Simple spectrum of sunlight
Using prism, needle and sunlight to observe a spectrum.
Apparatus and materials
For each student or group of students
Health & Safety and Technical notes
For safety reasons, you might prefer to mount the needle in some cork.
a Hold the needle at arm's length in one hand and the prism in the other hand, close to your eye. The needle should be parallel to the refracting edge of the prism and brightly illuminated by sunlight.
b Twist the prism until you can see the needle by refraction through the prism.
1 The needle would appear as a bright line, but the dispersion makes it appear drawn out into a bright, white-light spectrum.
2 The higher the dispersive power of the prism material, the better.
3 It may be possible to see absorption lines, otherwise known as Fraunhofer lines. This is because the needle forms a bright, narrow line-image of the sun, which serves as a slit. The eye, viewing the needle from some distance, provides the lens system that is needed.
This experiment was safety-checked in January 2007