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The Giant Planets

The Giant Planets

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THE GIANT PLANETS :

 

 

 

 

 


 

 

 

 

 

 

 

 

 

1. WHAT ARE SATURN’S RINGS?
Ans: Saturn’s rings are the planets shining halo, first seen by Galileo Galilei ( I564-1642), who invented the first simple telescope in 1609. The rings are made of countless billions of tiny chips of ice and dust, few bigger than a refrigerator and most the size of ice cubes. The rings are incredibly thin no more than 50 m deep yet they stretch from 7,000 km to 74,000 km out into space. One of Saturn’s rings is as thin as a piece of tissue paper being stretched over a football pitch.

 


2. HOW WINDY IS SATURN?
Ans: Saturn’s winds are even faster th roar round up to I,800 km/h. But Neptune’s a even faster!Jupiter’s and the planet.

 


3. WHAT IS THE CASSINI DIVISION?

Ans: Saturn’s rings occur in broad bands referred to by the letters A to G. In 167 the astronomer Cassini spotted a dart gap between rings A and B. This is now called the Cassini division, after him.

 


4. HOW MANY MOONS HAS SATURN?

Ans: Saturn has at least 18 moons, including Lapetus, which is dark on one side and light on the other.

 


5. HOW BIG IS JUPITER?
Ans: Very big. Even though Jupiter is largely gas, it weighs 320 times as much as the Earth and is 142,984 km in diameter.

 


6. HOW LONG DOES IT TAKE JUPITER TO ORBIT THE SUN?
Ans: Jupiter takes 11 years and 314 days (by our Earth calendar) to complete its journey around the Sun.

 


7. WHAT ARE THE GIANT PLANETS?
Ans: Jupiter and Saturn, the fifth and sixth planets out from the Sun, are the giants of the solar system. Jupiter is twice as heavy as all the planets put together. Saturn is almost as big. Unlike the inner planets, they are both made largely of gas, and only their very core is rocky. This does not mean they are vast cloud balls. The enormous pressure of gravity means the gas is squeezed until it becomes liquid, and even solid.

 


8. COULD YOU LAND ON JUPITER?
Ans: No. Even if your spaceship could withstand the enormous pressures, there is no surface to land on – the atmosphere merges unnoticeably into deep oceans of liquid hydrogen.

 


9. HOW FAST DOES JUPITER SPIN?
Ans: Jupiter spins faster than any other planet. Despite its huge size, it turns right around in just 9 hours 55 minutes, which means the surface is moving at 45,000 km/h!

 

 

The Inner Planets

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THE INNER PLANETS :

 

 

 

 

 

 

 

 

 

1. WHAT ARE THE INNER PLANETS?
Ans: The inner planets are the four planets in the solar system that are nearest to the Sun. These planets- Mercury, Venus, Earth and Mars are small planets made of rock, unlike the bigger planets further out, which are made mostly of gas. Because they are made of rock, they have a hard surface a spaceship could land on, which is why they are sometimes called terrestrial(earth) planets. They all have a thin atmosphere, but each is very different.

 

2. WHY IS MARS RED?
Ans: Mars is red because it is rusty.The surface contains a high proportion of iron dust, and this has been oxidized in the carbon dioxide atmosphere.

 

3. WHAT’S FRIGHTENING ABOUT MARS’S MOONS?
Ans: One night American astronomer Asaph Hall got fed up with studying Mars and decided to go to bed.But his domineering wife bullied him into staying up and that night he discovered Mars’s two moons.Mocking his fear of his wife,he named the moons Phobos (fear) and Deimos (panic), after the attendants of the Roman war god, Mars.

 

4. LIFE ON MARS?
Ans: The Viking landers of the 1970 s found no trace of life. Then, in 1996, microscopic fossils of what might be mini-viruses were found in a rock from Mars.These turned out not to be signs of life after all.

 

5. WHY IS VENUS CALLED THE EVENING STAR?
Ans: Venus reflects sunlight so well it shines like a star. But because it is quite close to the Sun, we can see it in the evening, just after the Sun sets.We can also see it just before sunrise.

 

6. WHAT ARE THE INNER PLANETS MADE OF?
Ans: Each of the inner planets is formed a little bit like an egg with a hard “shell” or crust of rock, a “white” or mantle of soft, semi-molten rock, and a”yolk” or core of hot, often molten, iron and nickel.

