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The Nature of Light. Physical Science. What is Light?. Light is an Electromagnetic Wave (EM Wave) that can travel through space or matter that has changing electric or magnetic fields.
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The Nature of Light Physical Science
What is Light? • Light is an Electromagnetic Wave (EM Wave) that can travel through space or matter that has changing electric or magnetic fields. • A field is a region that exerts a force that is a push or pull on another object without touching that object. Example: magnetic field. The fields of the EM wave are perpendicular to the direction of the wave.
How is Light Produced? • An EM wave is produced by the vibration of an electrically charged particle. When a particle vibrates, the electric field around it vibrates. Energy in the form of radiation is released. • When electrons transfer from one level to another, it is unstable. So the electron will jump back to it’s original leve. This “jumping back” releases little packs of energy called photons.
The Electromagnetic Spectrum • There are many different types of light. The light we use to illuminate (see) things is just one type of light called visible light. • Other EM waves include X-rays, radio waves and microwaves.
Electromagnetic Spectrum • Every light wave travels at 300,000,000 m/sec in a vacuum. Each wave has it’s own wavelength. However, longer wavelengths have greater frequencies, so the travel speed is the same. • Waves can be categorized by their wavelengths on the electromagnetic spectrum.
Electromagnetic Spectrum • The electromagnetic spectrum is arranged from long wavelengths (EM waves that become sound waves) that have less energy, to short wavelengths (X-rays and gamma rays) that have a lot of energy. • It has the color pattern of visible light, based on long to short wavelengths: • Red, orange, yellow, green, lightblue, dark blue, violet.
Radio Waves Have the longest wavelengths and lowest frequencies. Receivers change them from EM waves to sound waves. Radio stations: AM (amplitude modulations – longer wavelengths); FM (frequency modulations carry more information.) Although AM stations can travel further than FM station waves, most stations prefer FM. Why?
Television • TV uses both FM and AM. The FM carries sound waves, and the AM carries the information for the visual picture (Again, first travel as EM waves) Cable TV: Info is sent to satellites, which is relayed back to ground atennae, then sent through cables. What are pluses/minuses of cable vs. dish receiver?
Microwaves • Microwaves have short wavelengths and much more energy than radio waves. Microwaves can be used in a radar gun to send bursts of microwaves to an object and get return information (like a police officer using a radar gun to catch speeders).
Infrared Waves • Sun rays are infrared waves. When rays hit your skin, the particles in your skin begin moving (this makes you feel warm). Infrared can be used to keep things warm and free of bacteria. • Many warm objects emit infrared waves (thermal imaging).
Visible Light • Visible light is a very narrow range of wavelengths on the electromagnetic spectrum, and the only light humans see. • The range of color we see is called the visible spectrum. Each color has a specific wavelength that our eyes receive and our brain determines which color(s) we are seeing.
Colors: Roy G. Biv • The wavelengths of light transfer to the following colors in order of greatest to shortest: Red orange yellow Green Blue Indigo violet
Ultraviolet Light • Ultraviolet light is the high frequency light put out by the sun (or a tanning booth ) • Good stuff: can be used to kill harmful bacteria and produces Vitamin D • Bad stuff: can cause sunburn, skin cancer, eye damage, wrinkles and premature aging.
X-rays and Gamma Rays • X-rays can travel through skin and muscle, but is absorbed by bones. • Gamma rays – very, very powerful x-rays that can be used for medical purposes. • Both can cause great harm if there is too much exposure
Interaction of Light Waves • The interaction of light waves should be familiar to you: reflection (bouncing off) , refraction (bending between 2 different mediums) and diffraction (bending around barriers or corners). There are some special types of these interactions when it comes to light.
Reflection • Law of reflection: the angle of incidence equals the angle of reflection. • If you throw a ball straight down, it comes back straight up. If you throw a ball at an angle, it will bounce back at an angle.
