1. It should have suitable colour.
2. It should be able to fix itself or be capable of being fixed to the fabric.
3. It should be fast to light.
4. It should be resistant to the action of water, dilute acids and alkalies.

After the Big Bang, clouds of gases began to compress under gravity to form the building blocks of galaxies. A galaxy is a massive collection of gas, dust and millions of stars and their solar systems.

1. Echo, which is used to find the distance between the two distant objects.
2. Sound navigation and ranging (SONAR) which is used to find the depth of the Ocean.

The upward force acting on an object immersed in a liquid is known as upthrust or buoyant force.
It depends on
i) volume of the object immersed in the liquid.
ii) Density of the liquid.

Many salts are found as crystals with water molecules they contain. These water molecules are known as water of crystallisation.

Bond which is formed between atoms by the mutual sharing of electrons is known as covalent bond.

The memory unit in the computer saves all data and information temporarily. The data is measured in units which is called as Bit.

Mass of iron m = 5 kg
Initial temperature t₁ = 30°C
Final temperature t₂ = 130°C
Rise is temperature \(\triangle \)t₂ = (t₂ - t₁) = 130 - 30 = 100°C
Specific heat capacity of iron c = 483 Jkg^-10C^-1
Q = m x c x \(\triangle \)t
= 5 x 483 x 100
= 2,41,500 J

Atoms of different elements with different atomic numbers and different mass numbers, but with the same number of neutrons are called isotones.
Example : Atomic number of Boron is 5, atomic number of carbon is 6. But Boron and Carbon has the same number of neutrons that is 6.

When two elements A and B combine together to form more than one compound, then masses of A which separately combines with a fixed mass of B are in simple ratio.
This law was stated by John Dalton

(i) Green Plants, (ii) B - Glucose, H - Starch
C = CO₂ ; D = H₂O
E = Chlorophyll ; F - Green leaf Photosynthesis.

Human skin can synthesize vitamin D when exposed to sunlight. When sun rays falls on the skin dehydrocholesterol is converted into vitamin D.

Uses of concave mirror:
(1) As a dentist's head mirror: A circular mirror attached to a band tied to the forehead of the dentist / ENT specialist. A parallel beam of light is made to fall on the concave mirror; this mirror focuses the light beam on a small area of the body.
(2) As a make-up mirror: When a concave mirror is held near the face, an upright and magnified image is seen. Here, our face will be seen magnified.
(3) Concave mirrors are also used as reflectors in torches, head lights in vehicles and search lights to get powerful beams of light.
(4) Concave reflectors are also used in room heaters.
(5) Large concave mirrors are used in solar heaters.
Uses of convex mirror:
(1) Convex mirrors are used as rear-view mirrors in vehicles.
(2) A convex mirror provides a much wider field of view compared to plane mirror.
(3) Convex mirrors are installed on public roads as traffic safety device.

In change of states, increase in pressure alone cannot bring about change of states from gas to liquid to solid. Apart from high pressure, low temperature is also necessary for a gas to be converted into liquid.

1. The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
2. The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a light of given colour and for the given pair of media.
\(\frac { \sin { i } }{ \sin { r } } =\)constant

When an object moves with constant speed along a circular path the motion is called uniform circular motion.
Examples:
I . Revolution of earth around the sun.
2. Revolution of moon around the earth.

i) Standard units give the correct and accurate measurement like length, mass, time etc...
ii) It is not varied from person to person, place to place and object to object. So we need standard unit.

Total distance travelled by the object = 16 m + 16 m = 32m
Total time taken = 4s + 2s = 6s
Average speed \(=\frac { Total\ distance\ travelled }{ total\ time\ taken } =\frac { 32m }{ 6s } =\frac { 32 }{ 6 } =5.33m{ s }^{ -1 }\)
Therefore, the average speed of the object is 5.33ms^-1