Lenses A quick recap of some of the major points from last lesson Principal focus Incident rays (parallel) C F f (focal length)When parallel rays of light fall on a converging lens1) They all meet at the Principal focus2) The ray passing through the centre of the lens (C) does not change directionThese two simple ideas allow us to predict where an image will be formed
Lenses Ray Diagrams for convex lenses Rule 1: Parallel rays of light are refracted through the principal focus F Central axis of lens 2F F F 2FIn Ray Diagrams and only in Ray Diagrams we don’tdraw the rays bending on entering and leaving then lens.We draw on change of direction when the ray reachesthe central axis of the lens.
Lenses Ray Diagrams for convex lenses Rule 2: Rays of light passing through the centre of the lens travel straight on Central axis The centre of the lens 2F F F 2FIt is best to draw ray diagrams on graph paper.Ray diagrams can be drawn to scale with 1cm on the graphpaper representing a much larger distance.
Lenses Ray Diagrams for convex lensesLets get drawingYou will need1. A 30 cm ruler2. A sharp pencil3. A4 graph paper Draw a line along the centre of the graph paper A4 Graph paper
Lenses You are going to draw a ray diagram to show the position and size of the image of an object 4cm tall that is placed 30 cm from a lens which has a focal length of 10cm. First you need to write down all the relevant information Click on reveal when you have done so.Reveal
Lenses You are going to draw a ray diagram to show the position and size of the image of an object 10cm tall that is placed 30 cm from a lens which has a focal length of 10cm. First you need to write down all the relevant information Click on reveal when you have done so.Focal length of lens = 10cmObject height = 10cmObject to lens distance = 30cmThis is the information we need to draw the ray diagram.Our scale will let 2 cm on the graph paper represent 5cm.
LensesDraw in the central axis of the lensAnd mark on either side of the lens the postion of F and2F Central axis Graph paper 2F F F 2F 4cm 4cm 4cm 4cm on graph paper represents 10 cm
Lenses Now draw in our object to scale. 30cm from the lens is represented by 12cm on our scale 10cm height is represented by 4cm on the graph paper Object Central axis Graph paper4cm 2F F F 2F 12cm on graph paper represents 30 cm
LensesNow follow rule 1.Draw in the path of a parallel ray from the top of theobject Object Central axis Graph paper 2F F F 2F The parallel ray must pass through F on leaving the lens
Lenses Now draw in a ray from the top of the object that passes through the centre of the lens Remember this doesn’t change direction. Graph paperObject Central axis 2F F F 2F
LensesThe image will form where the two rays meet.The size of the object and its distance from the lens canbe measured on the graph paper.Object Central axis Graph paper The image 2F F F 2F The image is inverted (upside down) and diminshed
Lenses On the graph paper measure from the centre of the lens to the image. Remembering the scale we used predict what the real distance would be. Write it down Now measure the height of the object and calculate the magnification Magnification = height of image height of objectWrite this down.Your teacher will have set up this experiment for you.Measure the actual lens to image distance and the magnification.How close were your predictions?
LensesIf you have time you can draw some more diagrams.Try them with the object 20cm, 15cm and 5cm away fromthe lens.Now test your predictions by setting out the experiment asdemonstrated by your teacher.Remember all distances are measured to or from the lens. Some real examples of ray tracing
Lenses Object further than 2F away from lens Image is real, inverted and diminishedUses: in a camera in your eye – yes the image on your retina is upside down but your brain corrects for this!!
Lenses Object placed at exactly 2F Image is real, inverted but the same size as the object
LensesObject placed between F and 2F from the lens Image is real, inverted and larger than the object (magnified) Uses: projectors in cinemas and in the classroom
Lenses The object is placed between the lens and F.The image is virtual (can not be put on a screen, forms on the sameside of the lens as the object), it is upright and magnified. Uses: Magnifying glass