Question

The threshold wavelength for photoelectric emission from a material is $5200\mathop {\rm A}\limits^ \circ$ . Under what condition photoelectrons will be emitted when this material is illuminated with monochromatic radiation?
(A) 50 W infrared lamp
(B) 1 W infrared lamp
(C) 1 W ultraviolet lamp
(D) 60 W infrared lamp

Answer 1

Hint : From the knowledge of the electromagnetic spectrum we can solve this question. For photoelectric effect to occur, the wavelength of the light should be less than $5200\mathop {\rm A}\limits^ \circ$ .

Complete step by step answer
The photoelectric effect refers to the phenomenon in which a body that has absorbed electromagnetic radiation emits electrons. Each particle of light, called a photon, on collision with an electron, removes the electron. The rest of the photon's energy, remaining after knocking out the electron, is transferred to the free negative charge. This is called a photoelectron.
The effect is often defined as the emission of electrons from a metal plate when light falls on it. The radiant energy may be infrared, visible, or ultraviolet light, X-rays, or gamma rays; the material may be a solid, liquid, or gas; and the released particles may be ions (electrically charged atoms or molecules) as well as electrons.
Given that the threshold wavelength for photoelectric emission from a material is $5200\mathop {\rm A}\limits^ \circ$ . This implies that radiation of the wavelength greater than $5200\mathop {\rm A}\limits^ \circ$ will not be able to produce the photoelectric effects. Thus electrons will not be emitted. As a result, frequency increases and hence the energy of incident photons increases.
We know from the electromagnetic spectrum that, ultraviolet light has less wavelength that $5200\mathop {\rm A}\limits^ \circ$
$\therefore$ Among the given options, only Option C has a 1W ultraviolet light.
Thus, the correct answer is Option C.

Note
Albert Einstein was the first to describe the photoelectric effect and received a Nobel Prize for his work. This discovery was a breakthrough in science because it demonstrated the particle nature of light. Light also behaves as waves, as illustrated by Clark Maxwell, thus light has dual nature.