CHAPTER 14 REFRACTION Ms. Hanan Chapter 14 Overview 14-1 Refraction Investigate which direction light will bend when it enters another medium and uses Snell’s law to solve problems. 14-2 Thin lenses Solves problems involving image formation by converging and diverging lenses using ray diagrams and the thin-lens equation, explores eye disorders and eyeglasses, and examines the positioning of lenses in microscopes and refracting telescopes. 14-3 Optical phenomena Calculates critical angle; predicts when total internal reflection will occur; explains atmospheric phenomena, including mirages and rainbows; and briefly describes lens aberration. 14-3 Optical phenomena Objectives: • Predict whether light will be refracted or undergo total internal reflection. • Recognize atmospheric conditions that cause refraction. • Explain dispersion and phenomena such as rainbows in terms of the relationship between the index of refraction and the wavelength. Optical phenomena • Total Internal Reflection. • Atmospheric Refraction. • Dispersion. • Lens Aberration. Total Internal Reflection The complete reflection of light at the boundary between two transparent mediums when the angle of incident exceeds the critical angle. So the 60* angle has total internal reflection http://phet.colorado.edu/en/simulation/bending-light At the Critical Angle The refracted ray skims along the boundary (in more technical language) The ray is refracted parallel to the edge of the block Total Internal Reflection only happens when……… • The ray is travelling through a medium of higher refractive index and meets a boundary with a medium of lower refractive index (ie the light is slow moving and meets a medium where it would be fast moving) Critical angle and refractive index critical angles of different materials Medium Glass Refractive index Critical angle 1.50–1.70 30–42 Water 1.33 49 Perspex 1.5 42 Diamond 2.42 24 Applications of Total Internal Reflection Fiber Optics…light signal is internally reflected. OPTICAL FIBRES Fiber optics are used in communications. In many places, thinner optical fibers have replaced thicker copper cables to carry telephone/internet communication… more efficient… The 2009 Nobel Prize in Physics was awarded for the development of fiber optic cable… Fiber optic cables are used in medicine… -exploratory surgeries: bronchoscopes and colonoscopes consist of two cables; one provides light and the other sends back an image for viewing. Arthroscopic knee surgery Arthroscopic knee surgery Bronchoscope - Colonoscope Examples of total internal reflection Mirage In a desert. On a dry road. Mirage • On a hot day, you may see a pool of water at a distance in a desert. Hurray! Sorry! What you see is just a MIRAGE. • It happens due to total internal reflections. Image of the sky layers of air near the ground – hot – less dense than upper air – lower refractive indices • light refracted more and more towards the horizontal • When light meets a layer of air near the ground at an angle greater than C, Image of the sky Total internal reflections occurs. Example 1 A ray of light travelling in the direction EO in air enters a rectangular block. E angle of incidence = 30 30 angle of refraction = 18 O 18 (a) Find the refractive index n of the block. (b) Find the critical angle C of the ray for the block. Solution 1 (a) By Snell’s law, n = (b) C = sin1 1 n n sin 18 = 1 sin 30 sin 30 sin 18 = sin1 1 1.62 = 1.62 = 38.1 Example 2 (c) If the ray is incident on surface BC of the same block in example 1, from which surface and at what angle will the ray leave the block? D C 30 A B Solution 2 (c) The ray comes out from surface AD. The outgoing angle from normal is 60. C D 30 60 32.3 57.7 32.3 A B Question……. Which of the following angles is the critical angle of glass? A B C D Prisms totally internally reflect light in optical instruments such as binoculars Diamonds “sparkle” because the index of refraction is so large (n = 2.42), so the critical angle is relatively small _______. So light inside a diamond is more likely to be totally internally reflected than to escape…. numerous reflections before light exits. Dispersion • Prism is a transparent block with a triangular cross section. • Dispersion is an effect in which white light separates into its component colors. DISPERSION OF WHITE LIGHT THROUGH A PRISM The phenomenon of splitting a ray of white light into its constituent colours (wavelengths) is called dispersion and the band of colours from violet to red is called spectrum (VIBGYOR). A N White light B C Screen So, the colours are refracted at different angles and hence get separated. are a product of refraction and total internal reflection… Indexes of refraction actually vary slightly with different wavelengths/frequencies of light, so light of different frequencies refracts angles at different _______. Since “white” light is a mixture of all colors, the colors are spread out when refracted through a medium such as glass DEMO are typically seen as a storm is leaving, if you look in the direction of the departing rain with the sun at your back. Why? Refraction of the white light occurs when the light enters a raindrop. The light then undergoes total internal reflection at the back of the drop, before refracting back out into the air. Each drop disperses a full spectrum of light, but a person only sees one color from an individual drop. When we see a rainbow, we are seeing the refraction of light from many different drops at different elevations. • Sunlight is dispersed and internally reflected by water droplets to form a rainbow. Lens Aberration Departures of real images from the ideal predicted by simple theory are called aberrations. Types of aberrations To reduce spherical aberration, parabolic mirrors are used instead of spherical. To reduce chromatic aberration, combinations of converging and diverging lenses are used.