 

7. WHAT CANYON IS BIGGER THAN THE GRAND CANYON?
Ans: A canyon on Mars! The surface of Mars is more stable than Earth’s, and there is no rain or running water to wear down the landscape. It has a volcano called Olympus Mons which is 17 miles (27 km s)high three times higher than Mount Everest. It also has a great chasm, discovered by the Mariner 9 space probe and called the Valles Marineris. This is
over 4,000 km long and four times as deep as America’s Grand Canyon.

 

8. COULD YOU BREATHE ON MERCURY?
Ans: Not without your own oxygen supply. Mercury has almost no atmosphere-just a few wisps of sodium because gases are burned off by the nearby Sun.

 

9. WHAT IS THE AIR ON VENUS?
Ans: Venus’s atmosphere would be deadly for humans. It is very deep, so the pressure on the ground is huge. It is made mainly of poisonous carbon dioxide and is also filled with clouds of sulphuric acid.

 

10. HOW HOT IS MERCURY?
Ans: Temperatures on Mercury veer from one extreme to the other because it has too thin an atmosphere to insulate it. In the day, temperatures soar to 400° C; at night they plunge to -175°C.

 

 

THE SUN

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THE SUN :

 

 

 

 

 

 

 

 

 

 

 

 

 

1. WHAT IS THE SUN?
Ans: The Sun is an average star, just like countless others in the universe. It formed from gas left behind after an earlier, much larger star blew up and now, in middle-age, burns yellow and fairly steadily – giving the Earth daylight and remarkably constant temperatures. Besides heat and light, the Sun sends out deadly gamma rays, X-rays and ultraviolet, as well as infrared and radio waves. Fortunately we are shielded from these by Earth’s magnetic field and atmosphere.

 

 

2. WHAT IS A SOLAR ECLIPSE?

Ans: A solar eclipse is when the Moon moves in between the Sun and the Earth, creating a shadow a few hundred kilometres wide on the Earth.

 

 

3. WHAT MAKES THE SUN BURN?

Ans: The Sun gets its heat from nuclear fusion.Huge pressures deep inside the Sun force the nuclei (cores) of hydrogen atoms to fuse together to make helium atoms, releasing huge amounts of nuclear energy.

 

 

4. WHAT IS THE SUN’S CROWN?

Ans: The Sun’s crown is its corona, its glowing white hot atmosphere seen only as a halo when the rest of the Sun’s disc is blotted out by the Moon in a solar eclipse.

 


5. HOW HOT IS THE SUN?
Ans: The surface of the Sun is a phenomenal 5500″C, and would melt absolutely anything. But its core is thousands of times hotter at over 15 million C.

 


6. WHAT IS THE SOLAR WIND?
Ans: The solar wind is the stream of radioactive particles constantly blowing out from the Sun at hundreds of kilometres per second. The Earth is protected from the solar wind by its magnetic field, but at the Poles the solar wind interacts with Earth’s atmosphere to create the aurora borealis or northern lights.

 


7. HOW OLD IS THE SUN?
Ans: The Sun is a middle-aged star. It probably formed about 4.6 billion years ago. It will probably burn for another five billion years and then die in a blaze so bright that the Earth will be scorched right out of existence.

 


8. WHAT ARE SUNSPOTS?
Ans: Sunspots are dark blotches seen on the Sun’s surface. They are thousands of kilometres across, and usually occur in pairs. They are dark because they are slightly less hot than the rest of the surface. As the Sun rotates, they slowly cross its face – in about 37 days at the equator and 26 days at the Poles. The average number of spots seems to reach a maximum every I| years, and many scientists believe these sunspot maximums are linked to periods of stormier weather on Earth.

 


9. WHAT ARE SOLAR FLARES?
Ans: Flares are eruptions from the Sun’s surface that fountain into space with the energy of one million atom bombs for about five minutes. They are similar to solar prominences, the giant flame-like tongues of hot hydrogen that loop 32,000 km into space.

 


10. HOW BIG IS THE SUN?
Ans: The Sun is a small-to-medium-sized star 1,392,000 km in diameter. It weighs just under 2,000 trillion trillion tonnes.

 

 

The Earth

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THE EARTH 

 

 

 

 

 

 

 

 

 

 

1.HOW DID THE EARTH BEGIN ?
Ans: Around 4.6 billion years ago, neither the Earth nor any of the other planets existed. There was just this vast, dark, very hot cloud of gas and dust swirling around the newly formed Sun. Gradually,the cloud cooled and the gas began to condense into billions of droplets. Slowly these droplets were pulled together into clumps by their own gravity and they carried on clumping until all the planets, including the Earth, were formed.But it took another half a billion years before the Earth had cooled enough to form a solid crust with an atmosphere around it.

