*1-The periodic time decreases with the increase in frequency.
Because The periodic time inversely proportional to frequency.
*2. Sound waves are mechanical longitudinal waves.
because they need a medium to propagate through it and
its particles vibrate along the direction of the wave propagation in form of compressions and rarefactions.
*3. Water waves are mechanical transverse waves.
because they need a medium to propagate through it and
its particles vibrate perpendicular to the direction of the wave propagation in form of crests and troughs..
*4. Light (or radio) waves are electromagnetic waves.
Because they do not need a medium to propagate through it.
5. On the moon surface, astronauts use electromagnetic waves instead of mechanical waves.
Because the mechanical waves need a medium to propagate through it, while the electromagnetic waves propagate in vacuum
*6. Sound does not travel in vacuum.
Because the sound is mechanical waves need a medium to propagate through it.
*7-. We hear sound from all the directions that surround the sound source.
Because sound travels through air as pulses of compressions and rarefactions whose center is the source of sound. ;
8-The flame of a candle vibrates forward and backward if we put the candle in front of a loudspeaker.
Due to the vibration of particles of air between the candle and the loudspeaker causing the vibration of the candle’s flame.
*9-. Some sound waves cannot be heard. Because the frequencies of these waves are lower than 20 Hz or more than 20000 Hz, but the human ear can hear only the waves that their frequencies range between (20 : 20000) Hz.
10- The human ear can hear sounds of frequencies ranging from 20 to 20000 Hz
Because the ear transmits the effect of these waves to the biain which translates them to sound and audible tones.
11-. Some animals can hear the sounds which cannot be heard by man.
Because they can hear non-audible sounds that the human brain can’t translate them into audible tones.
12- The ultrasonic waves are important to sailors.
Because they use these waves in estimating the depth of the sea beds and other bodies (fish) under the water.
13-. We cannot hear echo in small rooms.
Because for hearing the echo, the distance between the source of sound and the reflecting surface should not be less than 17 meters.
14- Echo cannot be heard if the distance between the sound source and the reflecting surface is less than 17 meters.
Because the time interval between hearing the original sound and its echo will be less than 0.1 sec. and the human ear cannot distinguish between the two sounds.
15-. The reflection of ultrasonic waves can be used in detecting the defects of welding of metals.
Because the reflected waves from defect areas of welding have different intensity than those from well welded areas.
16- Ceilings and walls of old mosques are concave.
to make the sound more clear and to increase its intensity.
*17-. Bats can fly in the dark without hitting (colliding) an obstacle.
Because they produce ultrasonic waves that reflect on the surfaces and barriers, then they receive them back to locate the positions of surfaces and avoid them.
*18-. Noise pollution is very harmful for human’s health.
Because it causes fatigue of the nervous system, raising of blood pressure, headache, general loss of concentration, reacting violently and complete or partial deafness.
*19- Noisy factories are moved outside the residential areas.
To limit the noise pollution, since they produce sounds whose intensity is more than 70 decibels which have dangerous effects on human’s health.
*20. Corona is seen around the Sun (or the Moon).
Due to the refraction of light rays when passing through ice crystals found in air and its deviation with definite angles, then the light reaches the eye as if it was coming from a circle around the Sun or the Moon and the corona can be seen.
*21. We see lightning first, then we hear thunder.
Because the velocity of light is greater than the velocity of sound.
22. The energy of violet light quanta is larger than that of the red one.
Because the frequency of violet light is greater than that of the red one.
*23. The perpendicular incident ray on a reflecting surface reflects on itself.
Because the angle of incidence = the angle of reflection = zero.
24-When a light ray passes through a glass, it deviates.
Due to the difference of the velocity of light through the different transparent media.
25- The pencil appears broken when a part of it is placed in water.
Due to the refraction of light when transfers from water (high optical density) to air (low optical density).
*26. • A coin appears in a higher position than its true position when it is seen from air.
Because the eye see the extensions of the refracted rays, so the object appears in an apparent position higher than the real position.
27- The inaccurate estimation of the eye to the position of a fish in water.
Because the eye see the extensions of the refracted rays, so the object appears in an apparent position higher than the real position.
*28-. To enable Fatma to pick up a coin falling
in a deep beaker, she must look at it in the position
shown in the opposite shape.
Because the light ray which is incident perpendicular to the separating surface between the air and water passes without refraction, so the true position of the coin can be determined.
*29-. The refractive index of any transparent medium is always greater than one.
Because the velocity of light through air is always greater than its velocity through any other transparent medium.
30- When you look at a plane mirror, you find that you hold the pen by the left hand which is inverse the real position.
Because the formed image in the plane mirror is reversed (laterally inverted in position).
31-. The word AMBULANCE is written in a reversed way on the car equipped to carry sick persons.
To let the drivers of the front cars see the word correctly.
*32. Silver spoon is considered as a spherical mirror
Because its internal face is a concave mirror and its external face is a convex mirror.
