Starter Number the sentences in the correct order to explain why we see the pencil as bent in the water. 5 2 3 1 4 6

Learning Goals By the end of this lesson you should be able to… Describe the path of light when it enters a thin converging or a thin diverging lens. Describe the images formed by convex and concave lenses. Draw simple ray diagrams to show the principle axis, focal point, focal length and focus. Explain the effect of increasing the thickness of a lens on its strength Apply the equation for magnification of a lens.

What have these all got in common?

Diverging Converging Causes rays to disperse from a central focus. Brings parallel rays into focus (causes them to converge to a point)

Will each lens be diverging or converging?

What do you notice about the lenses which are diverging / converging?

All diverging lenses are wider at the edges than in the centre. All converging lenses are wider in the centre than at the edges.

Light travelling through a lens is refracted twice; once when it enters the lens and once when it leaves. In the case of a thin lens, it is possible to neglect this double bending.

What effect does a convex lens have on parallel rays of light? Principal Axis Principal Axis F Focal point They will converge to a point which will call the focal point (F). If you put a screen at the focal point you would focus a real image onto the screen.

What effect does a convex lens have on parallel rays of light? Principal Axis Principal Axis F Focal Length The distance from the center of the lens to the focal point is called the focal length of the lens (f).

What effect does a convex lens have on diverging rays of light? If a light is placed at the focal point of a convex lens it will produce parallel light

What effect does a concave lens have on parallel rays of light? Principal Axis F Causes them to disperse, so that they will never converge. Thus when we talk about the focal point for a diverging lens it is a virtual image and must be found by extending the diverging rays backwards.

What effect does a concave lens have on parallel rays of light? Focal Length The distance from the centre of the lens to the focal point is called the focal length of the lens (f).

Real Virtual Which type of image do you need in a projector or camera?

Converging / diverging lens Principal axis Principal focus Continue the 3 rays on each diagram through the lens and label the following: Converging / diverging lens Principal axis Principal focus Focal length

Experiment: Finding the focal length of a lens Experiment: Finding the focal length of a lens. Method Find the focal length (F) of each of the lenses by focussing a distant object ‘at infinity’ onto the white screen. The distance between the midpoint of the lens and the screen is the focal length.

SAFETY Be careful the ray boxes get very hot when on, so do not touch the bulbs.

Experiment: Investigating the image size and position with a convex lens. Method: Using the lens with focal length F=15cm investigate the image formed to complete the table. Use the words, enlarged, same size, diminished, upright or inverted. Object position Object height Image Height Magnification (image height / object height) Description of image and position Further than 2F Diminished, real, inverted Between 2F and F At 2F Between 2F and F At F Closer than F

Object position Type of image Real/Virtual Inverted/ Non-inverted Position of image Further than 2F Smaller / Diminished Real Inverted Between 2F and F At 2F Same sized Larger / Enlarged At F More than 7F Closer than F Larger/ Enlarged Virtual Non-inverted/ Upright On the same side as the object

Lens Simulation http://ophysics.com/l12.html Move position of rhs Principal Focus to lhs to convert to concave lens.

Extension Using p152/3 (Complete Physics) draw a ray diagram to show how a convex lens produces an image if the object is at 2F and as the object moves closer or further away from the lens.

Modelling a lens worksheet

Diverging or Converging Lens