# CONVERGING LENSES. Lenses Lenses REFRACT light and are usually used to form IMAGES 2 types convexconcave bi-convex plano-concave plano-convex bi-concave

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• CONVERGING LENSES

• LensesLenses REFRACT light and are usually used to form IMAGES2 typesconvexconcave bi-convex plano-concave plano-convex bi-concave

• ActualDraw asActualDraw asIn practice, light is refracted at both surfaces of the lens but for simplicity we draw only one refraction as if it happened at the centre line

• FFOCUSaxisPARALLEL rays from distant objectfocal lengthConvexConvex lenses bring the rays together (converge) at a focus. Convex lenses are CONVERGING LENSES

• FFOCUS(Virtual)PARALLEL rays from distant objectfocal lengthConcaveConcave lenses spread the rays out (diverge). The rays seem to come from a virtual focus on the other side.Concave lenses are DIVERGING LENSESFJump back to ray diagramsNext

• Ray diagramsLight is reflected off ALL POINTS of a non-luminous object in LOTS OF DIFFERENT DIRECTIONS To work out what sort of image a lens will produce, we select 2 rays only from the top point of the object: one parallel to the axisone to the centre of the lens

• FF2F2Ffocal length2 x focal lengthOBJECT (simplified)RAY DIAGRAMS:RULES1. A ray parallel to the axis is refracted through the focus2. A ray to the centre of the lens passes through undeflected(3. A ray through the focus is refracted parallel to the axis)Image diagram gives position and size

• FF2F2FRAY DIAGRAMS:IMAGESIf image LARGER than object: MAGNIFIED If image SMALLER than object: DIMINSHEDMAGNIFICATION = IMAGE HEIGHT OBJECT HEIGHTIf image SAME WAY UP as object: UPRIGHT If image UPSIDE DOWN: INVERTEDIf rays pass through object: REALIf rays only seem to come from object (see diverging lens ): VIRTUALThis is:MAGNIFIEDINVERTEDREALMag = 1.25

• Image DIMISHED INVERTED REALImage MAGNIFIED UPRIGHT VIRTUALObjectObjectImageImageIMAGE TYPESREAL images can be PROJECTED ON A SCREENVIRTUAL images cannot be projected

• 1. Draw the following on graph paper2. Draw an object outside 2F at the position shown and at the size shown2cm large square3. Apply the ray diagram rules and draw in the image4. Classify the image by filling in the table below. Repeat for other positions

• 1. OBJECT OUTSIDE 2FIMAGE: REAL, INVERTED, DIMINSHEDIMAGE POSITION: between F and 2Fusesnext diagramArrow key to animate slide

• 2. OBJECT AT 2FIMAGE: REAL, INVERTED, SAME SIZEIMAGE POSITION: at 2Fusesnext diagramArrow key to animate slide

• 3. OBJECT BETWEEN F AND 2FIMAGE: REAL, INVERTED, MAGNIFIEDIMAGE POSITION: outside 2Fusesnext diagramArrow key to animate slide

• 4. OBJECT AT FIMAGE: NO IMAGE FORMED (rays dont meet)IMAGE POSITION: none (or at infinity)usesnext diagramArrow key to animate slide

• 5. OBJECT INSIDE FIMAGE: VIRTUAL*, UPRIGHT, MAGNIFIEDIMAGE POSITION: inside 2F & SAME SIDE AS OBJECTusesend show*VIRTUAL Image: Light does NOT actually pass through it cannot be projected onto a screenArrow key to animate slide

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