B-2 Physics Optics (Light) PDF

Module: B-2 Physics Topic 2.4 Optics  INTRODUCTION On completion of this topic you should be able to: 2.4.1 Describe the nature of light and state the speed of light 2.4.2 Describe the laws of reflection and refraction...

Module: B-2 Physics Topic 2.4 Optics  INTRODUCTION On completion of this topic you should be able to: 2.4.1 Describe the nature of light and state the speed of light 2.4.2 Describe the laws of reflection and refraction reflection at plane surfaces reflection by spherical surfaces refraction of light through various media the use of lenses 2.4.3 Describe the nature and use of fibre optics 30-03-2024 Slide No. 2  ELECTROMAGNETIC RADIATION and LIGHT Electromagnetic radiation is defined as energy resulting from the acceleration of electric charge and the associated electric and magnetic fields. Electromagnetic radiation flows from the source and consists of waves of differing wavelength and frequency, but they all travel through at the speed of light (2.998 x 108 m/sec in a vacuum ~ 300,000 km/sec). 30-03-2024 Slide No. 3  ELECTROMAGNETIC RADIATION A model of the transmission of EMR. Remember there is no medium required. 30-03-2024 Slide No. 4  ELECTROMAGNETIC RADIATION and LIGHT The spectrum of electromagnetic radiation. Gamma rays, X rays, Ultraviolet light, Visible light, Infrared light, Microwaves/Radar and Radio waves. Visible Light is defined as electromagnetic radiation of a frequency range detectable by the human eye. 30-03-2024 Slide No. 5  THE NATURE OF LIGHT The Quantum Theory suggests that light exists in packets of energy that have both particle and wave properties. Light is assumed to be wave- based but there is also evidence that light is composed of particles with mass. (photons). Some of the evidence that light is composed of particles is: Light is affected by gravity (bent around large planets) Light exerts a force, light from the sun causes the deflection of comet tails Light can generate a photoelectric effect. 30-03-2024 Slide No. 6  THE NATURE OF LIGHT Some of the evidence that light is composed of waves is that light can be : reflected refracted dispersed, broken down into spectral components meaning that each colour has a different wavelength Polarisation and Polaroid lenses blocking out one plane of light waves The Doppler effect (red shift) suggests that light travels in waves, like sound. 30-03-2024 Slide No. 7  REFLECTION Reflection of light and other electromagnetic radiation occurs when waves encounter a boundary that does not absorb the radiation’s energy and bounces the waves off the surface. The incoming wave is known as the incident wave and the wave that is bounced from the surface is called the reflected wave. The law of reflection states that the angle of incidence equals the angle of reflection. The angles are measured against a line perpendicular to the surface of the reflective material, called the ‘normal’. 30-03-2024 Slide No. 8  REFRACTION The speed of light, as stated, is 2.998 x 108 m/sec in a vacuum. The speed of light is less in various transparent substances, including air. Most transparent substances have a refraction index which gives an indication of their density, how much the light slows down and, therefore, how much the light bends through the substance. The higher the refractive index number, the denser the material and the more the light will slow down and refract or bend as it passes through the substance. 30-03-2024 Slide No. 9  REFRACTION When light waves pass from one medium to another, they often have to change velocity. For example, when light travels through air and then enters a glass lens, it slows down. When waves change velocity, there is an associated change in direction. This change in direction is called refraction. The angle of refraction is dependent on the density of the material through which the light passes and also the angle of incidence to the ‘normal’. 30-03-2024 Slide No. 10  REFRACTION As light travels through different media, it is refracted (bent) by different amounts depending on the angle of incidence and also the refraction index of the material. A person may see a fish in the water but, in reality, the fish is in a different position because the light from the image is refracted as it leaves the water. 30-03-2024 Slide No. 11  REFRACTION The index of refraction for: Air is 1.00029; Diamond is 2.42; Glass is 1.5 to 1.7; Ice is 1.31; Water is 1.33. 30-03-2024 Slide No. 12  TOTAL INTERNAL REFLECTION As the incidence angle increases, less and less energy is refracted and more is reflected. At the Critical Angle, 100% of the light is reflected and Total Internal Reflection is occurring. This phenomenon is important in the design of fibre optic cable. 30-03-2024 Slide No. 13  REFRACTION The index of refraction also varies with the wavelength of the radiation. If white light enters a prism, the different wavelengths of the component colours are refracted by different amounts. This is termed dispersion. 