We can explain how convex and concave lenses work.

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Presentation transcript:

We can explain how convex and concave lenses work. We know when the first lenses were made and what the simplest lens in the world is.

Recap Answer these questions on your white board: What colour do you get: - if you shine red and green lights at a white surface? - if you shine blue and red lights at a white surface? - if you mix cyan and magenta inks? How do you make these colours from a printer? red? green? What colour will they look, - if you look at blue trousers in red light? if you look at blue trousers in magenta light? if you look at yellow trousers in green light?

Quick quiz 1. Name three things that we use lenses for. Microscopes, glasses, telescopes, magnifying glasses 2. How old is the oldest lens that we have? About 2700 years old 3. How old is the oldest picture we have which shows a lens? About 2800 years old (ancient Egypt) 4. What is the simplest magnifying glass there is? A drop of water. 5. When did people first wear glasses? In the 13th century. 6. What property of light do lenses use? Refraction. 7. What kind of lenses do you know? Convex, concave.

Make notes We use lenses in microscopes, telescopes, glasses and magnifying glasses. The oldest lenses are over 2800 years old. They were made our of a natural crystal. People have worn glasses since the 13th Century. Lenses use the phenomenon of refraction. They refract the light to give an image. There are two main forms of lens:

What‘s this? It‘s the Nimrud lens. When was it made? About 750 BC. What did they use it for? Maybe to start fires.

Refraction What do you remember? http://www.neok12.com/php/watch.php?v=zX0a6a7b01014b4e727a7f7b&t=Light-Optics

Do the questions So what happens when light hits and goes through these lenses? Draw it on the board.

Now do the experiment Set up the ray box so that you can see three or five rays of light. Make sure that the middle ray goes straight through the middle of the lens. It should pass straight through. Draw on your diagrams what happens to the light when it goes through the different lenses. Draw round the lenses and put dots on the rays of light before and after the lenses. Do the rays of light meet anywhere?

Convex FOCUS F PARALLEL rays from distant object focal length axis F PARALLEL rays from distant object focal length Convex lenses bring the rays together (‘converge’) at a focus. Convex lenses are CONVERGING LENSES

Concave PARALLEL rays from distant object F F FOCUS (‘Virtual’) focal length Concave lenses spread the rays out (‘diverge’). The rays seem to come from a ‘virtual focus’ on the other side. Concave lenses are DIVERGING LENSES Jump back to ray diagrams Next

Recap. What is a convex lens? Where do parallel rays meet? Can you think of any uses of convex lenses? What is a concave lens? Can you think of any uses of concave lenses?