A semiconductor has a small forbidden energy gap, so heating supplies thermal energy that pushes more electrons from the valence band into the conduction band, creating more electron-hole pairs.

A gravitational wave detector is an L-shaped interferometer whose two arms, each four kilometres long, are watched for the minute change in length produced when a wave from colliding black holes or neutron stars passes through the Earth.

Their instruments, such as the giant sundial called the Samrat Yantra and the hemispherical Jai Prakash Yantra, measured time, declination and the positions of the planets with shadow and sightline alone, and the Jaipur site was inscribed on the UNESCO World Heritage List in 2010.

At that point the gravitational pulls of the Sun and the Earth combine with the orbital motion so that a spacecraft keeps station with the Earth, and from there the Sun can be watched continuously without any eclipse or occultation.

AstroSat was launched on 28 September 2015 by PSLV-C30 from Sriharikota into a near equatorial low Earth orbit.

In 1977 astronomers watched Uranus pass in front of a distant star from the observatory in the Javadi hills, and the dips in the star's light before and after the planet itself crossed showed that rings surround Uranus.

It is now part of the Indian Institute of Astrophysics, and its digitised archive of daily photographs of the solar disc, sunspots and the chromosphere is among the longest continuous records of the Sun anywhere, which makes it valuable for the study of long solar cycles.

The Indian Astronomical Observatory sits there at about 4,500 metres in a cold desert, one of the highest and driest observing sites in the world, where very little water vapour and almost no artificial light make the sky exceptionally clear and dark.

The telescope was built in collaboration with Belgium and activated in 2016 on the Devasthal ridge at about 2,450 metres, and the same site carries a 1.3 metre fast optical telescope and the four metre liquid mirror telescope.

Fourteen of them stand close together in a central square of about one kilometre, and the remaining sixteen are spread along three arms in a Y shape stretching some twenty-five kilometres, which gives the array both the sensitivity of a filled aperture and the resolution of a wide one.