30-03-2024 Slide No. 14  POLARISATION Light is actually radiating like a length of rope being shaken vigorously in all directions. What emerges from the slit could be described as "plane polarized radiation", because the vibrations are only in a single (vertical) plane. 30-03-2024 Slide No. 15  POLARISATION Put a second slit on the string. If it is aligned the same way as the first one, the vibrations will still get through. 30-03-2024 Slide No. 16  POLARISATION The second slit will only let through horizontal vibrations - and there aren't any. The energy is completely polarized. Polarising sunglasses etc. do the optical equivalent of this using certain materials in place of the slits, to reduce the energy of the radiation, and cut glare. 30-03-2024 Slide No. 17  PLAIN and CURVED MIRRORS A mirror is device used to reflect light or other electromagnetic radiation. A plain mirror is one whose reflective surface is flat, however various types of curve can be used, for example: A spherical mirror has a reflective surface that forms part of a sphere. Curved mirrors can be concave or convex. Concave mirrors converge light while convex mirrors diverge the light. Flat Mirror. Concave Mirror. Convex Mirror. 30-03-2024 Slide No. 18  MIRROR FOCAL LENGTH Consider a source at the centre of a curved mirror, a plane wave is reflected. Focal length of a mirror is given by: f = R/2 (Approx) (R = radius) This means that light from infinity is focused a distance f away. The effect in this case is to converge and focus the light to a real image. A real image is one that can be projected on to a screen 30-03-2024 Slide No. 19  VIRTUAL IMAGES Convex mirror Note that these behave as if there is a focus behind the mirror giving a virtual image. Driving and security mirrors are convex to increase coverage. For accurate focusing, a parabolic mirror is required. 30-03-2024 Slide No. 20  LENS CONSTRUCTION We can build up a lens from a series of prisms: sinθ1 = n.sinθ2 entering the prism and, sinθ3 = n.sinθ4 exiting the prism More prisms are added and ground smooth. 30-03-2024 Slide No. 21  LENSES There are a variety of lenses, but essentially they are: Converging or positive (convex) Diverging or negative (concave) 30-03-2024 Slide No. 22  LENSES A Convex lens has ‘real’ image. Focal length f: Is how far from the lens parallel rays get focused. A Concave lens has virtual image. Concave lenses cause light to diverge, but the rays can be traced back to an imaginary focus point. 30-03-2024 Slide No. 23  REAL IMAGES Real images are those where light actually converges, and can be projected on to a screen. Real images occur when objects are placed outside the focal length of a converging lens or outside the focal length of a converging mirror. Note that it is magnified, but inverted. A real image has to be where the light is, which means in front of a mirror, or behind a lens.) 30-03-2024 Slide No. 24  VIRTUAL IMAGES Virtual images are locations from where light appears to have converged. A virtual image is formed by diverging lenses. Image is upright but diminished. 30-03-2024 Slide No. 25  CONVEX LENS If a convex lens does not focus the light passing through it at a single focal point, the image will not be sharp. This is termed spherical aberration and is common in less expensive lenses. Sometimes the human eye does not focus images well enough on the back of the eye, the retina. In these cases spectacles, contact lenses or corrective surgery can be used. 30-03-2024 Slide No. 26  OPTICAL FIBRE COMMUNICATIONS Imagine a long flexible plastic pipe – inside surface coated with a perfect mirror. Shine a torch in one end – light is visible at the other end even if several kilometres away – light reflects off sides – even though pipe may curve & twist. Making a cable out of mirrored tube would work, but – too bulky & difficult to coat interior of tube with perfect mirror. 30-03-2024 Slide No. 27  OPTICAL FIBRE COMMUNICATIONS Real optical fibre – glass cable so pure that light is visible through it, even when many kilometers long – thickness comparable to that of single human hair. Laser at end of cable switches on & off to send digital bits – billions of bits per second. Multiple lasers – different colors (frequencies) – multiple signals on same fibre. Capable of carrying a signal quite a distance ≈ 100 km 30-03-2024 Slide No. 28  OPTICAL FIBRE CABLE An optical fibre is a thin strand of high quality glass. Very little light is absorbed in the glass. Light getting in at one end is totally internally reflected, even if fibre is bent. 30-03-2024 Slide No. 29  CONCLUSION Now that you have completed this topic, you should be able to: 2.4.1 Describe the nature of light and state the speed of light. 2.4.2 Describe the laws of reflection and refraction reflection at plane surfaces reflection by spherical surfaces refraction of light through various media the use of lenses. 2.4.3 Describe the nature and use of fibre optics. 30-03-2024 Slide No. 30  This concludes: Module: B-2 Physics Topic 2.4 Optics