2 Refraction of LightWhen light travels through a surface between two different media, the light will be refracted if the angle of incidence is greater than zero.If light is passing into a more dense media, it will bend towards the normal.
3 Law of Refraction (Snell’s Law) The ratio of the sine of the angle of incidence to the angle of refraction is a constant.n1 sin1 = n2 sin2Where:n1, n2 = index of refraction1 = Angle of incidence2 = Angle of refractionspeed of light in a vacuum cspeed of light in the material vn = =
5 Lenses and Their Uses Eyeglasses first made around the 13th century. Galileo used them as a telescope to discover the moons of Jupiter and the phases of Venus.Other applications include microscopes, overhead projectors and cameras.A special type of lens, called the fresnel lens, is used in lighthouses, traffic lights, rear windows of motor homes and overhead projectors.
6 Definition of a Lens What is a lens? A lens is made of a transparent material such as glass or plastic such that the index of refraction is greater than that of air.
7 Types of Thin Lenses What types of lenses are there? Convex (Converging): A lens that is thicker in the middle than at the edges. Converging lenses cause incident parallel rays to converge at a point.Concave (Diverging): A lens that is thinner in the middle than at the edges. Diverging lenses cause parallel rays of light to diverge when leaving the lens.Fresnel: A lens comprised of rings of glass prisms positioned above and below a lamp to bend and concentrate light into a bright beam.
9 Image Formation by Converging Thin Lens 1RealImage3Principle Axis2FFF2F2ObjectAn object placed more than 2X the focal distance before the lens will produce an inverted and smaller real image.This type of lens is similar to those used in cameras.
10 Image Formation by Converging Thin Lens RealImage13Principle Axis2FFF2F2ObjectAn object placed between F and 2F will produce an inverted and larger real image.This type of lens is similar to those used in projectors.
11 Image Formation by Converging Thin Lens 1Principle Axis2FFF2F2VirtualImageObjectAn object placed between F and the lens will produce an upright and larger virtual image.This type of lens is similar to a magnifying lens.
12 Image Formation by Diverging Thin Lens 123FFVirtualImageObjectA diverging lens always produces a virtual image that is upright and smaller than the object.This type of lens is used in glasses to correct for myopia (near sighted).
13 Image Formation for Converging and Diverging Thin Lenses Image formation for diverging lenses.Image formation for converging lenses.
14 The Thin Lens Equations do di fWhere:do and di are the distances of the object and image from the mirror, respectively.f = focal length.Image height, hi diObject height, ho do=m = = -
15 Example 1ImagehiPrinciple Axis2FFF2FhiObjectfdodiAn object is placed at a distance of 6 cm from a converging lens. The focal length of the lens is 2 cm. The distance of the image to the lens is:a. 1.0 cm b. 1.5 cm c. 3.0 cm d. 4.5 cm e. 6.0 cm
16 Example 2 & 3An object is placed between the focal point and twice the focal length of a converging lens. The image formed will be:a. real and upright b. real and invertedc. virtual and upright d. virtual and invertede. located at the focal lengthAn object is placed at a distance of 20 cm from a converging lens. The resulting image appears at a distance of 80 cm from the lens. The image is magnified by a factor of:a b c d e. 16.0
17 Sign Conventions for Thin Lenses Focal Lengthf is positive for a converging lens.f is negative for a diverging lens.Object Distancedo is + if the object is to the left of the lens (real object).do is - if the object is to the right of the lens (virtual object).Image Distancedi is + for an image (real) formed to the right of the lens by a real object to the left.di is – for an image (virtual) formed to the left of the lens by a real object.Magnificationm is + for an image that is upright with respect to the object.m is – for an image that is inverted with respect to the object.
18 Key IdeasSnell’s Law / Law of Refraction: Light will bend toward the normal when transitioning from a media with a low index of refraction (e.g. air) to a media with a higher index of refraction.Paraxial light rays parallel to the principle axis of a converging lens will come to a point called the focus.Paraxial light rays parallel to the principle axis of a diverging lens will appear to have originated from a point called the focus.Diverging lenses always form virtual images.
19 Key IdeasThe thin lens equation can be used to determine the distance an image forms from a lens and is the same as that used for spherical mirrors.Ray diagrams can be used to determine where images will form.