 

 

 2.HOW BIG IS THE EARTH ?
Ans: Satellite measurements show it is 40,075 km around the equator and 12,757 km across. The diameter between the poles is slightly less by 43 km.

 

 

 3.HOW OLD IS THE EARTH?
Ans: The Earth is about 4.6 billion years old. The oldest rock is about 3.8 billion years old. Scientists have also dated meteorites that have fallen from space, and must have formed at the same time as the Earth.

 

 

4.WHAT SHAPE IS THE EARTH?
Ans: The Earth is not quite a perfect sphere. Because it spins faster at the equator than at the Poles,Earth bulges at the equator. Scientists describe Earth’s shape as “geoid”,which simply means Earth-shaped!

 

 

5.WHAT’S SO SPECIAL ABOUT THE EARTH?
Ans: The Earth is the only planet with temperatures at which liquid can exist on the surface and is the only planet with an atmosphere containing oxygen. Water and oxygen are both needed for life.

 

 

6.WHY DOES THE EARTH SPIN?
Ans: Earth spins because there is nothing to stop it spinning. The Sun’s gravity keeps it in orbit.

 

 

7.EXACTLY HOW LONG IS A YEAR?
Ans: Every year the Earth travels once around the Sun. This epic journey covers a distance of 938,886,400 km and takes exactly 365.24 days,which gives us our calendar year of 365 days. To make up the extra 0.24 days, we add an extra day to our calendar at the end of February in every fourth year which is called the leap year and then we have to knock off a leap year every four centuries.

 

 

8. HOW LONG IS A DAY?
Ans: A day is the time Earth takes to turn once. The stars move to the same place in the sky every 23 hours, 56 minutes and 4.09 seconds (the sidereal day). Our day (the solar day)is 24 hours, because Earth is moving around the Sun, and must turn an extra for the Sun to be in the same place in the sky.

 

 

9. WHO WAS COPERNICUS?
Ans: In the 1500 s, most people thought the Earth was fixed in the centre of the universe, with the Sun and the stars revolving around it. Nicolaus Copernicus(1473-1543) was the Polish astronomer who first suggested the Earth was moving around the Sun.

 

 

 

 

 

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THE MOON

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THE MOON 











 

 

 

1. WHAT ARE THE MOON SEAS?

Ans: The large, dark patches visible on the Moon’s surface are called seas, but in fact they are not seas at all. They are huge plains formed by lava flowing from inside the Moon.

2.WHO WERE THE FIRST MEN ON THE MOON?

Ans: The first men on the Moon were Neil Armstrong and Buzz Aldrin of the US Apollo II mission. They landed on the Moon on July 20,1969.As Armstrong set foot on the Moon, he said: “That’s one small step for (a) man, one
giant leap for mankind.”

3. WHAT IS MOONLIGHT?

Ans: The Moon is by far the brightest thing in the night sky.But it has no light of its own.Moonlight is simply the Sun’s
light reflected off the white dust on the Moon’s surface.

4. WHAT IS A LUNAR ECLIPSE?

Ans: As the Moon goes around the Earth,sometimes it passes right into Earth’s shadow, where sunlight is blocked off. This is a lunar eclipse. If you look at the Moon during this time, you can see the dark disc of the Earth’s shadow creeping across the Moon.

5. WHAT IS THE MOON?

Ans: The Moon is the Earth’s natural satellite. It has circled around the Earth for at least four billion years. It is a rocky ball about a quarter of Earth’s size and is held in its orbit by mutual gravitational attraction. Most scientists believe that the Moon formed when, early in Earth’s history, a planet smashed into it. The impact was so tremendous that nothing was left of the planet but a few hot splashes thrown back up into space.Within a day of the smash, these splashes had been drawn together by gravity to form the Moon.

6. WHAT IS A HARVEST MOON?

Ans: The harvest moon is the full moon nearest the autumnal equinox (when night and day are of equal length). This moon hangs bright above the eastern horizon for several evenings,providing a good light for harvesters.

7. WHY DOES THE MOON LOOK LIKE CHEESE?

Ans: The moon looks like Swiss cheese because it is full of holes and can appear yellowish. The holes are craters in the surface created when it was bombarded by huge rocks early on in its history.

8. WHAT IS A NEW MOON?

Ans: The Moon appears to change shape during the month because, as it circles the Earth, we see its bright, sunny side from a different angle. At the new moon, the Moon lies between the Earth and the Sun, and we catch only a crescent – shaped glimpse of its bright side. Over the first two weeks of the month, we see more and more of the bright side (waxing) until full moon,When we see all its sunny side. Cover the next two weeks, we see less and less(waning), until we get back to just a sliver-the old moon.

