Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Demonstration spectrum

Demonstration

Class demonstration of a spectrum, and the appearance of coloured objects in coloured light.

Apparatus and materials

High-dispersion prism (flint or lead glass)

Convex lens

Lens holder

Light source, compact

Power supply for light source, low voltage, variable

Retort stands and bosses, 2

Pieces of coloured filter

Health & Safety and Technical notes


Treat the compact light source with respect as it is a significant source of UV; do not look directly at the bulb. The lens will effectively filter out the UV.

Note that no slit is needed if the source is compact or if it is a line filament, parallel to the prism's edge. It will be advantageous to reduce the light level in the lab. 

If you don't have a compact light source (quartz iodine lamp), use a 36 W 12 V lamp overrun to 14 or 16 V. 

Apparatus set-up

Procedure


a Set up the light source at one end of the laboratory so as to produce an intense line of light. Place the lens so that a sharp image is produced on a distant screen about 3 m away. Place the glass prism in the beam, just beyond the lens, and rotate it to show how the prism swings the rays round. Move the screen so that a suitable spectrum is produced; this need not be at minimum deviation. 

b Look at the spectrum through a selection of coloured filters. The effect of the primary and secondary coloured filters on the white light spectrum should be noted. Look at a selection of coloured materials - a sheet of stamps for example - and hold them in different parts of the spectrum.


Teaching notes


1 Many prisms used in school are made of low dispersion glass in order to reduce the colours on the emergent ray. Here a high dispersion prism is needed, because the dispersion is what is required. The prism should be large enough to make use of the whole of the incident beam from the bright light source. 

2 The position of minimum deviation produces a bright, clear spectrum without a white band in the middle. However this is not the point of maximum dispersion, and so a compromise has to be made between clarity and size of the spectrum. (To find the position of minimum deviation, rotate the prism until the spectrum rotation stops and begins to retrace its path as the prism continues to rotate in its initial direction.) 
 
This spectrum is a series of images of the lamp filament in a progression of colours side by side. The screen should be slanted to spread out the colours even further. The blue light is refracted more than the red light. The wider the filament, the more overlapping there is and the less 'pure' the spectrum, but the brighter it is. 

White light to spectrum back to white light 
3 Demonstrations of the recombination of coloured light to form white light are easy to set up. The prism version is less fiddly than the mirrors, but not so impressive. Remember that there is little blue light in a tungsten filament lamp, and so distance travelled by the blue light in particular should be as short as possible. 
 
The reverse prism: If the lab has a second prism of the same dispersion as the one used for the spectrum demonstration, place it in the demonstration arrangement, next to the first prism but the other way round. 
 
Reassembly by mirrors: Form a spectrum. Place strips of plane mirror in the spectrum: one to catch the red, one to catch the orange, etc. Twist each mirror so that all the reflected beams fall at the same place on a white screen. 
 
4 Newton was delighted, and his contemporaries amazed, when he analyzed a beam of sunlight with a glass prism. Let your students share those feelings. Without any complicated optical system, let a streak of sunlight fall onto a high dispersion prism and then travel to a white wall of a darkened room. The sun's disc subtends an angle of 1/2° therefore each colour of the spectrum will make at least 1/2° patch in the spectrum at any distance. 
 
Ask students what Newton, a very good scientist, did as his next experiment. The splitting up of light into colours was a delight, and it must have been noted by many, although they didn't think about it as Newton did. But it was an act of scientific genius to pierce a hole in the card on which he caught the spectrum, and try a second prism with the light of one colour that came through the hole: no further production of a new range of colours. That is not only sensible experimenting: it gives a strong hint that the colours were there in a mixture, waiting to be sorted 'once and for all'. 
 
This experiment was safety-checked in January 2007

 

Related guidance


The electromagnetic spectrum

Learning from spectra