Exam practice

The wave travels along the rope (horizontally), while each part of the rope moves up and down (vertically), at right angles to the direction the wave travels. A wave in which the particles vibrate perpendicular to the direction of travel is a transverse wave.

Seismic waves are mechanical waves. As they travel through the rocks, (1) the rocks absorb part of their energy, and (2) the energy spreads out over a larger and larger area. So the farther a place is from the epicenter, the less energy reaches each part of the ground there, and the weaker the shaking.

To make a picture of a slice of the body, the computer needs to know how much X-ray passes through the body from many directions. By rotating the source (and the detectors opposite it) through 360°, the machine measures from every direction, so nothing stays hidden behind anything else, and the computer can build a detailed cross-sectional (and 3D) image.

A CT scan uses a fairly large dose of X-rays, which can damage cells that are dividing fast, and could harm the growing baby. Ultrasonography uses ultrasound, a sound wave that does no harm to cells, while still showing the baby’s size, position, heartbeat and movements clearly, and it can be repeated as often as needed.

A wave is a periodic disturbance that carries energy from one place to another, without carrying the particles of the medium along with it. For example, a ripple carries energy across a pond while the water only moves up and down.

• i.A transverse wave, as in Figure 10.2: draw a smooth up-and-down curve along a dotted mean line. Label the crest, trough, mean position, amplitude (from the mean line to a crest), wavelength (from one crest to the next), the direction of the wave (along the surface) and the direction of vibration of the water (up and down).
• ii.A longitudinal wave, as in Figure 10.1: draw the mouth on the left and vertical lines across the air, crowded together in some places and spread apart in others. Label the compressions, rarefactions, wavelength (from the middle of one compression to the middle of the next), the direction of the wave (away from the mouth), and the direction of vibration of the air (back and forth, along the same line).

The electromagnetic spectrum is the whole range of electromagnetic waves, arranged in order of wavelength (or frequency): radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays and gamma rays. Radio waves have the longest wavelength and the lowest frequency; gamma rays the shortest wavelength, the highest frequency and the most energy.

Highest frequency: gamma rays. Uses: radiotherapy (destroying cancer cells); sterilizing surgical instruments and food; tracers in nuclear medicine.
Lowest frequency: radio waves. Uses: radio and TV broadcasting; radio telescopes to study the universe.

• i.A is the CT scan picture: it is a 3D image built by a computer from many slices, so the bones can be seen in their real shape, from any side. B is the X-ray photograph: a single flat shadow picture, in which bones show white and overlap one another.
• ii.X-ray photography: the patient is placed between the X-ray tube and the film or detector; a short burst of X-rays passes through the body; bones absorb most, soft tissue less, air almost none; the shadows are recorded on the film or detector and shown on a screen. CT scan: the patient lies on a table that slides into the ring of the scanner; an X-ray source and the detectors opposite it rotate around the body, measuring from every direction; the computer turns the measurements into slice images and, if needed, a 3D image.
• iii.X-ray photography: broken bones, teeth roots, lung infections, swallowed objects. CT scan: internal injuries and bleeding (for example in the brain), tumors, blood clots, and the condition of the lungs, brain and other organs.

1. Echolocation: bats and dolphins find their way and their prey.
2. SONAR and the fathometer: measuring the depth of the sea and finding fish, submarines and shipwrecks.
3. Ultrasonography: seeing organs inside the body and a baby in the womb.
4. Ultrasonic rulers, parking sensors and burglar alarms: measuring distances and detecting obstacles and intruders.

(Also: stethoscopes and megaphones guide sound by reflection; and concert halls are designed so reflections reach the audience well.)

