Visible Light and Its Sources The Visible Spectrum
Visible Light and Its Sources
The Visible Spectrum • Light is visible at wavelengths of about 780 nm to 380 nm. • We see light as colors, not as waves.
1020 gamma x-ray Frequency (Hz) UV IR 1010 UHF RADAR TV FM VHF AM VLF 101 ELF
Frequency (THz) 700 650 600 550 500 450 550 600 650 Wavelength (nm) 700 400 750
The Visible Spectrum • Luminous objects produce light. § Ideally – emits a continuous spectrum (all possible wavelengths of visible light) • The sun comes close to emitting a continuous spectrum, but it does have some gaps.
The Visible Spectrum • Luminous objects produce light. § Monochromatic – emits a single color (a single wavelength of visible light) § Other light sources – emit multiple distinct colors
The Visible Spectrum • Light projected through a prism produces a line spectrum. § line spectrum – distinctly visible lines of color can be seen at specific wavelengths aligned along a scale in a spectrograph
The Visible Spectrum § spectroscope – an instrument designed to display the line spectrum § spectrograph – a photo of the spectrum
Which of the following is a luminous object? 1. 2. 3. 4. Light bulb T-shirt Piece of paper Window Question
The Speed of Light • depends on the material through which it travels • defined as 299, 792, 458 m/s (can be rounded to 300 million m/s)
The Speed of Light • If 300 million m/s is 18 million km/min, then sunlight takes …about 8. 3 light-minutes to reach Earth. …about 4. 2 light-hours to reach Neptune.
The Speed of Light • If 300 million m/s is 18 million km/min, then sunlight takes …about 4. 4 light-years to reach the nearest star. …about 100, 000 light-years to travel the diameter of the Milky Way galaxy.
Intensity of Light • The intensity, power, or brightness of a light source is the rate at which it radiates energy. • Intensity is always measured at the source.
Intensity of Light • SI unit – candela (cd) § It is just slightly brighter than an average candle.
Intensity of Light • SI unit – candela (cd) § It is just slightly brighter than an average candle. § An ordinary incandescent bulb makes just less than 1 cd/watt. § Fluorescent lights emit about 4 cd/watt.
Approximately how many candles would be needed to give as much light as a 60 W light bulb? 1. 2. 3. 4. 10 60 95 130 Question
Intensity of Light • Illumination is the amount of light received at a distance from the source. • The amount of illumination depends on the brightness of the light source and the distance from it.
Intensity of Light • Illumination and distance from the light source are related by the inverse-square law. § Doubling the distance reduces the illumination to one-fourth of the original illumination.
Intensity of Light § Tripling the distance reduces the illumination to one-ninth of the original. Distance and its effect on forces are often related by the inverse-square law.
1 m 2 4 m 2 9 m 2 16 m 2 1 m 2 m 3 m 4 m
If the distance is cut in half, the illumination will be 1. 2. 3. 4. quadrupled. doubled. cut in half. reduced to one-fourth. Question
Intensity of Light • Photometers measure the illumination of light using a “solar cell. ” • More light equals more current.
Sources of Light 1. Incandescent 2. Fluorescent 3. Phosphorescent
Incandescent • Incandescent bulbs have materials which are heated until they release visible energy. • As the filament gets hotter, the light goes from red to yellow and on up the spectrum.
Relative Intensity 4000 K 3000 K 2000 K 0 1000 2000 3000 4000 Wavelength (nm)
Fluorescent • Fluorescent tubes contain mercury gas which emits light as the electricity passes through it. • The light is in the UV range. • Phosphors absorb this light and reemit it at a lower energy: the visible light range.
Fluorescent • A UV lamp is simply a regular fluorescent tube with little or no phosphors.
electrode mercury low pressure argon and mercury gas phosphor coating glass tube
Phosphorescence • Phosphorescence occurs when the material continues to emit light when the source is removed.
Coherent Light • Coherent light is “in step” or “in phase” and of the same color.
Coherent Light • Lasers and certain LED (lightemitting diodes) produce coherent light. • Since it is in phase, it does not spread out like ordinary visible light produced from a flashlight.
T/F Laser light is monochromatic. True Question
Bioluminescence • Bioluminescence is light given off by living creatures like fireflies. • It is produced from a chemical reaction inside the creature. • Bioluminescence is also called cold light.
Chemiluminescence • Chemiluminescence is cold light produced non-biologically as in light sticks.
The Nature of Color
Introduction • Look at Image 15 -14 on p. 361. • Is the ball red because you perceive it as red or because it is red? • This is a question of philosophy, not science.
Introduction Example: If a tree falls in the forest and nobody hears it, did it make a sound?
Introduction • Scientifically, an object’s color depends on how your eyes perceive it.
Color Mixing • There are two types of color mixing. 1. Additive (occurs in light) 2. Subtractive (occurs in paint)
Color Mixing Additive • Additive means that light is being emitted. • The human eye distinguishes three colors of light. These colors are called the additive primary colors or primary hues.
Color Mixing Additive • The additive primary colors are red, green, and blue. • When mixed they produce the additive secondary colors: yellow, magenta, and cyan. • If all three are mixed equally they produce white light.
Color Mixing monochromatic lamps
Color Mixing • Color TVs, computer monitors, and cell phone screens take advantage of the eye and brain’s ability to mix colors. • These screens display only red, green, and blue, yet the mixture of colors gives a multitude of shades of each color.
