Wave Interference Flashcards
(11 cards)
Phase
Phase is a way of comparing different points in wave cycles. A complete wave cycle is 360˚ or 2π.
Two waves are said to be in phase if if the medium of one wave is, at all times, travelling in the same direction as the medium of the other wave. They must have the same frequency but don’t need to have the same amplitude. When waves are out of phase, their media don’t undergo the same motion at the same time.
A special type of out-of-phase motion is when the particles of each medium are travelling in the opposite direction to each other - the waves are out of phase by 180˚.
Diffraction
Diffraction is the bending of a wave as it passes around the edge of a barrier. Larger wavelengths bend more than short.
Combining Waves (Superposition)
When two or more waves pass through each other, their media displacements of the medium by the waves combine to create a resultant wave. This is called interference. If both waves have the same frequency and amplitude, the resultant wave can form a regular pattern of troughs and crests. This is called an interference pattern.
If the particles are in phase, the crests and troughs add together to give a bigger amplitude. This is constructive interference. If the waves are in opposite phase, they subtract to give a smaller amplitude. This is destructive interference.
Two Point Source Interference
A point source of waves will produce a circular set of wavefronts. If two point sources have the same frequency and amplitude, the combined wave will have displacements that vary in a regular pattern between zero and double the original amplitude, creating an interference pattern.
Nodal lines
- Crests meet troughs
- Waves are out of phase
- Destructive interference
Antinodal lines
- Crests meet crests
- Troughs meet troughs
- Waves are in phase
- Constructive interference
Interference of Light Waves
For interference to occur, the waves must have the same frequency (colour), amplitude (brightness), and must be coherent (phase difference is constant). This can be done by diffracting a single beam of light through two slits. They can form an interference pattern.
The light will show up as bright and dark fringes, where constructive interference creates bright fringes and destructive interference creates dark fringes.
Path Difference
This is the difference in the distance between a selected point and each of the sources. Where the path difference is a whole number of wavelengths, there is an antinodal line. Where the path difference is something.5 wavelengths, there is a nodal line.
Interference Formula
For small angles (<10˚), we can make the small angle approximation, which assumes the antinodal lines intersect the screen at even intervals.
Order Number (n)
Antinodal lines have whole order numbers, while nodal lines are something.5.
Multiple Slit Interference
This is when there is more than one source
- Slits are very thin, so light diffracts at bigger angle.
- Slits are close together, so bright fringes are widely spaced.
- Bright fringes are narrower, because more sources are interfering, contributing to destructive interference.
- Fringes are brighter because more sources contribute to constructive interference.
Diffraction Grating and White Light
Diffraction gratings have thousands of lines, with each gap behaving like a slit. They are typically rated in lines per metre, and this can be related to slit spacing.
When white light is viewed through a diffraction grating, longer wavelengths are at a larger angle than smaller wavelengths. This occurs because longer wavelengths have a larger path difference (they don’t diffract more).
Secondary Maxima
In the case of a 3 source interference pattern, maxima (antinodal lines) occur when all 3 sources meet in phase. With 3 sources, we also have regions where 2 out of 3 sources meet in phase. This form smaller secondary maxima.