33. The image formed in a plane mirror is a virtual image.
Because it cannot be received on a screen.
34. The concave mirror has real focus, while the convex mirror has virtual focus.
Because the focus of the concave mirror is formed by the intersection of the reflected light rays, while the focus of the convex mirror is formed by the intersection of the extensions of the reflected light rays.
35. The light ray that is incident on a concave mirror passing through its center of curvature reflects on itself.
Because this ray falls perpendicular to the surface of the mirror and the angle of incidence the angle of reflection
*36. The image formed by a convex mirror cannot be received on a screen.
Because it is a virtual image as it is formed by the extensions of the reflected light rays.
37. A convex mirror is put at the left side of the car’s driver.
To give virtual, inverted and diminished images for the way behind the car.
38. Spherical mirror has one focus, while the lens has two foci.
Because the mirror is a part of a sphere, while the lens is a part of two spheres.
39. Convex lens has a real focus, but the concave lens has a virtual one.
Because the focus of the convex lens is formed by the intersection of the refracted light rays, while the focus of the concave lens is formed by the intersection of the extensions of the refracted light rays.
40. The image formed by the concave lens cannot be received on the screen.
Because it is virtual image is formed as a result of the intersection of the extensions of the refracted rays.
.41. The convex lens is used for burning paper with the sun’s rays, while the concave lens cannot be used.
..Because the convex lens converges the rays, while the concave lens diverges the rays.
42. The convex lens converges the light rays, but the concave lens diverges the light rays.
Because the convex lens refracts the rays towards the principal axis, but the concave lens refracts the rays away from the principal axis.
43. The images formed by the simple microscope are virtual.
Because they are formed due to the intersection of extensions of the refracted rays.
44. Telescopes provide a great help to scientists.
Because they are used to form enlarged images for the heavenly bodies.
.
*45- Bean flower is typical flower. .
Because it contains the four whorls of the flower.
46- The flower of gelly plant is bisexual flower. .
. Because it contains both male and female reproductive organs (androecium and gynoecium).
47-. The flowers of some plants are male.
Because they only contain the male reproductive organ (androecium).
*48. Petals of some flowers are coloured and contain plenty of nectar.
To attract insects which carry pollen grains from anthers.
*49-. Pollination in flowers whose anthers are hanging is carried out by air.
.Because the hanging anthers are opened by the effect of air movement (wind), therefore the pollen grains disperse from them in air.
*50-. Stigmas of air pollinated flowers are feather-like and sticky.
. To collect (pick) the pollen grains that spread in air
Because The periodic time inversely proportional to frequency.
*2. Sound waves are mechanical longitudinal waves.
because they need a medium to propagate through it and
its particles vibrate along the direction of the wave propagation in form of compressions and rarefactions.
*3. Water waves are mechanical transverse waves.
because they need a medium to propagate through it and
its particles vibrate perpendicular to the direction of the wave propagation in form of crests and troughs..
*4. Light (or radio) waves are electromagnetic waves.
Because they do not need a medium to propagate through it.
5. On the moon surface, astronauts use electromagnetic waves instead of mechanical waves.
Because the mechanical waves need a medium to propagate through it, while the electromagnetic waves propagate in vacuum
*6. Sound does not travel in vacuum.
Because the sound is mechanical waves need a medium to propagate through it.
*7-. We hear sound from all the directions that surround the sound source.
Because sound travels through air as pulses of compressions and rarefactions whose center is the source of sound. ;
8-The flame of a candle vibrates forward and backward if we put the candle in front of a loudspeaker.
Due to the vibration of particles of air between the candle and the loudspeaker causing the vibration of the candle’s flame.
*9-. Some sound waves cannot be heard. Because the frequencies of these waves are lower than 20 Hz or more than 20000 Hz, but the human ear can hear only the waves that their frequencies range between (20 : 20000) Hz.
10- The human ear can hear sounds of frequencies ranging from 20 to 20000 Hz
Because the ear transmits the effect of these waves to the biain which translates them to sound and audible tones.
11-. Some animals can hear the sounds which cannot be heard by man.
Because they can hear non-audible sounds that the human brain can’t translate them into audible tones.
12- The ultrasonic waves are important to sailors.
Because they use these waves in estimating the depth of the sea beds and other bodies (fish) under the water.
13-. We cannot hear echo in small rooms.
Because for hearing the echo, the distance between the source of sound and the reflecting surface should not be less than 17 meters.
14- Echo cannot be heard if the distance between the sound source and the reflecting surface is less than 17 meters.
Because the time interval between hearing the original sound and its echo will be less than 0.1 sec. and the human ear cannot distinguish between the two sounds.
15-. The reflection of ultrasonic waves can be used in detecting the defects of welding of metals.
Because the reflected waves from defect areas of welding have different intensity than those from well welded areas.
16- Ceilings and walls of old mosques are concave.
to make the sound more clear and to increase its intensity.
*17-. Bats can fly in the dark without hitting (colliding) an obstacle.
