Science Talkies - Limits of Light

This documentary was screened as a part of the “Science Talkies” initiative in our Education Resource Centres. The following article is a briefing of the content of the documentary.

Light is a fascinating physical phenomenon, which  kindled the interest of our ancestors and early scientists. The phenomenon of colour is attractive for children as well as adults. What causes these colours? Who was the first to understand the true nature of light and the colours we see around us? The answers to these questions are found in this documentary, which is about the blend of colours and the electromagnetic spectrum. How the different “unseen” radiations were discovered and what are its applications in our lives? The documentary also takes us through the social and cultural aspects which existed in the era, when colour pigments were discovered and used in paintings, industries etc., and questions such as: How were colours extracted?  What was their impact on the society? How did the social culture reserve the use of a certain coloured clothing for certain sects of people etc?

Interesting excerpts from the documentary –

Colour: Pigments or more?

The colour we see in different fabrics is the effect of dyes. Dyes impart colour because they contain molecules known as chromophores. Chromophores absorb incoming light and emit the resultant light (after absorption). However, the pigments or the chromophores are not solely responsible for producing the colour. The size of these particles, if they are in the range of a wavelength of a particular light, scatters the light, hence we see more of that particular colour. This is also called ‘Rayleigh Scattering’.

Will the shine last forever?

Have you ever wondered why some clothes have laundry instructions like “Dry in shade”? Why do some coloured clothes fade when exposed to the sun? The reason is that sunlight reacts with the pigment molecules and brings in structural changes to the molecules and they lose their property of radiating the colour. Vincent van Gogh’s famous painting ‘Sunflowers’ looks much different now than how it used to look previously. The paint he used was chrome yellow (Lead Chromate). This chemical oxidises and loses its colour over time. Nowadays, other alternatives have come up like cadmium yellow that do not lose their ability to radiate colour. It is a combination of cadmium sulphate and other chemicals.

Bright and Dark, how does it matter?

How do we see colours in the dark? Have you observed that under bright light, red looks brighter when compared to blue? In addition, when the intensity of light is reduced, blue looks brighter when compared to red. This is because of the photoreceptors present in the human retina – the rods and cones. The rods are responsible for vision at low light levels. They do not mediate colour vision. The cones are active at higher light levels and are capable of colour vision.

Discovering Electromagnetic Waves

Why is it important to ask questions? Most of the electromagnetic waves like X-rays and UV rays were accidental discoveries. In the year 1895, Wilhelm Roentgen was working on electric discharge in a vacuum tube. In one particular experiment, he wanted to block the light coming out of the discharge tube, hence he covered it with  cardboard to block all visible light. He then observed a photosensitive screen kept near the apparatus, shining. He wondered the reason for this illumination and later went on to discover the X-rays.

Observation, analytical thinking and perseverance led scientists towards such marvellous discoveries. These are the skills and values that we have to develop in our children.