M Manser WAVES : Optics (t riple science) By the end of this presentation you should be able to: Identify the differences between converging and diverging lenses. Draw diagrams showing the effect each lens on a parallel beam of light. Identify the positions of the focus of both a converging and a diverging lens. Know the difference between a real and virtual image. Construct ray diagrams to show the formation of real and virtual images by a converging lens. Explain the use of a converging lens as a magnifying glass, and in a camera.

M Manser Reviewing refraction Remember that when light crosses into a different medium, its speed is affected and the direction of the wave changes as a result. Two exceptions - If the wave is incident on the surface at 90 o, its direction will not change. - If light is travelling from glass into air, and hits the boundaryat an angle greater than the critical angle, it will undergo total internal reflection.

M Manser Refraction by non-rectangular prisms. Can you predict the path that this ray would follow through the glass prism shown? First draw the normal to the prism where the ray is incident on it. Then, considering that the ray will bend towards the normal… Now repeat the process, remembering that the ray will bend the other way as it emerges from the glass into the air.

M Manser Lenses The effect that lenses have on light can be explained by considering them to be constructed of different arrangements of prisms. 1. A converging lens 2. A diverging lens

M Manser How a parallel beam is affected by different lenses principal axis A parallel beam will converge and be focussed at the focal point: F. F A parallel beam will diverge. A virtual focal point, F, can be found by projecting the rays backward. F

M Manser Drawing ray diagrams for a convex lens Remember that light is coming from the object in all directions and from all points on the object, but we only need to consider a few rays to be able to determine the location, orientation, and size of the image. Light rays do change directions at each air/glass interface; but to make the ray diagrams easier to draw, we will only draw the rays changing direction at the imaginary line going vertically through the centre of the lens. Consider these rules: 1. Rays parallel to the principal axis are refracted through the focal point on the other side of the lens. 2. Rays through the focal point are refracted parallel to the principal axis. 3. Rays through the optical centre are undeflected.

M Manser Applying the rules to scale diagrams Choose the largest scale possible, given the data supplied (like object height, distance, focal length) and write it down. Do a rough sketch to determine where to position the lens and principal axis. Draw the principal axis and the lens. Use the scale to mark the focal point on each side of the principal axis. Now use the scale to accurately draw in the position and height of the object. FF

M Manser Using the rules for drawing ray diagrams 1. Rays parallel to the principal axis are refracted through the focal point on the other side of the lens. 2. Rays through the focal point are refracted parallel to the principal axis. 3. Rays through the optical centre are undeflected. FF Only two rays need to be drawn to determine where all rays would focus. Once this point is found, draw in the image from the principal axis to that point.

M Manser Uses of converging (or convex) lenses Cameras …..p. 30 Projectors….p. 31 (not on syllabus) Magnifying glasses…. p. 31 Telescopes …..p. 33 (not on syllabus)

M Manser A simple camera the outer case The lens will focus the rays from the object on the film. the film Rays from the top of the object will be focussed at the bottom of the film (and vice versa) The brightness of the image is controlled by the stop, which can be opened to make the aperture larger and let in more light. The sharpness of the image is controlled by the shutter speed. Taking pictures of moving objects require short exposures.

M Manser A magnifying glass FF Your eye will bring these rays to a focus on the same side as the object. It will be upright and magnified. A magnifying glass uses a convex lens to produce a “virtual image”. A virtual image is one where rays of light do not actually pass through the image (so a virtual image cannot be formed on a screen). This is because the object is very close to the lens (between the focal point and the lens). The rays diverge after being refracted.

M Manser More ray diagrams. A moving visual aidA moving visual aid