 Light travels along straight lines.

 Any polished or a shining surface acts as a mirror.







 An image which can be obtained on a screen is called a real image.

 An image which cannot be obtained on a screen is called a virtual image.



 The image formed by a plane mirror is erect. It is virtual and is of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it.



 In an image formed by a mirror, the left side of the object is seen on the right side in the image, and right side of the object appears to be on the left side in the image.



 A concave mirror can form a real and inverted image. When the object is placed very close to the mirror, the image formed is virtual, erect and magnified.



 Image formed by a convex mirror is erect, virtual and smaller in size than the object.



 A convex lens can form real and inverted image.



When the object is placed very close to the lens, the image formed is virtual, erect and magnified. When used to see objects magnified, the convex lens is called a magnifying glass.







 A concave lens always forms erect, virtual and smaller image than the object.

 White light is composed of seven colours.





FORCE AND PRESSURE


 a push or a pull on an object is called a force.

 The strength of a force is usually expressed by its magnitude.



 The state of motion of an object is described by its speed and the direction of motion.



 The state of rest is considered to be the state of zero speed.

 An object may be at rest or in motion; both are its states of motion.



 The force resulting due to the action of muscles is known as the muscular force.



 the muscular force that enables us to perform all activities involving movement or bending of our body.



 Since muscular force can be applied only when it is in contact with an object, it is also called a contact force.







 The force responsible for changing the state of motion of objects in all these examples is the force of friction.



 The force exerted by a magnet is an example of a non-contact force.

 The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.



 This force comes into play even when the bodies are not in contact.

 The electrostatic force, therefore, is another example of a non-contact force.



 Gravity is not a property of the earth alone.

 In fact, every object in the universe, whether small or large, exerts a force on every other object. This force is known as the gravitational force.



 The force acting on a unit area of a surface is called pressure.





FRICTION




 Friction opposes the relative motion between two surfaces in contact. It acts on both the surfaces.


 Friction depends on the nature of surfaces in contact.








 For a given pair of surfaces friction depends upon the state of smoothness of those surfaces.


 Friction depends on how hard the two surfaces press together.





 Static friction comes into play when we try to move an object at rest.


 Sliding friction comes with play when an object is sliding over another.





 Sliding friction is smaller than static friction.


 Friction is important for many of our activities.


 Friction can be increased by making a surface rough.





 The sole of the shoes and the tyres of the vehicle are treaded to increase friction.


 The friction is sometimes undesirable.





 Friction can be reduced by using lubricants.


 When one body rolls over another body, rolling friction comes into play. Rolling friction is smaller than the sliding friction.





 In many machines, friction is reduced by using ball bearings.


 Fluid friction can be minimised by giving suitable shapes to bodies moving in fluids.







SOUND




 Sound is produced by vibrating objects.


 In human beings, the vibration of the vocal cords produces sound.





 Sound travels through a medium (gas, liquid or solid). It cannot travel in vacuum.


 The eardrum senses the vibrations of sound, It sends the signals to the brain. This process is called hearing.








 The number of oscillations or vibrations per second is called the frequency of oscillation.


 The frequency is expressed in hertz (Hz)





 Larger the amplitude of vibration, louder is the sound.


 Higher the frequency of vibration, the higher is the pitch, and shriller is the sound.





 Unpleasant sounds are called noise.


 Excessive or unwanted sounds lead to noise pollution. Noise pollution may pose health problems for human beings.





 Attempts should be made to minimise noise pollution.


 Plantation on the roadside and elsewhere can reduce noise pollution.






CHEMICAL EFFECTS OF ELECTRIC CURRENT




 The process of depositing a layer of any desired metal on another material by means of electricity is called electroplating.








 It is one of the most common applications of chemical effects of electric current.


 The passage of an electric current through a conducting liquid causes chemical reactions.





 The resulting effects are called chemical effects of currents.


 Electroplating is a very useful process. It is widely used in industry for coating metal objects with a thin layer of a different metal.





 Chromium has a shiny appearance. It does not corrode.


 It resists scratches.



 However, chromium is expensive and it may not be economical to make the whole object out of chromium.