9. WHY DOES THE SEA HAVE TIDES?

Ans: The Moon’s gravity draws the oceans into an oval around the Earth, creating a bulge of water on each side of the world. These bulges stay beneath the Moon as the Earth spins round and so seem to run around the world making the tide rise and fall as they pass.

10. HOW LONG IS A MONTH?

Ans: It takes the Moon 27.3 days to circle the Earth, but 29.53 days from one full moon to the next, because the Earth moves as well. A lunar month is the 29.53 days cycle. Calendar months are entirely artificial.

 

 

 

 

 

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পূৰণ

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A. দুটা অংক ৰে গঠিত সংখ্যাৰ লগত দুটা অংক ৰে গঠিত সংখ্যাৰ পূৰণ:-

 

দুটা অংক ৰে গঠিত সংখ্যাৰ লগত দুটা অংক ৰে গঠিত সংখ্যাৰ পূৰণ:-

এই পূৰণৰ বাবে আমি তিনিশাৰীৰ এটা দীঘলীয়া পদ্ধতিৰে সমাধান কৰিব লাগিব।কিন্তু নিম্নলিখিত পদ্ধতিৰে একে শাৰীতে পূৰণফল নিৰ্ণয় কৰিব পৰা যাব।

ওপৰৰ পদ্ধতিৰ নাম RCL পদ্ধতি
R – Right (সো)
C – Cross (বজ্ৰ)
L – Left (বাওঁ)

a. সোঁফালে থকা অংক দুটাৰ পূৰণ

b. ( ওপৰৰ সোঁ ফালৰ অংক × তলৰ বাওঁ ফালৰ অংক ) + ( ওপৰৰ বাওঁ ফালৰ অংক × তলৰ সোঁ ফালৰ অংক )

c. বাওঁ ফালে থকা অংক দুটাৰ পূৰণ।
Examples :

45 × 62 = ?

i. 5 × 2 = 10 ৰ 0 হাতত থাকিল 1

ii. ( 4 × 2 ) + ( 6 × 5 ) = 38 হাতৰ 1 যোগ কৰি 38 + 1 = 39 এই 39 ৰ 9 লিখা হল হাতত থাকিল 3

iii. 4 × 6 = 24 হাতৰ 3 যোগ হলে 24 + 3 = 27 এই 27 ক 90 বাওঁ ফালে বহুৱাব লাগে।

45 × 62 = 2790.

B. 3 অংক আৰু 2 অংকৰ সংখ্যাৰ পূৰণ

 

. 3 অংক আৰু 2 অংকৰ সংখ্যাৰ পূৰণ

435×23 = ?

i. 5 × 3 = 15, 15 ৰ 5 লিখি হাতত থাকিল 1

ii. (3×3) + (2×5) = 19, 19 ৰ লগত হাতৰ 1 যোগ দিলে হব 20, 20 ৰ 0 লিখি হাতত থাকিব 2

iii. (4×3) + (2×3) = 18,18 + 2(হাতৰ) = 20,20 ৰ 0 হাতত থাকিব 2

iv. 4×2 = 8, 8+2(হাতৰ) = 10

435 × 23 = 10005.

C. তিনি অংক ৰে গঠিত দুই সংখ্যাৰ পূৰণ :

 

তিনি অংক ৰে গঠিত দুই সংখ্যাৰ পূৰণ :

432 × 241 = ?

i. 2×1 = 2 ইয়াক 2 লিখক

ii. (3×4) + (4×2) = 11, 11 ৰ 1 লিখক 1 হাতত থাকিব

iii. (4×1) + (2×2) + (4×3) = 20, হাতৰ 1 যোগ কৰি 20+1 = 21, 21 ৰ 1 লিখক হাতত থাকিব 2

iv. (4×4) + (2×3) = 22 ৰ লগত 2 যোগ কৰি 22+2 = 24,এই 24 ৰ 4 লিখি 2 হাতত থাকিব

v. 4×2 = 8 হাতৰ 2 যোগ কৰি 8 + 2 = 10,এই 10 ক ___4112 ৰ বাওঁ ফালে লিখক

432 × 241 = 104112

D. চাৰি অংকৰ সংখ্যা ৰে 2 অংকৰ সংখ্যাৰ পূৰণ :

 

চাৰি অংকৰ সংখ্যা ৰে 2 অংকৰ সংখ্যাৰ পূৰণ :

6395 × 42 = ?

i. 5×2 = 10 ৰ 0 লিখক 1 হাতত থাকিব

ii. (9×2) + (5×4) = 38 ৰ লগত হাতৰ 1 যোগ কৰি 39 ৰ 9 লিখক 3 হাতত থাকিব

iii. (3×2) + (9×4) = 42 + 3 ( হাতৰ) = 45, 45 ৰ 5 লিখক হাতত থাকিব 4

iv. (6×2) + (3×4) = 24 + 4 (হাতৰ) = 28 ,28 ৰ 8 লিখক হাতত থাকিব 2

v. 6×4 =24 + 2 (হাতৰ) =26 আৰু 26 ক বাওঁ ফালে লিখক

6395 × 42 = 268590.

