Producing fringes

To produce a good fringe pattern you need to have a coherent source of light so that the waves interfere with each other producing total destructive interference and total constructive interference - there will then be good contrast between the dark and light regions on the screen.

Sources need to be monochromatic - have the same frequency and constant phase difference - produce temporally coherent waves.

• You can use a laser - that produces monochromatic light.
• You can use a colour filter with a white light source - but filters are rarely able to absorb all but a single wavelength - they usually transmit a band of wavelengths - a laser is better!
• You can use a sodium lamp - that emits virtually monochromatic light - but you need total blackout to see the pattern clearly as the light is not very intense..

Sources need produce waves of the same amplitude so that total construction and destruction occurs - spatial coherence.

To get this to happen you need to pass sodium or white light through a single narrow slit. A laser produces waves of virtually constant amplitude without the need for the single slit arrangement so the single slit is not needed for the laser.

To get a double slit interference pattern (Young's slits) you then need to pass the light through a double slit arrangement.

If you just use two lasers (instead of passing the laser light from one laser through the double slit) they may not interfere with each other as well as a single one will - but interference will occur. They will be temporally coherent but might not be spatially coherent.

Dirac said, "Each photon then interferes only with itself. Interference between two different photons never occurs" but this has been shown to happen... the physics of why is beyond this site!

Young's double slits experiment

To observe interference of light, we can illuminate two closely spaced parallel slits (double slits) using a suitable light source (the light has to be coherent before it hits the double slits), as described below.

The two slits act as coherent sources of waves, which means they emit light waves with a constant phase difference and the same frequency (be temporally coherent). They should also be of the same amplitude (spacially coherent) .

A practical setup like the one used by Thomas Young is shown in the diagram. We could do this and use a bulb instead of a candle – for safety reasons! As filters would absorb most of the light we could use a monochromatic light source like a sodium lamp (gives out yellow light of two wavelengths that are very close to each other – virtually monochromatic) and dispense with the filter. We must have temporally coherent sources.

If the single slit is too wide the dark fringes of the double slit pattern become narrower than the bright fringes, and contrast is lost between the dark and the bright fringes. You therefore need to aim to have equally spaced ones.

The ‘double slits’ are illuminated by light from the narrow single slit. This produces spatially coherent waves but we could dispense with that too and use a laser instead.

Alternate bright and dark fringes, referred to as Young's fringes, can be seen on a white screen placed where the diffracted light from the double slits overlaps. The fringes are evenly spaced and parallel to the double slits. The diagram shows yellow ones (assuming a yellow filter was used – but remember that if you use a laser the fringes would be red!

The fringes are formed due to interference of light from the two slits:

• Where a bright fringe is formed, the light from one slit reinforces the light from the other slit. In other words, the light waves from each slit arrive in phase with each other.
• Where a dark fringe is formed, the light from one slit cancels the light from the other slit. In other words, the light waves from the two slits arrive 180° out of phase.

The distance from the centre of a bright fringe to the centre of the next bright fringe is called the fringe separation ‘w’. This depends on the slit spacing ‘s’ and the distance ‘D’ from the slits to the screen.

From the equation we can see that the fringes become more widely spaced if:

• the distance D from the slits to the screen is increased,
• the wavelength X of the light used is increased,
• the slit spacing, s, is reduced.