Color Mixing Subtractive (Paints) • Subtractive means that the colors are reflected.
white light other colors absorbed green and some yellow reflected
Color Mixing Subtractive (Paints) • Paints and pigments have three subtractive primary colors: cyan, magenta, and yellow.
Color Mixing Subtractive (Paints) • Mixing two of the three subtractive primary colors gives the subtractive secondary colors (which are also the additive primary colors): red, green, and blue.
Color Mixing Subtractive (Paints) • For example, mixing magenta and cyan paints gives blue paint.
Color Mixing Subtractive (Paints) • Mixing all three subtractive primary colors should give black. • Because no pigment absorbs colors completely, mixing all three colors often results in some shade of brown.
Color Perception Systems 1. HSV Hue Saturation Value H S V
Color Perception Systems 2. CMYK Cyan Magenta Yellow Black B M K C G R Y
If red, green, and blue lights are mixed, what color will result? 1. 2. 3. 4. black magenta yellow white Question
If red and blue lights are mixed, what color will result? 1. 2. 3. 4. cyan magenta yellow white Question
Reflection & Mirrors
Light Rays • A light beam is made up of many light waves traveling together in a straight line. • These waves are represented by lines called rays.
Types of Reflection • Diffuse reflection is caused by rough or uneven surfaces.
Types of Reflection • Specular reflection is caused by smooth surfaces.
Reflection Terms § incident ray – the ray going to the surface § reflected ray – the ray bouncing off the surface § normal – a line perpendicular to the surface
Reflection Terms § angle of incidence – the angle between the normal and the incident ray § angle of reflection – the angle between the normal and the reflected ray
Reflection Terms § Law of Reflection – states that the angle of incidence equals the angle of reflection v This law is true for all rays and all types of surfaces.
normal line angle of incidence angle of reflection incident ray point of incidence reflected ray
If the incident angle is 40°, what is the reflected angle? 1. 2. 3. 4. 20° 40° 50° 140° Question
Plane Mirror Reflection • A plane mirror is a flat mirror. • The reflected image appears to exist an equal distance away on the opposite side of (behind) the mirror. • No actual image exists at this location (behind the mirror), so it is called a virtual image. • The image is reversed left to right.
Curved Mirror Reflection • A real image is formed from focused rays that could be projected onto a screen.
Concave Mirrors • Concave mirrors are mirrors that “cave in” on the side exposed to light rays. • The light rays must also obey the law of reflection. • The light rays converge (meet) at one point called the principal focus or focal point.
center of mirror principal optical axis focal point
Concave Mirrors • If the object is located further from the mirror than the focal point, the image will be real and inverted.
1 2 object 3 image 2 1 3 focal point
Concave Mirrors • If the object is located at the focal point, no image will result.
Concave Mirrors • If the object is located closer to the mirror than the focal point, the image will be virtual and upright.
Concave Mirrors
Convex Mirrors • Convex mirrors reflect light to produce only virtual images of varying sizes, but the virtual images are always smaller than the original objects.
All mirrors (plane, concave, or convex) produce images by reflection only.
Refraction & Lenses
Light Refraction • Refraction is the bending of light as it passes from one medium to another medium of a different optical density. • Optical density is measured by comparing the speed of light in a vacuum to the speed of light in the material.
Light Refraction • If light enters a denser material, the light ray is refracted toward the normal line.
boundary surface angle of refraction angle of incidence angle of refraction
Light Refraction • If light enters a denser material, the light ray is refracted toward the normal line. • If light enters a less dense material, it bends away from the normal line.
angle of refraction angle of incidence
Light Refraction • As the angle of incidence of the light is increased, eventually the refraction will lie on the surface of the material. • This angle is called the critical angle of incidence.
angle of refraction = 90° critical angle of incidence
Light Refraction • As the angle of incidence is increased further, no light is refracted. • All the light is reflected back into the denser material. • When this occurs it is called total internal reflection.
totally internally reflected ray angle of incidence angle of reflection
Light Refraction • A fiber optic cable relies on the total internal reflection to transmit data and images. • Endoscopes are tiny cameras which use fiber optic cables to transmit light rays. • The use of endoscopes has reduced the risk and damage from exploratory surgery.
Lenses • With mirrors, light reflects off a surface. • With lenses, light refracts through the material. • The refracted light either converges to a point or diverges (spreads out).
Convex Lenses • Convex lenses are converging lenses. • The light converges to the focal point which is on the opposite side of the lens from the object. • Many of the terms from mirrors also apply to lenses.
Convex Lenses focal point principal optical axis focal point parallel rays lens
Concave Lenses • Concave lenses are diverging lenses.
Concave Lenses
Eyesight Correction • If a person is nearsighted, the focal point is in front of the retina. • Diverging lenses are used to correct this.
Eyesight Correction • If a person is farsighted, the focal point is behind the retina. • Converging lenses are used to correct this.
Normal Vision
Farsighted
Farsighted Corrected
Nearsighted
Nearsighted Corrected
Magnification • The images produced by lenses are usually different sizes from the objects. • If the image is larger than the object, the magnification factor is greater than 1.
Magnification • If an instrument has several lenses, the magnification factors are multiplied together to get the total magnification. • Telescopes and microscopes are examples of instruments where the magnification factors are combined.
Magnification • Binoculars are basically a pair of telescopes. – In this case, the magnification factor is the same for each side of the binocular. • Cameras also use several lenses.
T/F A lens that looks like this will focus light. True Question
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