UPSC Prelims Practice Questions — From light waves to quantum physics: the science behind smart washbasins
Q1. The Herschel Space Telescope, dedicated to studying infrared radiation from astronomical objects and named after the discoverer of infrared, was launched by which one of the following space agencies?
- A. European Space Agency (ESA)
- B. National Aeronautics and Space Administration (NASA)
- C. Indian Space Research Organisation (ISRO)
- D. Japan Aerospace Exploration Agency (JAXA)
Q2. Among the standard subdivisions of the infrared region of the electromagnetic spectrum, which one has the shortest wavelength range and lies closest to the visible spectrum?
- A. Near-infrared
- B. Middle-infrared
- C. Far-infrared
- D. Microwave
Q3. With reference to the photoelectric effect, consider the following:
1. It was first experimentally observed by Heinrich Hertz.
2. Albert Einstein received the Nobel Prize in Physics for its explanation.
3. In Einstein's photoelectric equation, the work function is the minimum energy required to liberate an electron from the surface.
4. Einstein explained the effect by treating light purely as a continuous electromagnetic wave.
Which of the above is/are correctly identified?
- It was first experimentally observed by Heinrich Hertz.
- Albert Einstein received the Nobel Prize in Physics for its explanation.
- In Einstein's photoelectric equation, the work function is the minimum energy required to liberate an electron from the surface.
- Einstein explained the effect by treating light purely as a continuous electromagnetic wave.
- A. 1, 2 and 3
- B. 2 and 4 only
- C. 1 and 3 only
- D. 1, 2, 3 and 4
Q4. The following statements compare Einstein's quantum explanation of the photoelectric effect with the earlier classical wave theory of light:
1. Unlike the classical wave picture, Einstein's explanation predicted that emission of photoelectrons depends on the frequency of the incident light rather than merely its intensity.
2. According to Einstein, sufficiently increasing the intensity of light below the threshold frequency will eventually eject electrons.
3. Einstein built upon Planck's earlier hypothesis that energy is emitted or absorbed in discrete quanta.
Which of the statements given above is/are correct?
- Unlike the classical wave picture, Einstein's explanation predicted that emission of photoelectrons depends on the frequency of the incident light rather than merely its intensity.
- According to Einstein, sufficiently increasing the intensity of light below the threshold frequency will eventually eject electrons.
- Einstein built upon Planck's earlier hypothesis that energy is emitted or absorbed in discrete quanta.
- A. 1 only
- B. 1 and 3 only
- C. 2 and 3 only
- D. 1, 2 and 3
Q5. Consider the following statements distinguishing the external and internal photoelectric effects:
1. In the external photoelectric effect, electrons are ejected from the surface of a material into the surrounding space.
2. The internal photoelectric effect involves excitation of electrons from the valence band to the conduction band within a semiconductor.
3. A vacuum phototube operates on the internal photoelectric effect.
4. The internal photoelectric effect requires photon energy greater than the band gap of the semiconductor.
Which of the above is/are correctly identified?
- In the external photoelectric effect, electrons are ejected from the surface of a material into the surrounding space.
- The internal photoelectric effect involves excitation of electrons from the valence band to the conduction band within a semiconductor.
- A vacuum phototube operates on the internal photoelectric effect.
- The internal photoelectric effect requires photon energy greater than the band gap of the semiconductor.
- A. 1 and 2 only
- B. 1, 2 and 4
- C. 2, 3 and 4
- D. 1, 2, 3 and 4
Q6. Which one of the following is the earliest type of light-detecting device, based on the external photoelectric effect, that has now been largely replaced by semiconductor photodiodes?
- A. Vacuum phototube
- B. Silicon photodiode
- C. Phototransistor
- D. Light-emitting diode
Q7. The following statements compare a semiconductor photodiode with the older vacuum phototube:
1. Unlike a vacuum phototube, a semiconductor photodiode operates at low voltage.
2. A photodiode converts incident photons into electron-hole pairs within a p-n junction.
3. Semiconductor photodiodes have largely replaced vacuum phototubes for light detection.
Which of the statements given above is/are correct?
- Unlike a vacuum phototube, a semiconductor photodiode operates at low voltage.
- A photodiode converts incident photons into electron-hole pairs within a p-n junction.
- Semiconductor photodiodes have largely replaced vacuum phototubes for light detection.
- A. 1 and 2 only
- B. 2 and 3 only
- C. 1 and 3 only
- D. 1, 2 and 3
Q8. With reference to the working of a semiconductor photodiode, consider the following:
1. It is essentially a p-n junction that generates a current when illuminated.
2. The photocurrent it produces is proportional to the intensity of the incident light.
3. It requires the incident photon energy to be below the band gap of the semiconductor to generate charge carriers.
4. It is used in fibre-optic communication and in solar energy conversion.
Which of the above is/are correctly identified?
- It is essentially a p-n junction that generates a current when illuminated.
- The photocurrent it produces is proportional to the intensity of the incident light.
- It requires the incident photon energy to be below the band gap of the semiconductor to generate charge carriers.
- It is used in fibre-optic communication and in solar energy conversion.
- A. 1, 2 and 3
- B. 1, 2 and 4
- C. 2, 3 and 4
- D. 1 and 4 only
Q9. The emission of infrared radiation by an infrared LED is primarily the result of which one of the following processes?
- A. Radiative recombination of electrons and holes at a p-n junction
- B. Thermal heating of a tungsten filament to incandescence
- C. Ejection of electrons from a heated metal cathode
- D. Excitation and de-excitation of gas atoms by an electric discharge
Q10. Forward-looking infrared (FLIR) / thermal-imaging cameras form images primarily by detecting which one of the following?
- A. Infrared radiation emitted by objects on account of their temperature
- B. Visible light reflected from objects
- C. Ultraviolet radiation emitted by objects
- D. Microwaves reflected from objects
Q11. Automatic 'touchless' washbasins and sensor-operated soap/sanitiser dispensers, as explained in a 2026 science feature, typically activate by detecting which one of the following when a hand is placed near them?
- A. Reflected infrared light from a built-in IR emitter
- B. Reflected ultraviolet light
- C. Emitted visible laser light from the hand
- D. Reflected microwave signals
Q12. The idea that electromagnetic energy can be emitted or absorbed only in discrete packets (quanta) — the hypothesis later used by Einstein to explain the photoelectric effect — was first introduced in 1900 by which one of the following scientists?
- A. Max Planck
- B. Albert Einstein
- C. Louis de Broglie
- D. Arthur Compton