Types of Reflection • Regular reflection: when light beam hits at an angle and bounces off at that same angle (i.e. shinning light in a mirror). • Diffused reflection: When a light beam hits at an angle, but the beam reflects at many different angles (shining light on rough surface). • Absorption of light occurs air particle absorb some of the particles. That is why lights go dim and disappear.
Refraction: Color Separation • Have you ever used a prism? Light waves enter a prism, bends the waves, and then separates by wavelength as they come out. You can then see the visible light spectrum (red, orange, yellow, green, blue and violet). When all the colors are mixed together (destructive interference), your eyes/brain see the color WHITE. • What causes a rainbow? What colors do you see?
Diffraction • Light wavelengths are short, so they don’t diffract around large obstacles (like buildings). They can diffract around your shadow. This is why the edges look “jagged”.
Light and Color • How do you see colors? • Remember, when light hits an object, some of the light rays are absorbed, and some of them are reflected. Your eyes pick up the waves that are reflected back. • The reflected waves will bounce back with a certain wavelength. That determines color!
The Color of Things • If the light wave that bounces off an object has very long wavelengths, you may see a shade of red or orange. If the wavelengths are very short, you may see blue, purple or violet. The pigmentation in paint, fabric, and other materials help to determine the wavelength that is reflected back.
Absorption of Light • What is a “good” color to wear outside in summer? What is a “bad” color in summer? • Light shades of the primary colors are best in sunny weather, because most of the light waves are reflected back (so you are cooler). What happens if you are wearing a dark color outside on a very hot day? Why?
Colors are Limited • Primary colors: red, blue and green. • When you combine them, you get the secondary colors: cyan (blue/green), magenta (blue/red), and yellow (red/green). Secondary colors are also known as primary pigments, because all color is a combination of these 3 pigments.
Pigmentation • Pigments in a material either absorb or reflect light. • Pigments always absorbs at least one color. The more colors mixed in, the more absorption takes place. • This constant absorption is called color subtraction.
Producing Light • An incandescent light has electrons traveling around tungsten wire, heating it until it glows. • A fluorescent light uses phosphorous gas to convert ultraviolet radiation to visible light. This makes it more efficient and less expensive than an incandescent light.
Neon • Similar to fluorescent light, the gas tubes are filled with neon gas. When an electric current flows through the tube, the electrons collide with gas molecules. These collisions produce a bright visible light of red. If other gases are added to the tube, you can get other colors.
Lasers • Lasers are multiple light waves emitted simultaneously. The large light wave is reflected off 2 mirrors, only one of which is fully reflective. • Coherent Light is light of only one wavelength that travels with crests and troughs aligned. • Incoherent Light waves are not aligned, so they are not as bright.
Using Light • Polarized light waves vibrate in only one direction. A special polarizing filter can be used (a group of parallel slits). A second filter can be used to stop certain wave motion. Polarized lenses are used to reduce glare. High reflection from a horizontal surface can be blocked with a vertical polarized lens.
Mirrors • Concave mirrors can form various images can be used to magnify objects or create beams of light by where the object is located (relative to the focal point of the mirror).
Convex Mirrors • Convex mirrors spread out reflected light to form a reduced image. Convex mirrors allow you to see large areas (like the mirrors used on the curve of a road.)
Lenses • Concave lenses diverge light rays to form virtual smaller, upright lenses. They are often used in combination with other lenses. Concave lenses make objects appear closer (for nearsighted people!)
Convex Lenses • Convex lenses converge light rays. They can form real (virtual) images, depending on the distance of the object to the lenses. Farsighted people wear convex lenses to make close objects appear clearer to them.
Telescopes and Microscopes • A refracting telescope uses a convex lenses to magnify distant objects. • A reflecting telescope uses concave mirrors and a convex lens to magnify distant objects. • A simple microscope uses a convex objective lens and eyepiece lenses with short focal lengths to magnify small objects.
