In which one of the following devices, the light energy is converted into the electrical energy?
- A.Light-emitting diode
- B.Laser diode
- C.Solar cell
- D.Transistor
Correct answer
Explanation
The correct answer is C, Solar cell. A solar cell turns the energy of sunlight directly into electricity through the photovoltaic effect.
A solar cell is a large p-n junction, usually made of silicon. When light falls on it, the photons free electrons and create electron-hole pairs near the junction, and the electric field there pushes them apart, so a voltage of about 0.5 to 0.6 V appears across each cell. Many cells joined together form a solar panel, used on rooftops, in calculators and on satellites. Photodiodes and photocells, used in light sensors and automatic street lights, work on the same idea of turning light into an electrical signal.
A is wrong because a light-emitting diode does the reverse: it turns electrical energy into light. B is wrong because a laser diode also turns electrical energy into a narrow beam of light. D is wrong because a transistor amplifies or switches electric signals and does not convert light.
Exam tip: solar cell and photodiode: light to electricity; LED and laser diode: electricity to light.
Practice Questions
View allWhich one of the following statements about sound is NOT correct?
- A.Sound travels at a speed slower than the speed of light
- B.Sound waves are transverse waves
- C.Sound waves are longitudinal waves
- D.Sound travels faster in moist air than in dry air
Show answer
This statement is false, because sound in air travels as longitudinal waves, not transverse waves. In a longitudinal wave, the particles of the medium vibrate back and forth along the direction in which the wave moves, forming compressions and rarefactions.
Read the full explanation →Power of a lens of focal length 25 cm is
- A.+2.5 Dioptre
- B.+3 Dioptre
- C.+4 Dioptre
- D.+5 Dioptre
Show answer
Power is the reciprocal of the focal length in metres, so P = 1 ÷ 0.25 m = +4 D. The power of a lens tells how strongly it converges or diverges light.
Read the full explanation →Two conducting wires of the same material and of equal lengths and equal diameters are first connected in parallel and then in series in a circuit across the same potential difference. The ratio of heat produced in parallel and series combinations is
- A.2:1
- B.4:1
- C.1:2
- D.1:4
Show answer
With the same voltage, heat varies inversely as resistance, and the parallel resistance is one-fourth of the series resistance. The two wires are identical, so each has the same resistance R. In series the total resistance is R + R = 2R; in parallel it is R/2.
Read the full explanation →The amount of air breathed in and out during a normal respiration by human lungs is referred to as
- A.Vital capacity
- B.Tidal volume
- C.Inspiratory volume
- D.Total lung capacity
Show answer
Tidal volume is the volume of air breathed in or out in one normal, quiet breath, about 500 mL in a healthy adult.
Read the full explanation →Cornea in human eye
- A.is a light sensitive screen.
- B.is a muscular diaphragm.
- C.contains blood vessels.
- D.is composed of proteins and cells.
Show answer
The cornea is a clear layer made of cells and proteins, chiefly collagen fibres laid out in a regular pattern that keeps it transparent.
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