Task 1: White and coloured light


Primary aim of this task is to show pupils that white light can consist of individual coloured lights. Another aims should encourage pupils to independently research and verify the effects of the surrounding world and to show them that for carrying out physical experiments there is no need for using specialist equipment in laboratories but it is possible to realize them in common environment with easily accessible objects, alternatively without the need of any equipment at all, only as simple observation of the surrounding reality.

Pupils can observe light refraction with the use of a variety of tools. The one easily most accessible is a common compact disc (CD) where the refraction of light occurs by interference of light on the grating which is formed by grooves on the recording area of the disc. If the teacher has a prism from any set of tools for teaching optics, he/she can show the light refraction with the help of this prism. However, in most cases light refraction with the use of a prism takes more careful preparation of experiments including the installation of a special light source and optical bench with a shade. It is not advisable to start with the experiment using the prism. It is better to let pupils experiment with the CD so that they try to examine the spectrums of various luminous sources independently. Pupils should conclude that the sun spectrum and the spectrum of a light bulb are continuous, i.e. they contain all colours while one passes into another continuously. The spectrum of LED bulbs also appears as continuous and looks the same as the Sun spectrum. The fact that the intensity of individual colours is different pupils cannot recognise. Pupils should name the basic colours of the Sun spectrum. Ideally, they should name Newton’s seven basic colours – red, orange, yellow, green, blue, indigo and violet. Nevertheless, it is not important to name them in this order

The spectrum of a fluorescent tube and energy saving light bulbs (compact fluorescent lamps) has a line or a band structure. It is suitable to use a mobile phone display as another source of light. If we shade a part of the display in order to observe a point or a line source, it is clearly visible that the light of the display, which we perceive as white is formed only by red, green and blue light in reality.

As a matter of interest, it is possible to try observing the spectrum of light emitted by a laser pointer. This light is monochromatic so no light refraction obviously occurs. Caution! Do not use any other stronger lasers but the ordinary pointers. If it by accident strikes an eye of the observer, it will blind him/her temporarily, but the eye will not be damaged. With lasers that are more powerful there is a risk of a permanent damage to the eye if the laser ray struck the unprotected eye.

Another spectrum, which can be well observed without any tools, is a rainbow. It is possible to observe the classical rainbow during the rain, the rainbow in a water spray above a waterfall or above a fountain, or when sprinkling water with a garden hose. It is likely that not always it will be possible to observe a real rainbow so a good-quality photo of this phenomenon can be examined. Pupils should again determine which colours appear in the spectrum and their order.

It is also important for pupils to realize that the order of colours in the spectrum is always the same no matter whether it is a continuous, line or band spectrum and the refraction is made by a reflection on the CD (alternatively refraction on a prism) or when passing through raindrops when the rainbow is formed.

Pupils can work individually or in small groups. It is advisable to have them discuss the findings and deduced conclusions. Some of the observation especially that carried out on an ordinary CD in combination with a planar light source does not have to be completely provable. It is always only a qualitative observation, at this level nothing can be quantified.


The observed source of light must have small angle dimensions otherwise the images of individual parts of the source, when observed, cover one another making the image unclear, potential bands or lines merge into one another and each spectrum appears as seemingly continuous. However, it is satisfactory if the small angle dimension is perpendicular to the observed band spectrum. (If the CD is horizontal and the spectrum shows up on the part of the CD away from the centre towards the observer, the violet colour will be the closest to the centre and the other colours in the direction towards the edge, closer to the observer, up to the red colour, the source of the light can appear as horizontal line.)

When observing, we place the ordinary CD in a way that we can see the recording layer roughly at an angle of 30–40° in order to see the reflection of the source in the more distant half of the disc. After tilting it slightly towards us, we can see the spectrum of the observed source in the closer half of the disc. For basic observation of angularly small enough sources, using a whole, unaltered CD is satisfactory.

When observing an intensive light source (the Sun, laser) it is possible to work in a common, lightened room. When observing light bulbs or fluorescent tubes it is suitable to darken the room at least partially. When observing a mobile phone display, it is suitable to darken the room adequately (however, it does not have to be perfect).

For a better quality observation, it is feasible to make simple spectroscopes with an inserted shard of a CD. Out of a black sheet of paper, cut out the spectroscope body according to a template, glue the CD shard on and glue the box together. Such spectroscope has three big advantages when compared to an unaltered CD. The inner blackening of the box shades surrounding light so the image is more contrasting. An aperture borders the source light so that we do not need to limit ourselves to observing angularly small sources of light. The construction of the box ensures the right angle of view without the need of complicated turning of the disc level in order to find the right reflection. More than 10 spectroscopes can be made out of one CD. It is advisable to separate the pieces of CD with a hot knife. A stronger pair of scissors can also be used but there is a risk of breaking the disc or peeling off the recording layer.

A DVD can be used as well if needed. However, the DVD has a violet colouring of the recording layer, which can distort the observation. In addition, it has a different density of the recording grooves so the image of the spectrum is created in a more complicated way and it is not so clear.

When using a light source from a mobile phone it is necessary to light the whole display to 100%. If it is a phone with an operational system, it is best to install an application that enables it. In the offer of Google Play or App Store there are various “Torchlights”, for Android “The brightest flashlight for free” proved to work well. It can light up both the LED of a camera and the display up to 100%.

Notes for SEN pupils:

  • Pupils with learning difficulties – it is satisfactory if they only observe the continuous spectrum of the Sun and the light bulb. The conclusion should be naming of the colours in the spectrum and saying that the spectrums of the Sun and the light bulb look the same.
  • Gifted pupils – can try to find fine differences between the Sun spectrum and the LED light bulb spectrum. Both spectrums are continuous but the LED light bulb spectrum is clearer due to the higher intensity of light in the blue part (especially with cool white LED) and lower intensity in the violet part (the sharp end after the blue colour). Also they can explain the existence of the secondary rainbow. (The secondary rainbow is formed by one more reflection than the primary rainbow so the colours are arranged in the reverse order, just as the mirror image.)