﻿ ﻿LIGHT - PHYSICS TOPIC REVIEW - SAT Subject Test Physics

## SAT Subject Test Physics (2012)

### Chapter 16. LIGHT

Light, microwaves, X-rays, and gamma rays all have something in common—they are kinds of electromagnetic waves. The properties of electromagnetic waves can help you understand not only their nature, but their behavior and interactions as well.

Electromagnetic Waves

An electromagnetic wave is produced by a vibration, just as mechanical waves are. However, rather than a hand moving the end of a rope or a pebble dropped in a lake of water, an electromagnetic wave is produced by harmonic motion of charged particles at the atomic level. The result is a wave consisting of vibrating electric and magnetic fields at right angles to one another and to the direction of motion of the wave. An electromagnetic wave, therefore, is a transverse wave that can travel through a vacuum.

Electromagnetic Spectrum

Like mechanical waves, electromagnetic waves can be described by properties of wavelength, frequency, and speed. In a vacuum, all electromagnetic waves travel at the same speed of 3.0 × 108 m/s. You will often see this speed described as the speed of light, c. Note that this speed is used for all electromagnetic waves in air for problems on SAT Physics unless otherwise indicated.

Example:

A sample of blue light has a wavelength of 550 nm. What is the frequency of the light?

Electromagnetic waves are commonly arranged in order according to wavelength. This arrangement is known as the electromagnetic spectrum. The waves with the longest wavelengths and shortest frequencies are radio waves. Gamma rays have the shortest wavelengths and greatest frequencies. Visible light makes only a very small portion of the spectrum.

Polarization

The vibrating nature of electromagnetic waves is somewhat different from the waves you examined in the previous chapter. The vibrations of an electromagnetic wave occur in more than one plane of vibration. Unpolarized light describes light vibrating in more than one plane. Light from the sun, a lamp, or a flashlight is unpolarized. It is common to represent unpolarized light as vibrating in all directions. Because roughly half the vibrations can be considered to be in a horizontal plane and the other half in a vertical plane, a more useful representation is simplified as shown.

Polarizing filters can be used to block out all but one direction of vibration so that the remaining vibrations are in only one plane. Two filters at 90° angles can block out all of the light.

Diffraction

Light waves can bend around an obstacle or through an opening in a process known as diffraction. The amount of bending depends on the relationship between the wavelength of light and the size of the obstacle or opening. If the obstacle or opening is much larger than the wavelength, the bending will be unnoticeable. If the sizes are closer to being equal or the wavelength is greater, the bending will be considerable.

The following diagram represents diffraction through a single slit, or aperture. As the light passes through the slit, they become semi-circular. A screen placed a distance from the slit shows a large, bright region in the center directly opposite the slit. In addition, a series of alternating light and dark spots are formed on either side. These regions, known as interference fringes, become smaller and fainter as the distance from the center increases.

The pattern on the screen changes somewhat if the light is passed through a double slit instead of a single slit. A series of light and dark lines again appears on the screen, but the two patterns overlap. The width of the bright central region is proportional to the wavelength of light.

Test-Taking Hint

Be familiar with various versions of the same units of measurement. For example, 1 Hz can also be written as 1/sec. This will help you cancel units during calculations and arrive at the correct unit in your answer.

REVIEW QUESTIONS

Select the choice that best answers the question or completes the statement.

1. What is one way that a light wave is similar to a water wave?

(A) They both require a medium through which to travel.

(B) They are both examples of transverse waves.

(C) They both travel at the same average speed.

(D) They both consist of electric fields in matter.

(E) They both cause vibrations in the same direction as the wave travels.

2. Which of the following arranges electromagnetic waves in order of increasing wavelength?

(A) radio wave, infrared, ultraviolet, gamma ray

(B) X-ray, gamma ray, radio wave, visible

(C) ultraviolet, visible, infrared, microwave

(D) microwave, radio wave, visible, infrared

(E) radio wave, visible, X-ray, gamma ray

3. What property is the same for all electromagnetic waves in a vacuum?

(A) frequency

(B) wavelength

(C) amplitude

(D) speed

(E) color

4. Which of the following colors of light has the longest wavelength?

(A) red

(B) orange

(C) yellow

(D) green

(E) blue

5. A purple light has a frequency of 7.42 × 1014 Hz. What is the wavelength of the light?

(A) 157 nm

(B) 223 nm

(C) 247 nm

(D) 300 nm

(E) 404 nm

6. A weather station broadcasts at 162 MHz. What is the wavelength of the radio waves in meters?

(A) 1.62 m

(B) 1.85 m

(C) 2.59 m

(D) 4.86 m

(E) 5.40 m

7. A color of light in the violet region has a wavelength of 413 nm. What is its frequency?

8. How does polarization affect light?

(A) It blocks vibrations in all but one plane.

(B) It magnifies the amplitude.

(C) It changes the color.

(D) It converts parallel waves to semi-circular fronts.

(E) It bends the light toward the normal of a surface.

Questions 9 and 10 relate to the diagram below, which shows a light source placed on one side of two slits in a grating.

9. Which process results in the bright regions of the diffraction pattern?

(A) virtual images

(B) internal reflection

(C) polarization

(D) constructive interference

(E) destructive interference

10. Which color of light will produce a central band of light with the greatest width?

(A) green

(B) orange

(C) yellow

(D) blue

(E) violet

1. B Both light waves and water waves are classified as transverse waves because the vibration involved is perpendicular to the direction of motion of the wave. However, the light wave consists of vibrating electric and magnetic fields, whereas the water wave consists of vibrating water molecules.

2. C Consult the electromagnetic spectrum on page 182.

3. D All electromagnetic waves travel at the same speed in a vacuum. All other properties listed vary with the type of wave.

4. A The visible spectrum is arranged from red light, with the longest wavelengths and lowest frequencies, to blue/violet, with the shortest wavelengths and greatest frequencies.

8. A Polarization is the process by which some of the vibrations of light are blocked such that the result light vibrates in a single plane.

9. D The light regions are caused by constructive interference, whereas the dark regions are caused by destructive interference.

10. B The width of the central region is directly proportional to the wavelength of light. Therefore, the color with the greatest wavelength will result in the greatest width. Orange has the longest wavelength of the colors listed.

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