LENSES

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

ActualDraw asActualDraw as In 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

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

F FOCUS (‘Virtual’) PARALLEL rays from distant object focal length Concave 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 F Jump back to ray diagrams Next

Ray diagrams Light 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 axis one to the centre of the lens

FF2F focal length 2 x focal length OBJECT (simplified) RAY DIAGRAMS:RULES 1. A ray parallel to the axis is refracted through the focus 2. 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

FF2F RAY DIAGRAMS:IMAGES If image LARGER than object: MAGNIFIED If image SMALLER than object: DIMINSHED MAGNIFICATION = IMAGE HEIGHT OBJECT HEIGHT If image SAME WAY UP as object: UPRIGHT If image UPSIDE DOWN: INVERTED If rays pass through object: REAL If rays only seem to come from object (see diverging lens ): VIRTUAL This is: MAGNIFIED INVERTED REAL Mag = 1.25

Image DIMISHED INVERTED REAL Image MAGNIFIED UPRIGHT VIRTUAL Object Image IMAGE TYPES REAL images can be PROJECTED ON A SCREEN VIRTUAL images cannot be projected

1. Draw the following on graph paper 2. Draw an object outside 2F at the position shown and at the size shown FF2F 2cm large square 3. Apply the ray diagram rules and draw in the image 4. Classify the image by filling in the table below. Repeat for other positions Object pos.Image pos.mag/dimupright/Invertedreal/virtual outside 2F at 2F between F & 2F at F inside F Uses

FF2F 1. OBJECT OUTSIDE 2F IMAGE: REAL, INVERTED, DIMINSHED IMAGE POSITION: between F and 2F uses next diagram Arrow key to animate slide

FF2F 2. OBJECT AT 2F IMAGE: REAL, INVERTED, SAME SIZE IMAGE POSITION: at 2F uses next diagram Arrow key to animate slide

FF2F 3. OBJECT BETWEEN F AND 2F IMAGE: REAL, INVERTED, MAGNIFIED IMAGE POSITION: outside 2F uses next diagram Arrow key to animate slide

FF2F 4. OBJECT AT F IMAGE: NO IMAGE FORMED (rays don’t meet) IMAGE POSITION: none (or at infinity) uses next diagram Arrow key to animate slide

FF2F 5. OBJECT INSIDE F IMAGE: VIRTUAL*, UPRIGHT, MAGNIFIED IMAGE POSITION: inside 2F & SAME SIDE AS OBJECT uses end show *VIRTUAL Image: Light does NOT actually pass through it – cannot be projected onto a screen Arrow key to animate slide

FF2F 6. CONCAVE (DIVERGING) LENS IMAGE: VIRTUAL, UPRIGHT, DIMINSHED IMAGE POSITION: INSIDE F SAME SIDE AS OBJECT uses end show Arrow key to animate slide Same type of image for all object positions STOP

1. OBJECT OUTSIDE 2F IMAGE: REAL, INVERTED, DIMINSHED IMAGE POSITION: between F and 2F Cameras, eyes Back to diagram

2. OBJECT AT 2F IMAGE: REAL, INVERTED, SAME SIZE IMAGE POSITION: at 2F Back to diagram Copying camera

3. OBJECT BETWEEN F AND 2F IMAGE: REAL, INVERTED, MAGNIFIED IMAGE POSITION: outside 2F Projectors Back to diagram

4. OBJECT AT F IMAGE: NO IMAGE FORMED (rays don’t meet) IMAGE POSITION: none (or at infinity) Searchlights Back to diagram

5. OBJECT INSIDE F IMAGE: VIRTUAL, UPRIGHT, MAGNIFIED IMAGE POSITION: inside 2F & SAME SIDE AS OBJECT Back to diagram Magnifying glass

6. CONCAVE (DIVERGING) LENS Used in spectacles: Short sight ‘Myopia’Long sight ‘Hyperopia’ Eye lens too strong: spectacle lens diverges light to bring back into focus Eye lens too weak: spectacle lens converges light to bring back into focus