Physics Notes Part 2

 v Waves can be drawn using lines called wavefronts. The to-and-fro movements of coils are oscillations. Transverse waves oscillate at right angles to the direction of travel. Light, electromagnetic waves, radio waves, microwaves, infrared, x-rays, ultraviolet and gamma rays are transverse waves.

v Bunched-up sections of coils are compressions and stretched-out sections of coils are rarefractions. The backward and forward oscillations are longitudinal waves, they oscillate parallel to the direction of travel. Sound wave is a longitudinal wave.

v Seismic waves are earthquake waves. Primary seismic waves can travel huge distances through the Earth. They are longitudinal. Secondary seismic waves are slower. They are transverse. Surface seismic waves are the most destructive. They shake the ground from side to side.

v The speed of waves is a metre per second. Frequency is the number of waves passing any point per second. Wavelength is the distance between any point on a wave and the equivalent point on the next. Amplitude is the maximum distance a point moves from its rest position when a wave passes.

v Reflection: A vertical surface is put in the path of waves. The waves reflected from the surface at the same angle as they strike it. Refraction: A flat piece of plastic makes the water more shallow, which slows the waves down. When waves slow, they change direction. Diffraction: The waves bend around the sides of an obstacle, or spread out as they pass through a gap. Wider the gap, less diffraction. It is only significant if the size of the gap is about the same as the wavelength.

v  Sound waves require a medium to travel through. Hard surfaces reflect sounds and can cause echoes. Sound waves can be displayed graphically using a microphone and oscilloscope. High frequency has high pitch while low frequency has low pitch. Oscillations in air are bigger and the amplitude of the waveform is greater.

v The human ear can detect sounds up to a frequency of about (20KHz) 20000Hz. Sounds above human hearing are called ultrasonic sounds or ultrasound. Ships use echo-sounders to measure the depth of water beneath them. Ultrasound is used for metal testing, scanning the womb and cleaning machinery, and breaking kidneys, and gall stones.

v Sun, lamp, laser and glowing TV screen emit their light. They are luminous. In most non-luminous objects, other light bounces off them, so they reflect light. The black surface absorbs nearly all light striking it. Transparent materials like glass, and water let light pass right through them. They transmit light. In the diagram, lines called rays are used to show which way light is going. Lasers emit light of a single wavelength and color is monochromatic light.

v Incoming ray is incident ray, outgoing ray is reflected ray. Line at right angles to the mirror’s surface is normal.

v The image seen in the mirror looks the same as the object apart from one important difference. The image is laterally inverted (back to front). The real image is formed when the rays meet on the screen. Virtual image is formed when no rays meet or when the image isn’t formed on the screen.

v The bending effect is refraction. The refractive index of a medium is the speed of light in a vacuum divided by the speed of light in the medium. A prism is a triangular block of glass or plastic. As the sides of the prism aren’t parallel, so light is refracted by the prism, when it comes out in a different direction. It is deviated. If a narrow beam of white light is passed through a prism, it splits into a range of colours called a spectrum. This effect is called dispersion. The diagrams which show three rays at different angles are called critical angles. The angle of incidence is greater than the critical angle, and there is no refracted ray. All light is reflected. This effect is total internal reflection.

v Reflecting prisms are used in periscopes, rear reflectors and binoculars. Ultraviolet, x-ray, and gamma-ray cause ionization.

v Optical fibres are very thin, flexible rods made of special glass or transparent plastic. They carry telephone calls and internet data. The signals are coded and sent along the fibre as pulses of laser light. Fewer booster stations are needed than with electrical cables.

v Endoscope an instrument used by surgeons to look inside the body. An endoscope contains a long, thin bundle of optical fibres.

v Snell’s Law states sin of incidence angle divided by the sin of refracted angle is constant.

v Convex lenses are thickest in the middle and thin around the edge. The point where they converge is called the principal focus. Its distance from the centre of the lens is focal length. A convex lens is known as a converging lens. A real image is produced in the focal plane if the object is far from the lens. If the object is closer to the lens than the principal focus, it forms a virtual image. It is often used as a magnifying glass and camera.

v Concave lenses are thin in the middle and thickest around the edge. The principal focus is the point from which the rays appear to diverge. A concave lens is known as a diverging lens. It always forms small, upright and virtual images.

v 3 standard rays: A ray through the centre passes straight through the lens. A ray parallel to the principal axis passes through F after leaving the lens. A ray through F’ leaves parallel to the principal axis.

v Short sight: Lens can’t be made thin to look at distant objects. To correct the fault, a concave lens. Long sight: Lens can’t be made thick to look at close objects. To correct the fault, a convex lens. In middle age, eye lenses become less flexible so people use bifocal spectacles whose lenses have a top part for distant objects and a bottom part for close ones. Progressive spectacles are for gradual changeover.

v Light waves belong to electromagnetic waves. The speed of light is 3 x 10⁸ m/s. (exact is 299792458 m/s). A full range of electromagnetic waves is called an electromagnetic spectrum.

v Radio waves: Stars are natural emitters. It can be detected by radio telescopes. Long and medium waves: It will diffract around hills and the round curved surface of the Earth. So a radio can catch signals. VHF and UHF waves: It has short wavelengths. VHF (Very High Frequency) is used for stereo radio and UHF (Ultra High Frequency) for TV broadcasts. It doesn’t diffract. So for good reception, a straight path is required. Microwaves: It has the shortest wavelength and highest frequencies. It is used by mobile phones, WiFi, beaming TV, data and telephone signals to and from satellites across the country.

v Infrared radiation and light: It produces a heating effect. All objects emit some infrared because of the motion of atoms. Most radiate a wide range of wavelengths. As an object heats up, it radiates more infrared and shorter wavelengths. Shorter wavelength infrared is often called infrared light, even though it is invisible. Security alarms and lamps are switched on due to infrared.

v Ultraviolet radiation: Very hot objects, such as Sun emit some of their radiation beyond the violet end of the visible spectrum. It is sometimes called ultraviolet light, even though it is invisible. Sun’s UV is harmful to living cells. It can cause skin cancer, damage the retina in the eye and cause blindness. Black/dark skin absorbs UV before it penetrates deep. It is used to sterilize equipment to kill bacteria. Water is sterilized too. Fluorescence: They convert their energy into visible light and glow. Security marker pens contain invisible ink, which glows when UV light is shone on it.

v Xrays: It is given off when fast-moving electrons lose energy quickly. Short-wavelength X-rays are highly penetrating. A dense metal reduces the speed of X-rays. Long wavelengths are less penetrating. It is used to detect flaws and weapons hidden in luggage. It is dangerous as it can cause cancer or genetic change (mutation). Concentrated X-ray beams treat cancer by destroying abnormal skills.

v Gamma rays: It comes from radioactive materials. It is produced when unstable nuclei break up or lose energy. They have shorter wavelengths than X-rays. It is used for the same purposes as xrays.

v An encoder(microphone) turns incoming information(speech) into transmitted form(electrical signals). They pass along the transmission path(wires) to a decoder(earphone). This turns signals back to speech. Sound waves entering the microphone, make the voltage vary. A continuous variation is called an analogue signal. It is converted to digital signals represented by numbers. The measurement is changed into binary codes (0’s and 1’s).

v Signals lose power as they travel along is attenuation. They are spoilt by noises(electrical interference). To restore their power and quality, digital pulses can be ‘cleaned up’ and amplified at different stages by regenerators.

v Optical fibres: At the transmitting end, electrical signals are encoded into light by an LED(light-emitting diode) or laser diode. At the receiving end, signals are decoded by the photodiode, which turns them back into electric cables.