Physics Chapter 14 PDF

# Chapter 14: Waves ## 14.1: How Does Light Enable Us to See? ### Learning Outcomes - Recall and use the terms normal, angle of incidence and angle of reflection to describe the reflection of light. - State that, for reflection, the angle of incidence is equal to the angle of reflection and use t...

# Chapter 14: Waves ## 14.1: How Does Light Enable Us to See? ### Learning Outcomes - Recall and use the terms normal, angle of incidence and angle of reflection to describe the reflection of light. - State that, for reflection, the angle of incidence is equal to the angle of reflection and use this principle in constructions, measurements and calculations. ### Reflection - Luminous objects emit light. - Non-luminous objects reflect light from luminous objects. ## 14.2: How Is Light Refracted? ### Learning Outcomes - Recall and use the terms normal, angle of incidence and angle of refraction to describe the refraction of light. - Recall and apply the relationship sin i/sin r = constant to new situations or to solve related problems. - Define refractive index of a medium in terms of the ratio of speed of light in vacuum and in the medium. ### Refraction - Refraction is the bending of light as it passes from one optical medium to another. - The refracted ray is the light ray that enters a medium and undergoes a change of direction. - The angle of refraction is the angle between the refracted ray and the normal at the point of incidence. The incident angle is labelled i while the angle of refraction is labelled r. ### Laws of Refraction - The incident ray, refracted ray and the normal at the point of incidence all lie in the same plane. - The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for two given media, that is, *sin i / sin r* is constant. ### Refractive Index - The refractive index *n* of a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in that medium. - *n = c/v* where c = speed of light in a vacuum - v = speed of light in the medium - The speed of light in a vacuum is constant over all frequencies. However, in different media, the speed of light varies with the frequency of light. Hence, the value of refractive index is different for different media. - For a light ray passing from a vacuum into a given medium, the refractive index *n* of the medium is also given by the ratio, *sin i / sin r* (Figure 14.23). - *n= sin i / sin r* where i = angle of incidence in a vacuum - r = angle of refraction in the medium ### Principle of Reversibility of Rays - The principle of reversibility of light rays states that regardless of how many times a light ray has been reflected or refracted, it will follow the same path when its direction is reversed. ### Effects of Refraction: "Bent" Objects - The pencil appears "bent" due to refraction. ## 14.3: What Is Total Internal Reflection? ### Learning Outcomes - Explain the terms critical angle and total internal reflection. - Apply total internal reflection to the use of optical fibres in telecommunication and medicine, stating the advantages of such use. ### Total Internal Reflection - The angle of incidence in an optically denser medium for which the angle of refraction in the less dense medium is 90°. - The complete reflection of a light ray in an optically denser medium at the boundary with an optically less dense medium. #### Conditions for Total Internal Reflection - The incident ray must travel from an optically denser medium to an optically less dense medium. - The angle of incidence must be greater than the critical angle. ### Applications of Total Internal Reflection #### Glass Prisms in Binoculars - Glass prisms are used to reflect light in binoculars (Figure 14.40(a)). Good-quality binoculars use prisms instead of mirrors as prisms are more durable and have better image quality. - After two total internal reflections due to the prisms, the resulting image is in the same orientation as the object (Figure 14.40(b)). #### Optical Fibres - Optical fibers made from glass or plastic can carry information in the form of coded light pulses (Figure 14.41). The fibers rely on total internal reflection of light to transmit signals. ## 14.4: How Does a Converging Lens Work? ### Learning Outcomes - Describe the action of a thin converging lens on a beam of light. - Define the focal length for a converging lens. - Draw ray diagrams to illustrate the formation of real and virtual images of an object by a thin converging lens. - Describe the characteristics of images formed by a thin converging lens. ### Converging Lenses - A converging lens is thicker in the centre than at the rim. - A device made of glass or other transparent material that is able to concentrate light rays is called a converging lens (Figure 14.51). - A converging lens converges the parallel rays to a point. ### Terms Used to Describe Thin Converging Lenses **Principal axis:** The line which passes through the centre of the lens and which is perpendicular to the plane of the lens. **Optical centre C:** The point on the principal axis that is the midpoint between the surfaces of the lens. **Principal focal point F:** The point on the principal axis where all the rays parallel to the principal axis meet after passing through the lens. **Focal plane:** The plane perpendicular to the principal axis on which all parallel rays meet after passing through the lens (It is a plane of all the possible focal points.). **Focal length f:** The distance between the optical centre C and the principal focus point F. ### Ray Diagrams for Thin Converging Lenses - Ray 1: Passes Through Optical Centre C - An Incident ray will pass through the optical centre without bending. Recall that the emergent ray Is parallel to the Incident ray and that the shift Is small If the lens Is thin. - Ray 2: Parallel to Principal Axis - An Incident ray parallel to the principal axis Is refracted to pass through the principal focal point F. - Ray 3: Passing Through Focal Point F - An Incident ray passing through the principal focal point F before passing through the lens will emerge parallel to the principal axis (principal of reversibility). ### Applications of Converging Lenses #### Visual Correction for Long-sightedness - People who are long-sighted are unable to clearly see objects close to their eyes. Their eyes focus the rays from each object point to a point beyond the retina (Figure 14.61). As the light rays are not focused on the retina, the rays from a single object point form an extended patch instead of a sharply focused point on the retina. When different object points form many overlapping patches on the retina, the overall image becomes unclear. #### Forming Image in a Camera - A converging lens is used to form an inverted, real and diminished image on the sensor of a film camera. The focusing ring is used to adjust the lens to sensor distance (Figure 14.63). ## Summary of Image Characteristics of Converging Lens - The distance of an object from a thin converging lens determines the type of image formed. - Table 14.2 shows the ray diagrams and types of images formed when an object is placed at different distances from the lens. ## Image Characteristics - **Real image:** A real image can be formed on a screen placed at the image plane. - **Virtual image:** A virtual image cannot be formed on a screen placed at its image plane. # Let's Map It **Light** - **Undergoes Reflection:** - **Obeys:** - First law of reflection: The incident ray, reflected ray and the normal, all lie in the same plane. - Second law of reflection: Angle of incidence θi = Angle of reflection θr. - **Leads to:** Mirror image point is always as far behind the mirror as the object point is in front of the mirror. - **Happens:** - **Total internal reflection:** when Incident ray's angle (with respect to normal) in optically denser medium is greater than the critical angle θc, sin θc = 1 /n. - **Undergoes Refraction:** - **Happens:** At boundary between two different media - **Such that:** The light ray bends when passing from one medium to another medium. - **Obeys:** - First law of refraction: The incident ray, refracted ray and the normal, all lie in the same plane. - Second law of refraction: sin i / sin r = constant for given pair of media/materials. - **Used to define:** Material's refractive index *n = sin θi / sin θr* where i is angle in a vacuum, θr is angle in material. Also, *n = speed of light in vacuum / speed of light in material*. - **Governs:** Thin converging lens - **Described by:** - Focal length - Principal axis - Optical centre - Focal point - Focal plane - **Applications:** - Magnifying glass - Spectacle lenses - Camera