1. Rohan stands in front of his own house, facing the neighbor’s house, and makes a short, sharp sound (a clap, or by hitting two pieces of wood together).
2. A friend starts a stopwatch at the clap and stops it when the echo from the neighbor’s wall is heard. (To time it better, he can clap many times in a steady rhythm, so that each clap falls exactly on the echo of the one before; then the time between claps equals the echo time.)
3. He repeats it a few times and takes the average time t.
4. The sound travels to the house and back, so distance d = (speed of sound × t) ÷ 2. For example, if t = 0.3 s: d = 340 × 0.3 ÷ 2 = 51 m.

(The echo is heard separately only if the house is at least about 17 m away.)

An ultrasonic sensor module (HC-SR04) costs a few hundred rupees and has the sender and receiver side by side. Add a microcontroller board (such as an Arduino), a buzzer, and a battery.

Parts: an ultrasonic sensor (sender and receiver), a small microcontroller board, a buzzer (or an LED), wires, and a battery.
How it works:

1. The sender sends a short pulse of ultrasound towards the doorway, several times a second.
2. The pulse reflects off the opposite wall (or a person) and the receiver catches the echo.
3. The microcontroller measures the echo time and works out the distance (distance = 343 m/s × time ÷ 2).
4. Normally the distance is that of the wall. If someone walks through the doorway, the echo comes back sooner (the distance is shorter), and the microcontroller switches on the buzzer.

Diagram: a box labeled sender and receiver on one side of a doorway; arrows from the sender to the opposite wall and back to the receiver; a person crossing the beam; wires from the sensor to the controller board, and from it to a buzzer and a battery.

Acoustic protection is the use of soft, porous materials (foam boards, perforated panels, curtains, carpets, rough walls) on the walls, ceilings and floors of a room to absorb unwanted sound, so that sound isn’t reflected again and again.
Uses: it makes speech and music clear in cinema halls, auditoriums, music rooms, recording studios (such as Radio Nepal’s) and seminar halls, and it keeps noise from passing in or out.

Ultrasonography is the technology of examining the organs inside the body with ultrasound.

Procedure: gel is put on the skin and a transducer is pressed on it. The transducer sends high-frequency ultrasound into the body; part of it is reflected at each boundary between tissues, and the transducer receives the echoes. A computer uses the strength and the time of each echo to build a picture, the sonogram, on a screen.

Uses: checking stones in the kidneys and gall bladder; tumors; diseases of the liver, intestines and uterus; the growth, position and heartbeat of a baby in the womb; and the heart and its valves (echocardiography). It is safe, as it uses no X-rays.

Go through the spectrum in order, with two or three uses of each part, and end with a paragraph on safety.

The role of electromagnetic waves in modern life

From the moment our alarm rings to the moment we switch off the light, electromagnetic waves serve us. They travel at the speed of light, need no medium, and come in a huge range of wavelengths, from radio waves kilometers long to gamma rays smaller than an atom. Each part of this spectrum has its own job.

Radio waves carry radio and TV broadcasts to every corner of Nepal, even to villages with no roads. Microwaves carry our phone calls, Wi-Fi, Bluetooth and satellite signals, guide airplanes by radar, and heat our food in a microwave oven. Infrared rays switch our TVs from a remote, keep us warm in heaters, measure our temperature in a second, and let security cameras see at night; most of the internet’s data travels as infrared light in optical fibers.

Visible light lets us see, powers photosynthesis, and brings us every picture on a phone screen. Ultraviolet rays make vitamin D in our skin, disinfect drinking water by the SODIS method, and make tube lights glow. X-rays show doctors broken bones and lung disease, and check luggage at airports; in CT scanners they reveal the brain slice by slice. Gamma rays destroy cancer cells, sterilize medical instruments and keep food fresh.

Electromagnetic waves also let us explore the universe: radio, infrared and X-ray telescopes see what our eyes never could.

But the high-energy end of the spectrum must be used with care. Ultraviolet causes sunburn and skin cancer; X-rays and gamma rays can damage cells and genes. With sensible protection (sunscreen and sunglasses, lead aprons, short exposures), electromagnetic waves are among the most useful tools humans have ever had. Modern life, from communication and medicine to security and science, would be impossible without them.