E. 4 আৰু 3 অংকৰ সংখ্যাৰ পূৰণ :

 

. 4 আৰু 3 অংকৰ সংখ্যাৰ পূৰণ :

3572 × 849 = ?

i. 9×2 = 18 ৰ 8 লিখক হাতত থাকিব 1

ii. (7×9) + (2×4) = 71 ৰ লগত হাতৰ 1 যোগ কৰি 72, 72 ৰ 2 লিখিব হাতত থাকিব 7

iii. (2×8) + (7×4) + (5×9) = 89 + 7(হাতৰ) = 96, 96 ৰ 6 লিখিব হাতত থাকিব 9

iv. (7×8) + (5×4) + (3×9) = 103 + 9(হাতৰ) = 112, 112 ৰ 2 লিখিব হাতত থাকিব 11

v. (8×5) + (4×3) =52+11(হাতৰ) =63 , 63 ৰ 3 লিখিব হাতত থাকিব 6

vi. 3×8 = 24 + 6(হাতৰ) = 30. এই 30 ক একেবাৰে বাওঁ ফালে লিখিব ।

3572 × 849 = 3032628.

F. দুটা 4 অংকৰ সংখ্যাৰ পূৰণ :

 

দুটা 4 অংকৰ সংখ্যাৰ পূৰণ :

9246 × 2543 = ?

i. 6×3 = 18 ৰ 8 হাতত থাকিব 1

ii. (4×3) + (6×4) =36+1(হাতৰ) = 37 ৰ 7 লিখিব হাতত থাকিব 3

iii. (6×5) + (4×4) + (2×3) = 52 + 3(হাতৰ)= 55, 55 ৰ 5 লিখিব হাতত থাকিব 5

iv. (6×2) + (9×3) + (4×5) + (2×4) = 67 + 5(হাতৰ) =72, 72 ৰ 2 লিখিব হাতত থাকিব 7

v. (9×4) + (4×2) +(2×5) =54 + 7(হাতৰ)= 61, 61 ৰ 1 লিখিব হাতত থাকিব 6

vi. (9×5) + (2×2) = 49 + 6(হাতৰ) = 55, 55 ৰ 5 লিখিব হাতত থাকিব 5

vii. 9×2 =18 + 5(হাতৰ) = 23, এই 23 ক বাওঁ ফালে লিখক।

9246 × 2543 = 23512578.

 

 

 

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LIGHT:REFLECTION and REFRACTION

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 1.What is light?

Ans:Light is a form of energy which produces the sensation of vision.



2. What is beam?
Ans: A collection of rays moving in an organized manner is called beam.



3. What is photon?
Ans: Photon is a particle of light energy.



4.What is optics?What are the different kinds of optics?
Ans: The branch of physics that deals with the nature of light,it’s source, properties and effects is called optics. There are two kinds of optics.they are –
a) Physical optics.
b) geometrical optics.



5. How many types  are there?
Ans: There are three types of rays.they are-.
a) Divergent rays.
b) Convergent rays.
c) Parallel rays.



6. What do you mean by reflection of light?
Ans: The phenomenon in which a ray of light travels in a medium,comes back into the same medium when incident on a second medium is known as reflection of light.



7. State the laws of reflection.
Ans: a)The incident ray,the reflected ray and the normal at the point of incidence,all lie in the same plane.
b) The angle of incidence is always equal to the angle of reflection.



8. What is an image?what are the types of image and explain it.
Ans: When a number of rays starting from a point after reflection or refraction actually meet at or appear to do so at another point.the second point is called the image of the first.
There are two types of image.they are-

Real image – The image formed by actual intersection of the reflected rays or refracted rays is called a real image. Virtual image – The image formed by imaginary intersection of reflected rays or refracted rays is called virtual image.



9. Mention differences between real and virtual image.
Ans: a) The real image is formed by actual intersection of the reflected rays.on the other hand,The virtual image is formed by imaginary intersection of the reflected rays.
b) Real image can be projected on a screen.but,The virtual image can not be projected on a screen.
c) Real image is always inverted with respect to an object.on the other hand, virtual image is always erect with respect to an object.



