FIGURE shows the light intensity on a viewing screen behind a circular aperture. What happens to the width of the central maximum if
a. The wavelength of the light is increased?
b. The diameter of the aperture is increased?
c. How will the screen appear if the aperture diameter is less than the light wavelength?
(a) The width of the wavelength is increased.
(b) The width of the aperture is decreased.
(c) The screen appears like almost uniformly gray.
When it comes to waves like acoustic waves (sound) or electromagnetic waves like light or radio waves, intensity refers to the average power transfer across one period of the wave. Intensity can be used in a variety of situations where energy is delivered. For example, the intensity of the kinetic energy carried by drops of water from a garden sprinkler may be calculated.
We know that , so:
As is increased, the width grows.
As the diameter grows larger, the width shrinks.
For diffraction to occur, the field of view must be of the same order as the wavelength of light, resulting in a nearly evenly grey surface with no minima.
If sunlight shines straight onto a peacock feather, the feather appears bright blue when viewed from on either side of the incident beam of light. The blue color is due to diffraction from parallel rods of melanin in the feather barbules, as was shown in the photograph on page . Other wavelengths in the incident light are diffracted at different angles, leaving only the blue light to be seen. The average wavelength of blue light is . Assuming this to be the first-order diffraction, what is the spacing of the melanin rods in the feather?
The pinhole camera of FIGURE images distant objects by allowing only a narrow bundle of light rays to pass through the hole and strike the film. If light consisted of particles, you could make the image sharper and sharper (at the expense of getting dimmer and dimmer) by making the aperture smaller and smaller. In practice, diffraction of light by the circular aperture limits the maximum sharpness that can be obtained. Consider two distant points of light, such as two distant streetlights. Each will produce a circular diffraction pattern on the film. The two images can just barely be resolved if the central maximum of one image falls on the first dark fringe of the other image. (This is called Rayleigh’s criterion, and we will explore its implication for optical instruments in Chapter .)
a. Optimum sharpness of one image occurs when the diameter of the central maximum equals the diameter of the pinhole. What is the optimum hole size for a pinhole camera in which the film is behind the hole? Assume an average value for visible light.
b. For this hole size, what is the angle a (in degrees) between two distant sources that can barely be resolved?
c. What is the distance between two street lights away that can barely be resolved?
a. Green light shines through a -diameter hole and is observed on a screen. If the hole diameter is increased by , does the circular spot of light on the screen decrease in diameter, increase in diameter, or stay the same? Explain.
b. Green light shines through a -diameter hole and is observed on a screen. If the hole diameter is increased by , does the circular spot of light on the screen decrease in diameter, increase in diameter, or stay the same? Explain.
94% of StudySmarter users get better grades.Sign up for free