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# Lenses 1

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• 1. Lenses There are two different types of lenses. Convex lens Concave LensCopy the diagram of each lens into your book.You should have an example of each type of lens, lookthrough each lens at objects that are close and far away.Record what you see in your book. How does thischange if the lens is moved away from the eye?
• 2. LensesConvex lenses are also known asconverging lenses Concave lenses are also known as diverging lens To find out why you need to collect some equipment. A ray box, a power supply, a sheet of plain paper and two lenses. Your teacher will show you how to work the power supply. Start with the convex lens.
• 3. LensesPlace the lens in the centre of the paper. Use the raybox to produce 3 parallel rays of light which hit thelens. Make sure one of them hits the centre of thelens. What happens to the raysIncident rays when they pass through the lens? Produce a good diagram of what you see in your books and explain why concave lenses are also called converging lenses.
• 4. Lenses This is what you should have seen. Principal focus Incident rays (parallel) F f (focal length)Did you notice that the light rays bend on entering the lens and leaving thelens?Make sure you know what the princpal focus is and how this allows us tomeasure the focal length. What was the focal length of your lens?
• 5. Lenses Incident rays of light are parallel Principal focus Another convex Lens
• 6. Lenses Concave Lens – know do the same for the convex lens What happens to the rays when they pass throughIncident rays(parallel) the lens? Produce a good diagram of what you see in your books and explain why concave lenses are also called diverging lenses.
• 7. Lenses The light rays are spread out or DIVERGED by this lens. Incident rays (parallel) Parallel rays are diverged F so that they appear to come from the principal focus (F) of the concave lens. Objects always look smaller through a concave lens. f (focal length)Did you notice that the light rays bend on entering the lens and leaving the lens?Make sure you know what the principal focus and focal length are for a concavelens. Can you think of a way of measuring the focal length of your diverging lens? (Hint it involves drawing)l
• 8. Lenses Incident rays are parallel Rays leaving the lens are divergingThe black lines show where the diverging light appearsto have come from the Prinicpal Focus
• 9. Lenses Forming Images ScreenThe rays of light from the car are converged by the lens, to form an image onthe screen.The image is inverted (upside down) and real (the rays of light pass through the lens and an image is formed on the screen) and diminished (smaller than the object).
• 10. Lenses Cameras Film/screenObject In a camera the image is smaller than the object we Image: Real, Inverted and say the image is diminished diminished
• 11. Lenses Insert photograph hereProjectors The image is inverted, real (its on a screen ) and magnified (larger than the object) Magnification = height of image height of object
• 12. LensesQuestion 1. – Copy out and complete the followinga) A convex lens _____ rays of light, but a concave lens _____ rays of light.b) Parallel rays of light are refracted by a convex lens to a point called the _____ ____. The focal length is the distance from the _____ _____ to the _____ of the lens.c) A fat convex lens is a _____ lens, with a _____ focal length.d) A real image can be formed on a _______e) The image in a concave lens is always _______ than the ______. Centre, converges, diverges, focus, object, Answer principal, screen, short, smaller, strong
• 13. LensesQuestion 1. – Copy out and complete the followinga) A convex lens converges rays of light, but a concave lens diverges rays of light.b) Parallel rays of light are refracted by a convex lens to a point called the principal focus. The focal length is the distance from the principal focus to the center of the lens.c) A fat convex lens is a strong lens, with a short focal length.d) A real image can be formed on a screen.e) The image in a concave lens is always smaller than the object. Got something wrong ask your teacher!!
• 14. Lenses Question 2 – copy and complete the ray diagramsA BC D F Answer
• 15. Lenses Question 2 - answers A B C D FDid you remember to show the light bending onentering and leaving the lens? Got something wrong ask your teacher!!
• 16. LensesInsert picture here That’s all for now folks but You need to check through the work from today at home Next lesson: you will be using what you have learnt today to predict the size and position of an image and then checking your prediction.