Chapter 9 Optics
Light waves • Light generally refers to the narrow band of EM waves that can be seen by human beings. • These are transverse waves. • The frequencies range from 4×1014 to 7.5×1014 hertz.
Light waves, cont’d • We typically express the wavelengths in nanometers. • The wavelengths of visible light range from 750 nm (red light) to 400 nm (blue light).
Light waves, cont’d • Keep in mind that: • We perceive different frequencies of light as different colors, and • White light is typically a combination of all frequencies of the visible spectrum.
Light waves, cont’d • To describe various properties of light, we use two different methods: • wavefronts, or • light rays.
Reflection • Specular reflection is reflection off of a very smooth surface. • Specular reflection occurs when the direction the light wave is traveling changes.
Reflection, cont’d • To study reflection, we need some terminology. • The normal is an imaginary line drawn perpendicular to the mirror and touching it at the point where the incident ray strikes the mirror.
Reflection, cont’d • The angle of incident is the angle between the incident ray and the normal. • The angle of reflection is the angle between the reflected ray and the normal.
Reflection, cont’d • The law of reflection states that the angle of incidence equals the angle of reflection.
Reflection, cont’d • Diffuse reflection occurs when light strikes a surface that is not smooth and polished but rough. • The light rays reflect off the random bumps and nicks in the surface.
Reflection, cont’d • Objects have color because the light actually penetrates into the material. • Some of the light is reflected and some is absorbed.
Reflection, cont’d • A white surface takes the color of whatever light strikes the surface. • Shining a red light onto a white piece of paper makes the paper look red.
Reflection, cont’d • A colored surface only reflects light of the same “color” as the surface. • A red surface looks red when you shine white light on it. • It appears somewhat black if you shine blue on it.
Diffraction • Diffraction occurs when a wave passes through a hole or a slit. • It is only observable when the opening is not too much larger than the wavelength of the wave.
Diffraction, cont’d • Since the wavelength of visible light is so small, diffraction is not noticeable through a window. • You need a very narrow slight to see the diffraction of light. • This figure shows the result of laser light after passing through a slit 0.008 cm wide. • The screen was 10 meters from the slit.
Interference • Interference occurs when two waves overlap. • Two waves are said to be “in phase” when their peaks overlap. • When two waves are in phase, their amplitudes add. • This is called constructive interference.
Interference, cont’d • Two waves are said to be “out of phase” when the peaks of one wave overlap the valleys of the other.. • When two waves are out of phase, their amplitudes cancel. • This is called destructive interference.
Interference, cont’d • Here is a diagram illustrating a two-slit interference experiment. • Light is passed through two slits. • The light strikes a screen behind the slits. • An interference pattern appears on the slit.
Interference, cont’d • Bright regions on the screen indicate constructive interference. • The light from each slit constructively interfere. • Dark regions on the screen indicate destructive interference. • The light from each slit destructively interferes.
Interference, cont’d • Here is an actual photograph of two-slit interference using a laser.
Interference, cont’d • Thin-film interference occurs when light passes through a thin layer of one substance next to another substance. • Here, light passes from air, through a thin layer of oil floating on a surface of water.
Interference, cont’d • Some of the light is reflected off the top surface of the oil. • The rest of the light passes through the oil. • Some of the transmitted light is eventually reflected off the water’s surface and passes back into the air.
Interference, cont’d • The ray that passes through the oil travels a greater distance. • If the two waves emerge in step, you have constructive interference. • If the two waves emerge out of step, you have destructive interference.
Interference, cont’d • Important factors that determine whether the interference is constructive or destructive include: • The wavelength of the light, • The thickness of the film, and • The angle at which the light strikes the film. • If a single wavelength of light is used, you obtain bright and dark areas. • If several wavelengths of light are used (white light), you obtain different colors
Interference, cont’d • Here are two examples of thin-film interference using multi-wavelength light.
Polarization • Imagine a rope attached at one end to a wall. • The other end is free for you to move. • If you move your hand from side-to-side, we would say that the wave is horizontally polarized.
Polarization, cont’d • If you move your hand from up and down, we would say that the wave is vertically polarized. • Polarization is only possible with transverse waves.
Polarization, cont’d • Since light is a transverse wave, it can be polarized. • A Polaroid filter, absorbs light passing through it unless the light is polarized in a particular direction.
Polarization, cont’d • If vertically polarized light is passed through a vertical polarizer, all the light passes. • If light polarized to 45º from the vertical is passed through a vertical polarized, only some of the light passes.
Polarization, cont’d • If horizontally polarized light is passed through a vertical polarizer, none of the light passes through the polarizer.
Polarization, cont’d • Most light is unpolarized. • It contains components with random polarizations. • If unpolarized light is passed through a vertical polarizer, then only those components that were originally vertically polarized pass through the filter.
Polarization, cont’d • LCDs use perpendicular polarizers with a liquid crystal to either allow is block light. • The liquid crystal changes the polarization of the light.
Plane mirrors • Most mirrors are plane mirrors. • They are flat and almost perfect reflectors of light. • The reflection appears to originate from behind the mirror.
One-way mirrors • A “one-way mirror” is made by partially coating glass so that it reflects some of the light and allows the rest to pass through. • This is called a half-silvered mirror.
One-way mirrors, cont’d • The one-way mirror acts as a mirror for the side that is brightly lit. • It acts as a window on the dimly lit side. • If a bright light is turned on in the dimmer room, the “one-way” effect is lost.
Curved mirrors • A concave mirror is a mirror that curves inward on the reflecting side. • Parallel rays that reflect off the mirror are focused at a point called the focal point.
Curved mirrors, cont’d • A concave mirror can be used to create an image larger than that created by a plane mirror.
Curved mirrors, cont’d • A convex mirror is curved outward on the reflecting side. • It creates an image smaller than a plane mirror.
Curved mirrors, cont’d • An advantage of a convex mirror is that it has a wide field of view. • Images of things spread over a wide area can be viewed in it.
Curved mirrors, cont’d • Here is an example of the image from a convex mirror.
Astronomical mirrors • Many large telescopes use curved mirrors. • Here is a common design. • The large, concave mirror focus the object on the smaller, convex mirror, and then to the eye.
Astronomical mirrors, cont’d • A problem with large mirrors is spherical aberration. • This occurs when not all of the rays focus at the same point. • This can be overcome by using a parabolic mirror instead of a spherical mirror.
Astronomical mirrors, cont’d • The Hubble telescope originally had an aberration problem, as seen in these two images.
Astronomical mirrors, cont’d • An attempt to improve optical telescopes is called adaptive optics. • Here, the mirror is “deformable” so that imperfections can be electronically corrected.
Refraction • Imagine a light ray passing through air as it arrives at the surface of another transparent substance, e.g., glass. • The boundary between the air and the glass is called an interface.
Refraction, cont’d • Part of the ray passes into the glass while the rest reflects. • The reflected law obeys the law of reflection. • The ray that passes into the glass obeys the law of refraction.
Refraction, cont’d • The ray that passes into the glass is called the refracted ray. • The angle between the refracted ray and the normal is the angle of refraction.
Refraction, cont’d • The law of refraction states that: • a light ray is bent toward the normal when it enters a transparent medium in which light travels slower. • a light ray is bent away from the normal when it enters a transparent medium in which light travels faster.
Refraction, cont’d • This figure illustrates the effect as the light passes from the glass into the air. • Since light travels faster in air than glass, the ray is bent away from the normal. • The principle of reversibility states that the path of a light ray through a refracting surface is reversible.