10. What is spherical mirror? What are the types of spherical mirror?
Ans: A spherical mirror is a mirror whose reflecting surface forms a part of a hollow sphre of glass.
spherical mirror are of two types -.
a) Concave mirror – If the reflecting surface is curve inwards and the outer bulged surface is silvered then the mirror is a Concave.
b) Convex mirror – If the reflecting surface is bulged outwards and inner hollow surface is silvered then the mirror is a convex.



11. Define the following terms with a respect to spherical mirror.
a)Pole – The central point of the reflecting surface is called the pole of the spherical mirror.
b) Centre of curvature – The centre of the sphere of which the mirror surface is a part is called the centre curvature of the spherical mirror.
c) Radius of curvature – The radius of the sphere of which the mirror surface is a part is called the radius of curvature of the spherical mirror.
d) Principal axis – The line that joints the centre of curvature of the spherical mirror with its pole is called the principal axis of the spherical mirror.
e) Principal focus – For a Concave mirror it is a point on the principal axis such that rays of light parallel to the principal axis converge to it after reflection in the mirror. This point is called principal focus.
f) Focal length – The distance from the principal focus to the pole of the spherical mirror is called the focal length of the mirror.
g) Focal plane – A plane perpendicular to the principal axis and containing the principal focus is called the focal plane of the spherical mirror.



12. Write the uses of Concave mirror and Convex mirror.
Ans: Concave mirror – a) Concave mirrors are commonly used in torches, search lights,and vehicles headlights to get powerful parallel beams of light.
b) Concave mirrors are used by dentists to see large images of the teeth of patients.
c) Concave mirrors are used as shaving mirrors to see a larger image of the face. d) Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces.
Convex mirror – a) Convex mirrors are commonly used as rear view mirrors in vehicles.
b) Convex mirrors are used as security mirrors in public buildings.
c) Convex mirrors are used in magnifying glass.
d) Convex mirrors are also used in making lenses of sunglasses.



13. What do you mean by magnification of an image?
Ans: Magnification produces by a spherical mirror gives the relative extent to which the image of an object is magnified with respect to the object size. It is expressed as the ratio of the height of the image to the height of the object.It is usually represented by the letter ‘m’
m = – Height of the image (h’) / Height of the object (h).
The Magnification m is also related to the object distance (u) and image distance (v).It can be written as :.
Magnification (m) = – h’/h = -v/u
sign convention for magnification
(i) Magnification is negative for real images (since such images are inverted). (ii) Magnification is positive for virtual images (since such images are erect).



14. What do you mean by refraction of light?
Ans: When light travels from one medium to another medium,it speed is changed . consequently,at the interface of two medium,the direction of the passing ray changed. The change of direction of the incident ray is called refraction of light.



15. State the laws of refraction.
Ans: Laws of refraction
(a) The incident ray, the refracted ray and the normal to the interface of two transparent media at point of incidence all lie in the same plane.
(b) For a particular pair of media the angle of incidence’ i’ in medium l is related to the angle of refraction ‘r’ in medium 2 by the relation-
n Sin i = n sin r or,
Sin i / sin r =constant.
The constant value is called the refractive index of the second medium with respect to the first.
The second law of refraction is called Snell’s law of refraction.



16. What do you mean by refractive index of medium?
Ans: The refractive index (n) of a transparent medium is the relation of the speed of light in vaccum (c) to the speed of light in that medium (v).
Refractive index = speed of light in vaccum / speed of light in medium.
* n= c/v.



17. What is a lens? Name two types of lenses.
Ans: A lens is a transparent medium bounded by two surfaces of which at least one is spherical.
Lenses can be classified into two categories –
(i) Convex or converging lenses:A lens which is thick at the centre but thinner at the edges is a convex lens. There are three types of convex lens-
a) Double convex lens or biconvex lens. b) plano-convex lens.
c) Convex meniscus.
(ii) Concave lens or diverging lenses: A lens which is thin at the centre but thicker at the edges is a Concave lens. There are three of concave lens –
a) Double concave lens or biconcave lens b) Plano concave lens.
c) Concave meniscus.



18. Define the following term in connection with lens.
a) Centre of curvature : The centre of curvature of the surface of a lens is the centre of the sphere of which it forms a part because a lens has two surfaces so it has two centres of curvature.
b) Radius of curvature : The radius of curvature of the surface of a lens is the radius of the sphere of which the surface forms a part.
c) Principal axis : It is the line passing through the two centres of curvature of the lens.
d) Optical centre : It is a point situated within the lens through which a ray of light passes undeviated.
e) Focal length : It is the distance between the principal focus and the optical centre of the lens.
f) Aperture : It is the diameter of the circular boundary of the lens.



