GR11 Support pack for Waves sound and light

Study & Master Support Pack | Grade 11 CAPS Physical Sciences Waves, sound and light This support pack for the Waves, sound and light strand of the Physical Sciences Grade 11 CAPS curriculum provides revision summaries on the topic to help prepare for the examinations. Learners can work through these individually at home or these could form the basis of a catch-up class or online lesson. You have permission to print or photocopy this document or distribute it electronically via email or WhatsApp. Cambridge University Press Africa is a proudly South African publisher. For more information on our Study & Master CAPS-approved textbooks and valuable resource materials, visit www.cambridge.org Brighter thinking I Better learning www.cambridge.org

Unit 1: Geometrical optics Summary • Light is an electromagnetic wave that travels at constant speed of 3 × 108 m·s–1 in a vacuum. • We use ray diagrams to show the way that light rays travel. • Light rays travel in straight lines and cast shadows behind opaque objects. • An optical medium allows light to pass through. The more optically dense a material is, the slower the light wave will move through the medium and the lower the transmittance of light. • Reflection occurs when light rays bounce off the surface of an object and move back in the direction from which they came. • Rules of reflection: The angle of incidence is equal to the angle of reflection and the incident ray, normal and reflected ray all lie on the same plane. • The ratio of the speed of light in a vacuum to the speed in a given material c (n = v) is called the refractive index. • Refraction is the bending of light when it travels from one material into another in which it has a different velocity. (The frequency of the light remains constant.) • The normal is the line perpendicular to a surface. • The angle of incidence is the angle between incident ray and normal. • The angle of refraction is the angle between refracted ray and the normal. • Snell’s Law refers to the relationship between the angles of incidence and refraction when light passes through a boundary between two different optical media (n1 sin θ1 = n2 sin θ2) • The critical angle is that angle of incidence that provides an angle of refraction of 90° and the refracted ray is parallel to the boundary between the two media n (sin θc = n2 sin 90°) 1 • Total internal reflection occurs when light moves from an optically denser medium to an optically less dense medium and the angle of incidence is greater than the critical angle. WAV E S, S O U N D A N D L I G H T 23

Revision exercises: Unit 1 – Geometrical optics 1. A narrow beam of yellow light of wavelength 4. A small metal sphere is placed at the bottom 589 nm in air is incident on a smooth surface of a container of water. The critical angle for of water (refractive index of 1,33) at an angle water and air is 48,8°. Three of the light rays of incidence of 30°. emerging from the sphere are shown in the diagram below. a) What is the frequency of the light? b) Determine the speed of the light in the water. A c) What is the wavelength of the light in B C the water? d) At what angle will the light be reflected at the air-water surface? air e) Determine the angle at which the light will 1 2 3 be refracted in the water. 50˚ 2. The diagram below shows the path of a light water 20˚ ray through a rectangular block made of flint glass. N sphere air By drawing the refracted ray for each one of flint glass the incident rays, show on the diagram the 30˚ positions of the image of the sphere when viewed at the different angles of positions A, B and C. Note that an image may not always be formed. θ air 56˚ a) What is the value of the angle θ shown in the diagram? b) Determine the refractive index of the flint glass. 3. A beam of light is transmitted along a fibreglass cable (refractive index 1,50), which is submerged under water (refractive index 1,33). Determine the critical angle for which no light will emerge through the sides of the cable. 24 S T U DY & M A S T E R P H Y S I C A L S C I E N C E S S U P P O R T PA C K G R A D E 1 1

Unit 1: Answers c 1. a) f 5 λ Ray 3: Since the angle of incidence for ray 3 air 3 3 10 m·s 8 −1 is greater than the critical angle, total internal 5 589 3 10−9 m reflection will occur, and no image will be 5 5,1 3 1014 Hz observed when viewed from position C. c b) v 5 n A 3 3 108 m·s−1 B C 5 1,33 5 2,26 3 108 m·s−1 v c) λwater 5 f 2,26 3 108 m·s−1 5 5,1 3 1014 Hz 5 4,43 3 10−7 m air 1 2 3 d) Since the angle of reflection is equal to the angle of incidence, θr 5 30°. 50˚ water 2’ nair sin θair 20˚ e) sin θwater 5 nwater sin 30° sphere 5 1 3 1,33 5 0,38 The refracted angle 5 θwater 5 22,33° 2. a) Since the two normals are parallel to each other, θ 5 30° (alternate angles). sin 56° b) nglass 5 1 3 sin 30° 5 1,66 3. nglass sin θc 5 nwater sin 90° 5 1,33 1,33 1,33 sin θc 5 n 5 1,50 5 0,887 glass θc 5 62,46° 4. The refracted rays are shown in the diagram at the top right-hand side of the page. Ray 1: No refraction occurs for ray 1, since the incident ray is perpendicular to the refracting surface (since the angle of incidence is zero, the angle of refraction is also zero). The image of the sphere will be at the same position as the sphere itself. Ray 2: For ray 2, if the refracted ray is extended backwards it will cut ray 1 at position 2′, which is the position of the image when viewed from this direction. WAV E S, S O U N D A N D L I G H T 25

Unit 2: 2D and 3D wavefronts Summary • A wavefront is an imaginary line that joins all adjacent points that are in phase. • Huygens’ Principle states that every point on a wavefront acts as the source of secondary wavelets that spread out in a forward direction with the same speed as the wave. The new wavefront is the surface tangent to the secondary wavelets. • Diffraction is the process by which a wave spreads out after passing through a narrow aperture or around an obstruction. • Degree of diffraction α λ , where λ is the wavelength and w is the aperture width. w • When coherent light waves pass through a narrow slit, interference takes place. Constructive interference gives rise to a bright band of light and destructive interference produces a dark band. 26 S T U DY & M A S T E R P H Y S I C A L S C I E N C E S S U P P O R T PA C K G R A D E 1 1

Revision exercises: Unit 2 – 2D and 3D wavefronts 1. Give one term for the following descriptions. a) The spreading out of waves when they pass through an opening b) The quantity associated with a wave that needs to be increased in order to increase the amount of diffraction, without changing the width of the slit c) The line joining points that are in phase when a wave propagates in a medium d) The principle that says that every point in a wavefront is the source of a secondary wavelet that spreads out in all directions 2. Monochromatic green light is shone through a single slit (A), and the image is cast on a screen (B) in a darkened room. A B a) Define the term ‘monochromatic’. b) State and explain the phenomenon demonstrated by this set up. c) Describe the pattern observed on the screen. d) The green light is replaced by monochromatic red light. State two ways in which the pattern observed on the screen will be different. WAV E S, S O U N D A N D L I G H T 27

Unit 2: Answers 1. a) Diffraction b) Wavelength c) Wavefront d) Huygens’ Principle 2. a) Light of a single wavelength. b) Diffraction; when light passes through a single opening, it spreads out or travels around corners. This is the result of wavelets in a wavefront interfering with each other, so that dark and green bands appear on the screen. c) A broad central band of bright green light with alternating dark and green bands that become less intense as the light spreads away from the centre. d) There would be a broad central red band and this central band will be wider than the green band. 28 S T U DY & M A S T E R P H Y S I C A L S C I E N C E S S U P P O R T PA C K G R A D E 1 1