General Science MCQs – Page 13 (Q121-130)

The electromagnetic spectrum, ordered from longest to shortest wavelength, generally runs radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays, with wavelength decreasing and both frequency and photon energy increasing along this sequence. Radio waves can have wavelengths of kilometers, while gamma rays can have wavelengths smaller than an atomic nucleus. This ordering underlies countless applications, from long-wavelength radio waves used in broadcasting and communication to short-wavelength gamma rays used in cancer treatment and produced by radioactive decay. All electromagnetic waves, regardless of wavelength, travel at the same speed in a vacuum, about 3 x 10 to the 8 meters per second, differing only in wavelength and frequency.

X-rays are widely used in medicine for imaging bones and internal structures because their short wavelength and high energy allow them to penetrate soft tissue relatively easily while being absorbed more strongly by dense material like bone and teeth, creating a clear contrast on photographic film or digital detectors. Wilhelm Conrad Rontgen discovered X-rays in 1895, earning him the first Nobel Prize in Physics in 1901, and the technology quickly revolutionized medical diagnostics, later expanding into CT scans that combine many X-ray images into detailed cross-sectional views. Because X-rays are a form of ionizing radiation, exposure is carefully limited to minimize cellular damage risk.

Gamma rays have the highest frequency and photon energy of all electromagnetic waves, with wavelengths shorter than a hundred picometers, often smaller than an atomic nucleus, making them the most energetic and penetrating form of radiation in the electromagnetic spectrum. They are typically produced by radioactive decay of atomic nuclei, nuclear reactions, and violent astrophysical events like supernovae, and their high energy makes them useful in cancer radiotherapy and sterilizing medical equipment, but also hazardous, requiring thick shielding like lead or concrete to block. Unlike X-rays, which originate from electron transitions, gamma rays originate specifically from changes within the atomic nucleus.

Radio waves have the longest wavelength of all types of electromagnetic radiation, ranging from about one millimeter to over 100 kilometers, and correspondingly the lowest frequency and photon energy, making them the least harmful form of electromagnetic radiation to living tissue. Because of their long wavelength, radio waves can diffract, or bend, around large obstacles like hills and buildings, and travel long distances with relatively little absorption, which is why they are used for broadcasting, television, mobile communication, and radar. Heinrich Hertz first experimentally confirmed the existence of radio waves in 1887, following James Clerk Maxwell's theoretical prediction of electromagnetic waves two decades earlier.

Sound waves are classified as longitudinal waves because the particles of the medium they travel through vibrate back and forth parallel to the direction the wave itself is traveling, creating alternating regions of compression, where particles bunch closely together, and rarefaction, where particles spread apart. This differs from transverse waves, such as light and other electromagnetic waves, where oscillations occur perpendicular to the direction of wave travel. Longitudinal sound waves can travel through solids, liquids, and gases, since all these states of matter have particles capable of colliding and transmitting compressions, but not through a vacuum, which lacks particles altogether.

Light waves are classified as transverse waves because their oscillations, in this case the electric and magnetic fields, vibrate perpendicular to the direction the wave travels, unlike sound, which is a longitudinal wave with vibrations parallel to its direction of travel. As electromagnetic waves, light waves require no material medium and can travel through the vacuum of space, which is how energy from the Sun reaches Earth. Other examples of transverse waves include ripples on water and the waves generated by shaking a rope up and down. The transverse nature of light also allows it to be polarized, a property exploited in polarized sunglasses and LCD screens.

The pitch of a sound, how high or low it sounds to the human ear, is primarily determined by its frequency, measured in hertz, with higher frequencies producing higher-pitched sounds and lower frequencies producing deeper ones. This is why a small guitar string vibrating quickly produces a high note while a thick bass string vibrating slowly produces a low note. Pitch should not be confused with loudness, which depends instead on the amplitude, or intensity, of the sound wave. Musical instruments manipulate frequency through string length, tension, and thickness, or through the length of an air column in wind instruments, to produce specific pitches.

The loudness of a sound is primarily determined by its amplitude, which represents the maximum displacement of particles in the medium from their rest position, with larger amplitude corresponding to more energy carried by the wave and a louder perceived sound. Loudness is measured on the decibel scale, a logarithmic scale where every 10 decibel increase represents roughly a tenfold increase in sound intensity, which is why a jet engine at around 140 decibels is vastly more intense than normal conversation at about 60 decibels. Loudness is distinct from pitch, which depends on frequency rather than amplitude, so a sound can be loud and low-pitched, or soft and high-pitched.

The SI unit used to measure the frequency of any periodic wave, including sound waves, is the hertz (Hz), named after German physicist Heinrich Hertz, who first experimentally confirmed the existence of electromagnetic waves in 1887. One hertz represents one complete cycle or oscillation per second, and the unit applies universally across wave phenomena, from the low frequencies of ocean waves to the gigahertz frequencies of computer processors and radio signals. The decibel, by contrast, measures sound intensity or loudness rather than frequency, and the ohm measures electrical resistance, so it is important not to confuse these related but distinct units in exam questions.

The normal audible frequency range for humans is approximately 20 hertz to 20,000 hertz, though this upper limit typically declines with age, a natural process called presbycusis. Sounds below 20 Hz are called infrasonic, sometimes felt as vibration rather than heard, and are used by elephants and whales for long-distance communication, while sounds above 20,000 Hz are called ultrasonic, used by bats and dolphins for echolocation and by doctors for medical imaging. Many animals, such as dogs, can hear frequencies well above the human range, which is why silent dog whistles operate at ultrasonic frequencies inaudible to people but clearly audible to dogs.