19. Differences between Concave and a convex lens.
Ans: (i) Concave lens is thinner at the centre than the edges.and Convex lens is thicker at the centre than the edges.
(ii) Concave lens diverges a parallel beam of light on reflection through it.and Convex lens converges a parallel beam of light on refraction through it.
(iii) concave lens has a virtual focus.and convex lens has a real focus.



20. What do you mean by magnification of lens?
Ans: The magnification produced by a Lens, similar to that for spherical mirrors,is derived as the ratio of the height of the image and the height of the object.It is represented by the letter ‘m’ .If h is the height of the object and h’ is the height of the image given by lens,then the magnification produced by the lens is given by,
m = Height of the image / Height of the object = h’ / h.
Magnification produced by a lens is also related to the object (u) and the image distance (v).
This relationship is given by Magnification (m) = h’ / h = v / u.



21. Define the power of lens. What is its SI unit?
Ans: The ability of a convex lens to converge a parallel beam of light to point near the lens or the ability of a Concave lens to make the beam appear to diverge from a nearby point known as the power of the lens . If P denotes the power of. Lens of focal length f meter, then.               
P = 1 / f The SI unit of power lens is dioptre. It is denoted by the letter D. If f is expressed in metres, then power is expressed in dioptre.








TEXTUAL QUESTIONS




1. Define the principal focus of a concave mirror .
Ans: It is a point on the principal axis where the rays of light parallel to principal axis meet.



2. The radius of curvature of spherical mirror is 20 cm. What is its focal length?
Ans: Focal length=1/2 × Radius of curvature
=1/2 × 20 cm
=10 cm



3. Name a mirror that can give an erect and enlarged image of an object.
Ans : Concave mirror.



4. Why do we prefer a convex mirror as a rear-view mirror in vehicles?
Ans: Convex mirror can cover a wider range and give erect and diminished image Hence convex mirror is used as a rear-view mirror to get wider field of view.

 

5.Find the focal length of a convex mirror whose radius of curvature in 32 cm.
Ans: As the radius of curvature and focal length of a convex mirror are positive, so
R = +32 cm
f  = R/2
   =16 cm.



6. A concave mirror produces three times magnified (enlarged) real image of an object placed at 10 cm in front of it. Where is the image located?
Ans : Object distance, u =-10 cm (concave mirror),
magnification, m=-3
                  m=-v/u
therefore,v = – mu
                      =- (-3) (-10)
                      =-30 cm.
Image formed, v = 30 cm in front of the concave mirror.



7. A ray of light travelling in air enters obliquely into water. Does the light ray bend towards the normal or away from the normal? Why?
Ans: Light trends faster in rarer medium (air) and slower in denser medium (water). As a ray of light traveling in air enters obliquely into water, it slows down and bends towards the normal.



8. Light enter from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3×10⁸ m/s.
Ans: The speed of light in vacuum is 3×10⁸ m/s.
Refractive index of glass = 1.50
n= C/V
speed of light in glass, v ,= C/h
                                              =3×10 g /1.50
                                              =2 × 10⁸ m/s.



9. Find out, from table 10.3 the medium having highest optical density. Also find the medium with lowest optical density.
Ans: Diamond has highest refractive index ( 2.42), so it has largest optical density.
Air has lowest refractive index (= 1.0003), so it has lowest optical density.



10. You are given Kerosene, turpentine and water In which of these does the light travel fastest? Use the information given in table 10.3.
Ans: Refractive index of Kerosene=1.44
Refractive index of Turpentine.    =1.47 Refractive index of water              

=1.33 Lower the refractive index faster in the speed of light in that medium.

Hence, light will travel fastest in water.



11. The refractive index of diamond is 2.42. What is the meaning of this statement?
Ans: The refractive index of diamond is 2 .42 It indicates that the ratio of speed of light in air to that in diamond is 2.42.



