Light and sound are the two major ways that we receive information about the world. Of the two, light provides the greater variety of information. The eye can detect tiny changes in the size, brightness, and color of an object. Changes in position with time are quickly computed by our brains into velocities and accelerations
The Facts of Light Light is the range of frequencies of electromagnetic waves that stimulates the retina of the eye. Light waves have wavelengths from about 400 nm (4.00 x m) to 700 nm (7.00 x m). The shortest wavelengths are seen as violet light. As the wavelength increases, the colors change to blue, green, yellow, orange, and finally, red. We locate objects by assuming that light travels from them to our eyes in straight lines.
Visible Light’s position in the total ElectroMagnetic Spectrum
The Speed of Light Before the seventeenth century, most people believed that light traveled instantaneously. Galileo was the first to hypothesize that light had a finite speed and to suggest a method of determining it. His method, however, was not sensitive enough, and he was forced to conclude that the speed of light was too fast to be measured at all over a distance of a few kilometers.
The Danish astronomer Ole Roemer (1644—1710) was the first to determine that light did travel with a measurable speed. Between 1668 and 1674, Roemer made 70 careful measurements of the 42.5-hour orbital period of Io, one of the moons of Jupiter. He recorded the times when lo emerged from behind Jupiter and found that the period varied slightly. The variation was as much as 14 seconds longer when Earth was moving away from Jupiter and 14 seconds shorter when Earth was approaching Jupiter. He concluded that as Earth moved away from Jupiter, the light from each new appearance of lo took longer to travel the increasing distance to Earth. Thus the measured period was increased.IoJupiter Based on these data, in 1676 Roemer calculated that light took 22 minutes to cross a diameter of Earth's orbit. The speed of light was finite, but so fast that light took less than one second to cross the entire Earth!
Although many laboratory measurements have been made, the most notable was a series performed by the American physicist Albert A. Michelson ( ), Between 1880 and the 1920s, he developed Earth-based techniques of measuring the speed of light. In 1926, Michelson measured the time light required to make a round trip through a pipe from which all air had been removed. The pipe was constructed between two California mountains 35 km apart. Michelson's best result was ± x 10 8 m/s. For this work, he became the first American to receive the Nobel prize.
We see objects because they either EMIT light (due to their temperature), or they REFLECT light, due to their colors.
. The lumen is the unit of luminous flux and is a measure of the rate of flow of light energy from a source.
The illuminance on a surface varies inversely as the square of its distance from a light source.
There are two ways to increase the illumination on a surface. The luminous flux of a light source can be increased, or the distance between the source and surface can be decreased. Thus illuminance varies directly with the flux of the light source and inversely with the square of the distance from the source, Figure The illuminance, E, directly under a small light source, is given by
Objects can be seen clearly through air, glass, some plastics, and other materials. These materials transmit light waves and are called transparent materials. Other materials, such as frosted glass, transmit light but do not permit objects to be seen clearly through them. These materials are called translucent. Lampshades and frosted light bulbs are translucent. Materials such as brick transmit no light. They absorb or reflect all the light waves that fall on them. These materials are called opaque.
White light, when passed through a prism, is separated into a spectrum of colors. This occurs because different wavelengths (colors!) are bent differently by passing into and out of glass.
How Stuff Works CDs
Each color is reinforced where the soap film is 1/4 λ, 3/4 λ, 5/4 λ.... of the wavelength for that color. Since each color has a different wavelength, a series of color bands is seen reflected from the soap films.
Waves are polarized with respect to the vertical plane (a). Vertically polarized waves can't pass through a horizontal polarizer (b). Polarized Light
REVIEWING CONCEPTS 1. Sound doesn't travel through a vacuum; how do we know light does? 2. What is the range of wavelength, from shortest to longest, the human eye can detect? 3. What color of visible light has the shortest wavelength? 4. What was changed in the equation v = λf in this chapter? 5. Distinguish between a luminous body and an illuminated body. 6. Look carefully at an ordinary frosted incandescent bulb. Is it a luminous or an illuminated body? 7. Explain how we can see ordinary nonluminous classroom objects.
8. Distinguish among transparent, translucent, and opaque objects. 9. Of what colors does white light consist? 10. Is black a color? Why does an object appear black? 11. Why can sound waves not be polarized? 12. Why would a perfect polarizing filter transmit half the nonpolarized light incident on it?