Interference & Stationary Waves (Edexcel International A Level (IAL) Physics): Exam Questions

A student investigated stationary waves on a string, using the apparatus shown.

The frequency f of the vibrator was adjusted until a stationary wave was formed with a node at each end. This was repeated for various stationary waves on the string. A metre rule was used to measure the distance d between adjacent nodes on the string.

The resolution of the metre rule was 1 mm, but the measurements were recorded to the nearest 0.5 cm.

Suggest why.

The student plotted a graph of d against 1/f as shown.

i) Determine the total mass of the hanging masses used in the investigation. Mass per unit length of string = 4.5 × 10^–4 kg m^–1

Total mass of hanging masses = ............................(5)

ii) The string was replaced with one that had twice the mass per unit length.

The length of the string and the mass of the hanging masses did not change.

Add a line to the graph to show how d varied with 1/f for the new string.

(2)

The interference of microwaves can be investigated in the laboratory using a microwave transmitter and receiver. A coherent beam of microwaves is directed at two slits, S₁ and S₂, formed between metal plates, as shown.

Describe what is meant by a coherent beam.

As the receiver is moved along line AB, alternate points of maximum and minimum readings are detected.

Explain why maximum and minimum readings are detected.

The receiver detects a maximum reading at point O, which is equidistant from the two slits.

The next two maximum readings are detected at P and Q.

Point Q is $85\text{ cm}$ from S₁ and $75\text{ cm}$ from S₂.

Three common microwave bands and their corresponding frequencies are given in the table.

Deduce the band of the microwaves used in this experiment.

A student investigated the reflection of microwaves from a metal plate and a glass plate.

The metal plate reflects microwaves and the glass plate partially reflects microwaves.

A plan view of the apparatus is shown.

The metal plate, the transmitter and the receiver were kept in fixed positions.

The value of d was varied by moving the glass plate.

As d varied, the intensity of the microwaves detected by the receiver varied.

Explain why.

The student recorded values of d when the receiver showed a maximum value of intensity.

He recorded d for a sequence of five maxima.

i) Determine the wavelength of the microwaves being transmitted.

Wavelength = .......................................... (3)

ii) Calculate the frequency of the microwaves being transmitted.

Frequency = ...........................................(2)

A student uses a microwave oven to investigate standing waves. They remove the rotating turntable from the oven and place a bar of chocolate inside, so it lies flat and remains stationary, as shown.

The microwaves are switched on until a pattern of melted spots forms on the chocolate, as shown.

Explain how the standing wave pattern of melted spots forms.

Determine the frequency of the microwaves used by the oven.

Explain why it was necessary for the student to remove the rotating turntable before switching on the microwave oven.