Sound, Light - BPSC (Bihar) PDF Download

Sound

• Human ear can hear vibrations which are between 20 and 20,000 vibrations per second (Hertz). Any sound produced outside this range cannot be heard.
• During an earthquake infrasonic waves pass through the earth’s crust. Infrasonic waves are also produced by the heart. Infrasonic waves are the longitudinal waves of frequency less than 20 Hertz (Hz).
• Ultrasonics have the frequency greater than 20,000 Hz (vibrations per second)
• When electric current applied, certain crystals (quartz, zinc oxide, barium litanate, etc.) produce ultrasonic wave.
• Human ear cannot hear ultrasonics, but dogs, birds, bats and dolphin can hear them.
• Different types of human tissues like bone, fat, muscle have different reflective properties for ultrasonic waves. This fact is used in medical and surgical diagnosis. When a portion of the body is scanned by an ultrasonic beam, the varying echoes are recorded electronically on the screen to form a picture. This method serves a purpose similar to that of X-ray, but is preferable in circumstances where the use of X-ray is inadvisable for health reasons.
• In industry, a control check of brake linings are made by using ultrasonic apparatus.
• Sound waves need medium to propagate. Sound waves propagate in the form of longitudinal waves.
• Velocity of sound is more in solids, less in liquid and least in gases. In air at 0oC the velocity of sound is 330 metre per second, in water 1450 metre per second and iron 5100 metre per second.
• Velocity of sound is independent of pressure. However, increase in temperature increases the velocity of sound.

Similarly, lesser the density, greater will be the velocity of sound.

• For each degree centrigrade rise of temperature, the velocity of sound in air increases by 0.61 metre per second.
• If pitch is higher the sound is shrill and if less, flat. The voice of women have higher pitch than of men. Pitch depends on frequency ; higher frequency has higher pitch.
• Loudness depends upon the amplitude of vibration of sound-making object.
• Quality differentiates the sounds of two person. Quality of a sound depends on the wave form.
• The sound of single frequency is known as the tone. The sound of several frequencies are called a note.
• When two notes of nearly equal pitch are sounded together - a regular rise and fall occurs in the loudness of the tone heard. These alteration in loudness are called beats.
• Tape recorder records sound in the form of magnetic field.
• Humming of a bee is the sound of a higher pitch but lower intensity.
• The distance between two successive crests or, troughs is called its wavelength.
• Longitudinal waves are those, in which the vibrations are in the direction of propagation of the wave, like the sound waves. But in transverse waves vibrations are perpendicular to the direction of propagation of the waves.
• A combination of two or, more notes which produce a pleasant sound is called concord.
• Sweet sounds have greater number of overtones. Overtones are the notes of less intensity and high pitch.
• The greater the energy of a sound wave, the greater is the intensity of sound. The unit of intensity is decibel (db). Intensity of some common sound—ordinary conversation (40 db) ; heavy traffic (100 db) ; rock concert (120 db); jet aircraft (140-150 db) ; machine gunfire (170 db).
• Echoes are produced when sound is reflected or, thrown back. Reflection of sound waves obeys the same laws of reflection of light.
• In order to distinguish between a sound and its echo, the least time interval between them should be more than 0.1 second.
• The least distance between the observer and reflector to hear echo distinctly is about 55 feet or, about 17 metre.
• Smooth and hard surfaces are the best reflector of sound whereas rough, soft, thick, porous materials and heavy curtains are the bad reflector of sound.
• The continuous occurrence of echoes is known as Reverberation. Its common example is the reverberation of thunder, which is due to the continuous reflection of sound between two clouds.
• SONAR (Sound Navigation Ranging) is a method of determining the depth of submerged objects by transmitting ultrasonic waves and receiving the reflected waves.
• The phenomenon of beats is observed if the difference in frequencies of two notes is less than sixteen.
• Modern fishing vessels are equipped with fish lens, which can ultrasonically find the depth of fish shoals.
• The dolphin can detect an individual fish and also its kind from a distance of 50 metre.
• Bats can fly in darkness because of a sonar system of their own which enables them to detect up to 1,20,000 Hz.
• In 1939, in USA, the Tacoma suspension bridge collapsed because of mechanical resonance.
• Doppler effect is the change in observed frequency of sound, light, or other waves, caused by motion of the source or, of the observer.
• Doppler radar is a radar system used to measure the relative velocity of the system and the radar target.
• Supersonic body has the speed greater than that of sound.
• The speed of sound or, sonic speed is Mach 1.
• ‘Mach 2’ means any body having velocity twice to that of sound.

