Practical Activity for 14-16
This can be shown to a sixth form class as a fun demonstration of light interference.
Apparatus and Materials
- Laser or different colour lasers if available
- Small rectangular glass box as used for optics expts. (e.g. use a 6 cm hollow cube)
- Small right-angled 45 degree glass prism to fit inside the box with the right angle in one corner
- Bottle of (cheap) brandy - works better than ethanol
Health & Safety and Technical Notes
The laser should be positioned so the beam cannot fall onto the eyes either directly or indirectly.
Be careful of laser reflections from the face of the glass box.
Don't drink the brandy!!
The brandy can be re-used although eventually the alcohol content becomes too low.
It is possible to use a brandy glass and brandy rather than the apparatus used here but it is quite tricky to get working well.
The glass box and prism must be clean and dry.
- Arrange the box and prism on the bench with the laser set to shine in through the side and into the prism, passing perpendicularly through the prism face.
- The light will internally reflect from the back face and emerge from the third side of the prism; then pass onto the screen, which should be set around two metres from the glass box.
- Pour some brandy into the box until the level is about 1 cm below the point on the prism face where the laser beam is entering.
- After a while brandy tears will creep up the prism face.
- Adjust the laser so that it hits one of the tears (be patient).
- A striking interference pattern will be seen on the screen and will be continually changing. It is best seen in a darkened room.
- If lasers of different wavelengths are available, the different fringe separations are obvious.
- Without the brandy the laser totally internally reflects from the prism's back face.
- Brandy in contact with the face changes the critical angle at that point (because the change in the speed of light between glass and brandy is less than the speed change between glass and air) with the result that the light now enters the brandy tear, reflects from the back surface (a complex shape) and continues back through the prism to the screen, interfering with itself and producing the fringes on the screen.
- Alcohol continually evaporates from the brandy tear, changing its concentration and so the surface tension, and hence the angle of contact with the glass. As a result, the liquid in the tear is flowing up and down the glass all the time, changing the shape and thickness of the tear and so causing the interference patterns to shift in a fascinating way. The effect is rather beautiful and very soothing.
This experiment was submitted by Rod Smith from Cranbrook School in Kent.
This experiment was safety-tested in February 2008