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Science - Light
We had two great sessions experimenting with light. We didn’t do exactly the same thing with both groups but I’ll put what I can remember here so that everyone can refer back to it.
Ray Optics
We found out how light behaves in certain circumstances. Generally light travels in straight lines and rays of light diffuse, or spread out, as they travel. This is why a torch shone at the ceiling makes a bigger patch of light than if it’s shone at your hand right in front of the torch. We demonstrated how light travels in straight lines by puffing talcum powder into a box and shining the lasers through the cloud of talc. Reflections from the small particles in the cloud showed the path of the beam.
We all know that light can be reflected from a mirror or any other reflective surface, but there is some other terminology which is useful. Opaque objects don’t allow light to pass through them. Translucent objects allow light through, but you can’t see through them. Transparent objects are see-through. There’s a nice diagram of what this means in terms of light rays. When light passes through things we say it is transmitted.
As well as being reflected, light can also be refracted, or bent from its course. In air, light normally travels at around 3 million Km/second. In more dense media it slows down and when light hits the new medium at an angle, we see it change direction. This is called refraction. We experimented by shining lasers at an angle into beakers filled with a layer of oil and a layer of water. We could see the beam of light change direction slightly as it went from air into oil, then from oil into water. We also used a light ray box and placed acrylic prisms in the path of the beam to see if we could see any refraction. We found that the effect was most noticeable when the light shone through the longest side of the prism.
Light and Colour
We looked at how white light is composed of all the colours of the rainbow. In order to see these component colours we need dispersion to happen. We looked through a polarising lens and saw that daylight (didn’t seem right to call it sunlight..) looked different to artificial light from different light bulbs. The older group saw a small rainbow when we managed to get some dispersion using a light and a prism. The prism uses refraction to disperse white light, as different wavelengths of light are refracted slightly differently through it.
The older group looked at the electromagnetic spectrum, which contains visible light as well as X-rays, radio waves, microwaves and sound waves.
For more information have a look at this page on the electromagnetic spectrum from the Institute of Physics.
We used red and green lasers in these experiments. We discussed how lasers are different from other lights in that they produce pure colours (we checked this with the polarising lenses), and the beams do not spread out or diffuse like regular light, but instead are concentrated in one spot, or collimated. The waves are in phase or ‘coherent’
We watched some animations from the NASA information page on lasers.
What does it really mean when we see an object as a certain colour?
To help find out, we used lasers on gummy bears. When we shone a green laser on a green gummy bear, the light shone through it and on to the surrounding surface; it glowed. The same happened when we shone a red laser on a red gummy bear. But when we shone a green laser on a red gummy bear, it just had a spot on its tummy but little light shone through, and the same happened when we shone a red laser on a green gummy bear. We found that light could pass through gummy bears, so what happened to the green light when it shone onto the red gummy bear? It absorbed the light. When a thing looks red, that means it absorbs all other wavelengths of light, and reflects red light. Remember that the lasers are pure red or green light; the red gummy bear will only transmit or reflect red light, and absorbs all other light.
Lenses and parallel rays
We used the light ray box to show how concave and convex lenses alter the path of light. We set the box up with parallel rays of light showing, then placed a convex lens in its path. This caused the rays to bend towards the centre and they converged on a focal point. We talked about how the lenses in our eyes are convex, and remembered dissecting sheeps’ eyes and removing the lenses. Next we used a concave lens and saw how this made the rays spread further apart (diverge). When we placed the two lenses together they cancelled each other out. We talked about how corrective lenses work to help eyesight, with concave lenses used to correct long-sight and convex to correct short-sight.
