12.1 – Characteristics of Lenses

1. 12.1 – Characteristics of Lenses

2. Describing Lenses • A lens is a transparent object with at least one curved side that causes light to refract • Like mirrors, lenses have surfaces that are described as concave and convex, and you can predict characteristics of images formed by them using ray diagrams and equations

3. Describing Lenses • Lenses have two sides, and either side can be plane, concave, or convex • There are two general classes of lenses: • Converging lenses cause parallel light rays to come together • Diverging lenses cause parallel light rays to spread apart

4. Describing Lenses • Converging lenses have one or two convex surfaces and are thicker in the centre than on the edges • Diverging lenses have one or two concave surfaces and are thinner in the centre than on the edges

5. What About Windows? • When parallel light rays pass through a flat piece of glass like a window, the rays shift to the side but do not change their direction relative to each other • The rays first bend towards the normal, but when they exit on the other side, they bend back away from the normal the same amount that they first bent inwards • Therefore the image is not distorted, and a flat piece of glass is not considered to be a lens

6. Converging Lenses • Converging lenses bring parallel light rays toward a common point • This is most easily seen in a lens that is convex on both sides, or biconvex

7. Converging Lenses • When rays are incident on the surface on the left side of the lens, they move from a fast medium to a slow medium • Thus, the rays refract towards the normal, and converge slightly • When the light rays leave the second surface of the lens, they move from a slow medium to a fast medium • Thus, they refract away from the normals • Because of the directions of the normals at this surface, the rays continue to converge

8. Diverging Lenses • Diverging lenses cause parallel light rays to spread away from a common point • This is most easily seen in a lens that is concave on both sides, or biconcave

9. Focal Point and Focal Length of Lenses • The principal axis of a lens is a straight line that passes through the centre of the lens, normal to both surfaces of the lens (the same as with mirrors) • When rays that are parallel to the principal axis pass through a converging lens, the rays intersect at a point – the focal point, F, which sits some distance away (the focal length, f) • Because light can pass through from either side, there are actually two focal points for a lens

10. Focal Point and Focal Length of Lenses • After parallel rays pass through a diverging lens, the rays diverge • You must trace the rays backwards to see them converge to a point – this point is called a virtual focus

11. Focal Point and Focal Length of Lenses • The position of the focal point of a lens depends on both the index of refraction of the lens material and the curvature of the lens • Lenses with the same shape but higher indices of refraction bend rays more, making the focal length closer to the lens • Lenses with larger curvatures but with the same index of refraction have the same effect

12. Focal Point and Focal Length of Lenses