Chapter 7 - Rays and Waves Light Rays and Waves Objects which emit light is known as luminous. For example, sun, TV and lamps are luminous. Objects which do not emit light rays are nonluminous. White objects reflects most of the light whereas black / dark objects absorb most of the light. Uneven surfaces scatter light and reflects light in all direction. This phenomena is known as diffuse reflection. Smooth surfaces like mirror generates regular reflection. Features of Light Radiation Light transfer energy Travel as waves Features of Light Travel in straight line Speed of light is the fastest Travel through empty spaces Wavelength and Color The wave length range from 0.0004 mm (violet) and 0.0007 mm (red), and wavelength is made up all wavelength in this range. Most object emits a mixture of wavelength. Object that emit only a single wavelength is known monochromatic. For example, laser is monochromatic. as Quiz Reflection in Plane Mirrors The Law of Reflection: 1. The incident angle = The reflected angle 2. The incident ray, reflected ray and the normal all lie on the same plane. Image in Plane Mirror The Properties of Images on Plane Mirror: The image is exactly the same size as the object The image is the same as object The image is laterally inverted (back become front) The image formed is virtual image as no rays pass through the image. The distance of image from the plane mirror = The distance of object from the plane mirror. Real Vs. Virtual Image Real image is formed when rays meet to form the images. Virtual image is not form by the rays. It is formed simply from the reflection of object. Finding Position of Image from Plant Mirror Quiz Reflection in Plane Mirror (2) 1. From object O, draw a ray which strikes the mirror at an angle of incidence of 35º (or any value that you prefer). 2. Construct a normal at the point where the ray strikes the mirror. 3. Draw the reflected ray from this point, so that the angle of reflection is equal to the angle of incidence. 4. Repeat step 1 to 3 for a second ray with an angle of incidence of 55º (or any other value you choose). 5. Extend the two reflected rays backwards until they intersect (meet). The intersection point I, is the image position. Reflection in Plane Mirror (2) Reflection Problems Reflection Problems Quiz Refraction of Light Refraction of light occurs when the light ray travel from a medium to another medium with different density. Light will refracted towards the normal when incident light travel from a less dense medium to a denser medium. Oppositely, the light will be refracted away from the normal when light travel from a denser medium to less dense medium. Light travel more slowly when it travel in denser medium. Example of Refraction of Light Because of the refraction, the pebbles look less deep. Refractive Index The medium with higher refractive index has the greatest bending effect on light because it slow down the light the most. Speed of light in vacuum = 300000 m/s Refraction by Spectrum When a white light pass through the prism, it splits into a range of color called spectrum. This phenomena is known as dispersion of light. It occurs because the white light is not the single color. It is the combination of 7 colors which make up the rainbow color. Quiz Total Internal Reflection • Total internal reflection will occur when incident ray > critical angle • Different material has different critical angle. Application of Total Internal Reflection Quiz Refraction Calculations Examples Refractive Index Calculating Critical Angle C = critical angle n = Refractive index of a material Quiz Convex Lens Lens are thicker in the middle and thinner around the corner. When rays parallel to principal axis pass through convex lens, the ray bend inwards. The rays will converge at point F. Focal length = Distance of focal point to centre of the lens. Concave Lens Lens are thicker around the edge and thin at the centre. Rays bend outwards when rays parallel to the principal axis pass through the lens. Concave lens are diverging lens. Real Image form from Convex Lens Rays from distance object are focus by a convex lens on screen. The image formed on the screen is known as real image. In camera, is use to form an image on a piece of film / CCD. Image form in the same way in our eyes Drawing Ray Diagrams Predicting the Images form by Convex Lens The ray diagrams on the left show that the object is moved towards the lens, the image becomes bigger and further away. Film projector use convex lens to magnify real images on screen. Quiz Convex Lens as Magnifying Glass Drawing Ray Diagrams Images formed by Concave Lenses Quiz Application of Lenses Camera use convex lens to form small, inverted and real image on sensor. When shutter open, it capture image as a pattern of electric charge which can be stored as data on memory card. This can be process to produce final image on screen. Application of Lenses Human eyes use convex lens to form small, real and inverted image on retina. The light is converge by the cornea and liquid in the eyes. Application of Lens – Projector Projector use convex lens to form large, real and inverted image on screen. For projected image to be upright, the image on the panel must be up side down. For larger image, the panel must be outside the principal focus. Lens is move backward and forward to make focusing adjustment. Electromagnetic Waves Light waves are electromagnetic waves. It has the below characteristics: Can travel through vacuum. Travel at speed of 300 000 km/s Transverse waves Transfer energy Electromagnetic Spectrum Generation of Electromagnetic Waves The higher the frequency of oscillation, the shorter the wavelength of electromagnetic wave produced. Speed = Frequency (Hz) x Wavelength (m) Quiz Radio waves Stars are natural emitter of radio waves. Microwaves Shortest of radio waves Used by mobile phones, wifi and etc. Produce heating effects when absorbed. Infrareds X-Ray • X-rays are given off when fast moving electrons lose energy very quickly. • Short wavelength X-rays are extremely penetrating. • Dense metal like lead can reduce their strength. • X-ray can pass through flesh but not bone. Thus, bone will show up on an x-ray photograph. • X-ray can be use to check faulty welds in pipe joints, and airport security system. • X-ray is dangerous as they may damage the cells. Gamma Rays Sending and Storing An encoder turns incoming information (speech) into electrical signals. Signal is then pass through transmission path (wires) to decoder (earphone). This turns signal back into useful information. Signal may be changes in voltage, intensity of light or strength / frequency of radio waves. Signal can be transmitted using wire, optical fibres or radio waves. Analog and Digital Signal Quiz Quiz Quiz Quiz Quiz Quiz