• Radiowaves sent by radio stations are reflected by the ionosphere and can be received anywhere on the earth.
• At night radio reception is better because the ionosphere is more settled in the absence of sunlight.
• TV signals penerate the ionosphere and can be received only over a limited range : the earths’ curvature limits the range of reception. Use of satellite overcome this shortcoming.
• Application of plane mirrors is in kaleidoscope, periscope, and pepper’s ghost.
• Two mirrors face each other in kaleidoscope at an angle of 60^o whereas in periscope at 45^o.
• Periscope enables a person to see over the top of any obstacles.
• In submarine periscope prisms are used instead of mirrors and the tube supporting them incorporates a telescope to extend the range of vision.
• Pepper’s ghost is a method of producing the illusion of a ghost on the theatrical stage.
• Concave mirror forms an erect, magnified image and hence is used in shaving or makeup or in medical diagnosis.
• The first telescope made by Sir Issac Newton was of concave mirror.
• Parabolic mirror is the parabola shaped concave mirror which is used as reflectors in search lights, head lights of motor vehicles and reflecting astronomical telescope.
• Convex mirror always forms erect and diminished image and gives a wide range of view, hence, used as side mirror in the vehicle.
• Convex reflectors deverge the light over a large and extended surface, hence are used in street lamps.
• In refraction, rays passing from rarer medium to denser medium bent towards the normal.

Examples of Refraction—

A stick or spoon dipped in water appears short.

A coin dipped in water appears to be raised.

Twinkling of stars.

• In case of total internal reflection, reflection is 100% as compared to ordinary reflection where it is partial.
• At the refraction angle 90^o, the corresponding angle of incidence is called critical angle.
• Water has a critical angle of about 48.5°.
• Examples of total internal reflection—

Sparkling of a diamond.

Glittering of air bubble in water.

Formation of ‘mirage’ and ‘looming’.

• In mirage successive upward layers from ground are denser, hence the image formed of an object is inverted.
• In loom ing su cces si ve up ward l ayers from the ground are rarer and hence image formed seems to be floating in the air.
• Optical fibres, having great application in telecommunication, is made of fine strands of high quality glass is based on total internal reflection.
• Light waves are transverse.
• For seeing complete image of a man in a plane mirror, the minimum length of the mirror required is half the height of man.
• When an object is placed between two plane mirrors, placed parallel to each other, the number of image formed are infinite and the second image will be the brightest of all.
• Lunar eclipse is formed when the earth comes in between the Sun and the moon.
• Solar eclipse is formed when the moon comes in between the Sun and the earth.
• In torch the light source is placed at the focus of the concave mirror.
• The focal length of a plane mirror is infinite.
• The Sun and moon appear elliptical near the horizon because of refraction.
• Mercury is about 1/3 as far from the Sun as the earth is. Compared with the ellumination received on the earth, that received by mercury will be 1/(1/3)2 = 9 times as much.
• The power of lens = 1/focal length in metre.
• The unit of power of a lens is Dioptre (D). Convex lens has positive power and concave lens negative.
• For a normal human eye, the far point is infinity and near point ranges from 25 to 30 cm. In the case of infant the near point is only 5 to 8 cm.
• The image formed in the retina is real and inverted.
• Each human eye sees an object differently.
• With two eyes, one has better vision than with one eye, because image seen is in three dimensions and the object is visible in depth.
• Colour mixing —

• The splitting of white light into its component colours due to refraction through prism is called dispersion.
• In the spectrum the sequence of coloured lights, from bottom to top, are VIBGYOR.
• In prism, greater the wavelength of a light, greater will be the speed.
• A pure s pec trum is obtai ned by sp ect rome tre or spectroscope.
• Red, blue and green are primary colours; all other colours are secondary colours.
• Corpuscular theory of light assumes that light of different colours is made up of corpuscles of different sizes.
• The failure of lens to form a sharp and distinct image of white object is known as chromatic aberration.
• The technique of recording and reproducing threedimensional images of an object is known as holography.
• The phenomenon of polarisation demonstrates that light waves are transverse.
• The principal difference in construction between reflecting and refracting astronomical telescopes is that in reflecting instrument a concave mirror replaces the objective lens of refracting telescope.