Because they produce ultrasonic waves that reflect on the surfaces and barriers, then they receive them back to locate the positions of surfaces and avoid them.
*18-. Noise pollution is very harmful for human’s health.
Because it causes fatigue of the nervous system, raising of blood pressure, headache, general loss of concentration, reacting violently and complete or partial deafness.
*19- Noisy factories are moved outside the residential areas.
To limit the noise pollution, since they produce sounds whose intensity is more than 70 decibels which have dangerous effects on human’s health.
*20. Corona is seen around the Sun (or the Moon).
Due to the refraction of light rays when passing through ice crystals found in air and its deviation with definite angles, then the light reaches the eye as if it was coming from a circle around the Sun or the Moon and the corona can be seen.
*21. We see lightning first, then we hear thunder.
Because the velocity of light is greater than the velocity of sound.
22. The energy of violet light quanta is larger than that of the red one.
Because the frequency of violet light is greater than that of the red one.
*23. The perpendicular incident ray on a reflecting surface reflects on itself.
Because the angle of incidence = the angle of reflection = zero.
24-When a light ray passes through a glass, it deviates.
Due to the difference of the velocity of light through the different transparent media.
25- The pencil appears broken when a part of it is placed in water.
Due to the refraction of light when transfers from water (high optical density) to air (low optical density).
*26. • A coin appears in a higher position than its true position when it is seen from air.
Because the eye see the extensions of the refracted rays, so the object appears in an apparent position higher than the real position.
27- The inaccurate estimation of the eye to the position of a fish in water.
Because the eye see the extensions of the refracted rays, so the object appears in an apparent position higher than the real position.
*28-. To enable Fatma to pick up a coin falling
in a deep beaker, she must look at it in the position
shown in the opposite shape.
Because the light ray which is incident perpendicular to the separating surface between the air and water passes without refraction, so the true position of the coin can be determined.
*29-. The refractive index of any transparent medium is always greater than one.
Because the velocity of light through air is always greater than its velocity through any other transparent medium.
30- When you look at a plane mirror, you find that you hold the pen by the left hand which is inverse the real position.
Because the formed image in the plane mirror is reversed (laterally inverted in position).
31-. The word AMBULANCE is written in a reversed way on the car equipped to carry sick persons.
To let the drivers of the front cars see the word correctly.
*32. Silver spoon is considered as a spherical mirror
Because its internal face is a concave mirror and its external face is a convex mirror.
33. The image formed in a plane mirror is a virtual image.
Because it cannot be received on a screen.
34. The concave mirror has real focus, while the convex mirror has virtual focus.
Because the focus of the concave mirror is formed by the intersection of the reflected light rays, while the focus of the convex mirror is formed by the intersection of the extensions of the reflected light rays.
35. The light ray that is incident on a concave mirror passing through its center of curvature reflects on itself.
Because this ray falls perpendicular to the surface of the mirror and the angle of incidence the angle of reflection
*36. The image formed by a convex mirror cannot be received on a screen.
Because it is a virtual image as it is formed by the extensions of the reflected light rays.
37. A convex mirror is put at the left side of the car’s driver.
To give virtual, inverted and diminished images for the way behind the car.
38. Spherical mirror has one focus, while the lens has two foci.
Because the mirror is a part of a sphere, while the lens is a part of two spheres.
39. Convex lens has a real focus, but the concave lens has a virtual one.
Because the focus of the convex lens is formed by the intersection of the refracted light rays, while the focus of the concave lens is formed by the intersection of the extensions of the refracted light rays.
40. The image formed by the concave lens cannot be received on the screen.
Because it is virtual image is formed as a result of the intersection of the extensions of the refracted rays.
.41. The convex lens is used for burning paper with the sun’s rays, while the concave lens cannot be used.
..Because the convex lens converges the rays, while the concave lens diverges the rays.
42. The convex lens converges the light rays, but the concave lens diverges the light rays.
Because the convex lens refracts the rays towards the principal axis, but the concave lens refracts the rays away from the principal axis.
43. The images formed by the simple microscope are virtual.
Because they are formed due to the intersection of extensions of the refracted rays.
44. Telescopes provide a great help to scientists.
Because they are used to form enlarged images for the heavenly bodies.
.
*45- Bean flower is typical flower. .
Because it contains the four whorls of the flower.
46- The flower of gelly plant is bisexual flower. .
. Because it contains both male and female reproductive organs (androecium and gynoecium).
47-. The flowers of some plants are male.
Because they only contain the male reproductive organ (androecium).
*48. Petals of some flowers are coloured and contain plenty of nectar.
To attract insects which carry pollen grains from anthers.
*49-. Pollination in flowers whose anthers are hanging is carried out by air.
.Because the hanging anthers are opened by the effect of air movement (wind), therefore the pollen grains disperse from them in air.
*50-. Stigmas of air pollinated flowers are feather-like and sticky.
. To collect (pick) the pollen grains that spread in air
