LENSES Write down anything like this!

 Different types of lenses play an important part in our lives. They are used in cameras, telescopes, microscopes, and projectors. The also enable millions of people to read comfortably and to see clearly. LENSES

 A lens is a transparent object with at least one curved side that causes light to refract.  The curvature of the side can be described in 1 of three ways:  plane – flat side  concave – curves inward, away from the observer  Convex – curves outward, towards the observer

 Lenses have two sides, and either side can be plane, concave, or convex (i.e. can have two different types of sides). There are many types of lenses, but the ones that we will become familiar with are diverging and converging lenses.

 Diverging lenses cause parallel rays of light to spread away from a common point (i.e. diverge). TYPES OF LENSES Draw this picture

 Converging lenses causes parallel light rays to come together (i.e. converge) toward a common point. TYPE OF LENSES, CONT’D Draw this picture

 Principal axis is a straight line that passes through the centre of the lens, normal to both surfaces of the lens  When rays that are parallel to the principal axis pass through a converging lens, the rays intersect at a point—the focal point (F)  If rays that are parallel to the principal axis pass through a diverging lens, the rays will diverge. If the rays are traced backward, you will find the focal point (F) ***Because light can pass through both sides of a lens, lenses have two focal points!! SOME DEFINITIONS

 You can determine characteristics about how an image will be formed by drawing ray diagrams  Steps for converging lens: 1.Draw the object so it’s bottom is on the principal axis & is placed in the desired position relative to F 2.Draw an incident ray through the top of the object (parallel to the principal axis). In the centre of the lens, the ray refracts through F 3.Draw an incident ray from the top of the object through F (same side of lens). In the centre of lens, the ray refracts parallel to principal axis 4.Where the refracted rays cross is the top of the object (the bottom is always on the principal axis) – draw your image! DRAWING RAY DIAGRAMS FOR LENSES

 Draw the image for an object between F & 2F. (draw steps on your paper) EXAMPLE PA F F 2F

 Once you have an image drawn, you have to describe it relative to the object! You can do this using LOST:  L – location (in front of lens or behind the lens)  O – orientation (upright or inverted)  S – size (larger, smaller, or same size)  T – type (virtual or real)  Virtual images – cannot be projected onto a screen (in front of lens)  Real images – can be projected onto a screen (behind the lens) DESCRIBING THE IMAGE

 Now, use LOST to describe the image we drew! DESCRIBING THE IMAGE, CONT’D PAPA F F 2F L – behind lens O – inverted S – larger T – real

 Now draw & describe images for objects in these places:  Object is beyond 2F  Object is between F and the lens PRACTICE