Interactive Primary Newsletter 34 Nothin' but blue sky....

Sunset in a glass For this demonstration, you’ll need a large clear and colourless glass filled with water, a torch, a piece of white card or paper and a little semi-skimmed or skimmed milk. In the darkest corner of the classroom, place the glass of water on a table and put the piece of card approximately 10 cm behind it. Shine the torch beam through the glass so that it hits the card (Fig. 7). This demonstrates white light travelling through a substance with no scattering (or more accurately very little scattering). Slowly stir in a little semi-skimmed or skimmed milk. The light projected onto the card should slowly turn pink/red (Fig. 8). This happens because the fraction of light that is blue is preferentially scattered out of the beam by the particles in the milk. Hence the beam becomes depleted in blue light causing it to appear pink or red. Figure 7 - Torch beam shone through glass of water If the students are then asked to look at the glass containing the milk solution they should notice a slight bluish tinge to it (Fig. 9). This occurs because the observers are seeing the scattered blue light.                                        Figure 8 - Some semi-skimmed milk added                                 Fig.9 - Scattered light to side has bluish tinge If you add too much milk all the light gets scattered and the beam does not reach the screen. Make sure you try this demonstration before showing it to your class so as to find out how much milk to add. Milk is used in this demonstration because, like the atmosphere, it has tiny particles suspended in it. Just as light is scattered when it hits particles in the milk so also is it scattered on hitting air particles in the atmosphere. Why is the sea blue?