VISUAL PHYSICS ONLINE

7.1 THE NATURE OF LIGHT

P71 015

Consider the diagram below which shows the form of light (electromagnetic radiation) that can eject electrons from zinc, magnesium, lithium and cesium.

(A)

Which of the four metals would produce photoelectrons if the light source is an ordinary torch?

(C)

Which metal has the highest value for its work function and threshold frequency?

Which metal has the lowest value of its work function and threshold frequency.

(B)

A source of violet light shines on both lithium and cesium metals surfaces. From which metal would the ejected electrons have the greater maximum kinetic energy.

(D)

An electroscope is illuminated by ultra-violet light. If the leaves of the electroscope diverge, what is the sign of the charge on the leaves.

View solution below only after you have completed answering the question. The solution is not in a form that you would answer in an examination. The answers are often in more detail to help improve your appreciation and understanding of the physics.

Solution

(A)

An ordinary torch emits mostly infrared and visible light. We can assume zero emission of ultraviolet light. Therefore, no electrons would be emitted from the zinc and magnesium surfaces, but some electrons would be emitted from the surfaces of lithium and cesium.

(B)

Einstein explained the emission of electrons from a metal surface by assuming the that the light was composed of a stream of particles called photons. The energy of a photon being proportional to the frequency of the light . From the principle of conservation of energy, we can conclude that

The minimum energy to eject an electron from the surface is called the work function.

If then the threshold (critical) frequency of the election of electrons is

From the diagram, zinc has the highest values for its threshold frequency and work function whereas cesium has the lowest values for its threshold frequency and work function.

(C)

Cesium has a lower threshold frequency than lithium, therefore, the maximum kinetic energy of the ejected electrons is larger than from the lithium surface.

(D)

The ultraviolet light shining on the metal surface will cause electrons to be ejected. So, the electroscope will become negatively charged and the sign on the leaves is negative.