12. Define 1 diopter of power of a lens.
Ans: 1 dioptre is the power of a lens whose focal length is 1 metre.
1 D = 1 m–¹



13. A convex lens forms a real and inverted image of a needle at a distance of 50 cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also find the power of the lens.
Ans: Here, v = +50 cm [vis positive for real image]
As the real image is of the same size as the object, so
m= h’/h = V/U =1
or,u = -v
       =-50 cm
Now,1/f = 1/v-1/u
               = (1/+50) – (1/-50)
               = + 2 / 50
               = + 1/ 25
or, f = 25 cm
        = 0.25 m
therefore, P = 1/f
                       =1 / +0.25 m
                       = +4 D



14.Find the power of a concave lens of focal length 2m.
Ans: Focal length of concave lens, f = -2m Power of concave lens, P = 1/f
                       therefore, P = 1/-2
                                             = – 0.5 D



15. (i) Which one of the following materials cannot be used to make a lens?
(a) water (b) glass (c) plastic (d) clay
Ans: (d)clay.



(ii) The image formed by a concave mirror is observed to be virtual, erect and larger than the object. Where should be the position of the object?
(a) Between the principal focus and the centre of curvature.
(b) At the centre of curvature.
(c) Beyond the centre of curvature.
(d) Between the pole of the mirror and its principal focus.
Ans: (d) between the pole of the mirror and its principal focus.



(iii) A spherical mirror nd a then spherical lens have each focus length of 15 cm. The mirror and the lens are likely to be
(a) both Concave
(b) both convex
(c)the mirror is concave and the lens is convex,
(d) the mirror is convex,but the lens is concave
Ans : (a) both concave.



(iv) No matter how far you stand from a mirror, your image appears erect. The mirror Is likely to be.
(a) Plane (b) Concave (c) convex (d) either plane or convex.
Ans. (d) either plane or convex. 



(v) Which of the following lenses would you prefer to use while reading small letters found in a dictionary?
(a) A convex lens of focal length 50 cm.     
(b) A concave lens of focal length 50 cm.
(c) A convex lens of focal length 5 cm.
(d) A concave lens of focal length 5 cm.
Ans. (C) Convex lens of focal length 5 cm.



16. We Wish to obtain an erect image of an object, using a concave mirror of focal length 15 cm. What should be the range of distance of the object from the mirror? What is the nature of the image? Is the image larger or smaller than the object?Draw a ray diagram to show the image formation in this case.
Ans. Focal length of concave mirror=15 cm.
Object should be placed in front of green concave
lmage formed is virtual and erect.
image size is larger than object.









17. Name the type of mirror used in the following situations,
(a) Head light of a car.
(b) Side/rear view mirror of a vehicle.
(c) Solar furnace. Support your answer with reason.
Ans: (a) For head light of a car : Use Concave mirror, to get a powerful beam of light after reflection.
(b) A convex mirror is used a side/rear view mirror of a vehicle. Convex mirror forms an erect and diminished image of vehicles and gives wider view of rear.
(C) In solar furnance concave mirror is used as a reflector, it concentrates sun light at a point where the temperature increases sharply to 180°C-200°C.



18. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally? Explain your observation.
Ans. One-half of a convex lens when covered with a black paper, the lens produces a complete, full image of an object.
To verify experimentally:Take a convex lens, cover half part of it as shown in the figure, with a paper. Place it on a stand. Focus a distant object on a screen, the image obtained on the screen is complete.
Observation and conclusion : Image formed on the screen does not depend on the size of the lens. The brightness of the image decreases as less number of rays pass through the lens.



19. An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.
Ans:






20. A concave lens of local length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.
Ans. Concave lens
         f= – 15 cm
        V= -10 cm
We know,
             1/v – 1/u = 1/f
        =>1/-10 – 1/u = 1/-15
          1/u = -1/30
             u = -30 cm
Ray diagram,






21. An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm.Find the position and nature of the image.
Ans:






The image is formed 6 cm behind the mirror virtual image is formed.



22. The magnification produced by a plane mirror is +1. What does this mean?
Ans. Magnification,m=+ 1
                             +indicates virtual image.
                           1 indicates that the size of the object and size of the image is same.



23. An object 5.0 cm in length is placed at a distance of 20 cm in front of a convex mirror of radius of curvature 30 cm. Find the position of the image, its nature and size.

Ans:






24. An object of size  7.0 cm is placed at 27 cm in front of a concave mirror of focal length 18 cm.at what distance from the mirror should a screen be placed,so that a sharp focussed image can be obtained? Find the size and the nature of the image.
Ans:





25. Find the focal length of a lens of power -2.0 D. What type of lens is this?Ans. P=-2.0 D.
         P=1/f
         f=1/P
          =1/-2.0 D = -0.5 m.
The lens is concave lens as f= – ve.



26. A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?
Ans.p=+1.5 D,
  therefore,p=1/f
Focal length of the lens, f =1/P
                                              =1/+1.5 D
                                              =+0.67 m
Power of the lens is ve, and it is converging lens i.e., Convex lens.





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