VISUAL PHYSICS ONLINE

7.1 THE NATURE OF LIGHT

QUANTUM MODEL

P71 011

Hertz used a high voltage source to generate a spark and realised that when another spark was produced at a receiving coil, energy must have been transferred. Early in his experiments Hertz made a chance observation when he could increase the strength of the spark in the receiving coils. Which of the following correctly describes what Hertz had done that led to this observation? Comment on each alterative. What was the significance of Hertz’s observation?

(a) He had shone UV light on the receiving coil.

(b) He had slightly increased the gap between the terminals of the receiving coil.

(c) He had placed a glass plate between the transmitter and receiver.

View solution below only after you have completed the answering the question.

Solution

(a) UV radiation on metal – photoelectric effect – electrons more easily removed – larger spark.

(b) Increasing the gap between the terminals of the receiver decreases the strength of the spark across the terminals.

(c) A spark produced by the transmitter emits some UV – placing the glass plate reduced the UV from reaching the receiver – decrease in the strength of the spark.

Hertz’s observations led to the discovery of the photoelectric effect. Einstein was able to explain the photoelectric effect using a particle model of light. The particles are called photons and the energy of each photon is proportional to the frequency of the light .