## A Level Physics Session

Gravitation | Waves and Interference | Magnetic Force | Photoelectric Effect |

# Waves and Interference

Hi Everyone,

Welcome to this session on waves. The following are two videos on interference. I shall develop the idea using a ripple tank. Then I shall derive the conditions for constructive and destructive interference.

149 Constructive and destructive interference

### Exercises

Please try these questions.- Two narrow rods P and Q are held vertically at 10 cm apart. They
move up and down together 4 times per second into the water in a ripple tank.
Suppose that a peak of a ripple travels at 20 cm/s.
Find the wavelength of the ripples.
- The midpoint between the rods is O. Suppose that a point R is 20 cm from
P and 22.5 cm from Q. Sketch a diagram to show the positions of
O, P, Q and R. Label the distances.
- Calculate the number of wavelengths along the line PR and the line QR.
Describe and explain what happens at R.
- Draw a line through R that is parallel to PQ. Mark the point that is
equidistant from P and Q. Label it S. Describe and explain what happens at S.
- Describe and explain what happens if you start from S and move along the
line parallel to PQ to R, and carry on moving beyond R.
- Sketch a graph of the amplitude of the waves against distance from S along
this line.
- Describe the wave along the straight line joining the two rods if rod P is not there. Describe the wave if rod Q is not there. Describe and explain the wave if both rods are present.

### Past Exam Questions

In this video, I shall go through some questions from a past exam paper.2012 H2 Physics P1 23 Double slit

2012 H2 Physics P1 24 Diffraction grating

Thanks for attending this session.

Dr Hock

29 April 2014