# Physics facts for competitive exams

#### MIRRORS AND LENS

1) Light has the tendency to bend around very small opaque objects. This phenomenon is known as diffraction of light.
2) A mirror is an object that reflects almost all the light that falls on it. An ideal mirror reflects back 100% of the incident light.
3) A mirror is a highly polished surface.
4) Laws of reflection :i) Angle of incidence = Angle of reflection
ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray lies in the same plane.

5) All the mirrors are practically spherical.
6) Mirrors can be of two types.
Concave mirror- where the reflecting surface is curved inwards
Convex mirror- where the reflecting surface is curved outwards.

7) Some Important terms about mirrors :
a) Pole- The centre of the reflecting surface of the spherical mirror.
i) Lies on the surface of mirror.
ii) Usually represented by letter P.
b) Centre of curvature- the spherical reflecting surface forms a part of a sphere if we complete it’s boundary.
i) Represented by letter C.
ii) It does not form the part of they mirror.
iii) It lies in front of concave mirror and behind in the case of convex mirror.
c) Radius of curvature – the radius of the sphere, of which the reflecting surface forms a part of.
i) Represented by letter R.
d) Principal axis- the straight line passing through the pole and centre of curvature.
i) It is normal to the mirror at its pole.

#### USE OF CONCAVE MIRRORS

1. Commonly used in
a) Torches
b) Search lights

#### USES OF CONVEX MIRROR

1. Commonly used in
a) Rear view mirrors of vehicles
b) Hairpin turns ( to see the vehicles coming towards the turn)

### FORMULAS

1. Mirror formula : (1/v)+(1/u)=(1/f)
i) u- distance of object from pole
ii) v- distance of image from pole.
iii) f- distance of principal focus from pole.

2. Magnification : m = h’/h = v/u
i) h’- height of image
ii) h- height of object

### REFRACTION OF LIGHT

1. The phenomenon of bending of light when it travels from one transparent medium to another transparent medium.
a) This occurs due to the change in velocity of light when it enters a new medium.
b) Every medium has different refractive index.
c) Refractive index is the measure of the medium’s capability to bend the incident light.
2. Laws of refraction :
i) The incident ray, refracted ray and the normal to the interface of the two transparent medias will lie on the same plane.
ii) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of given color and the given pair of medias.
3. The second law of refraction is also called Snell’s law.
4. Refractive index, n = ( speed of light in medium 1)/( speed of light in medium 2)

### LENS

1. Lens are made of glass used to converge or diverge light rays
2. They are of two types :
i) Convex lens – both sides are bulging outwards.
(1) They are used to converge out focus light
ii) Concave lens – both sides are curved onwards.
(1) Mainly used to diverge rays.
3. Lens formula :
(1/v)-(1/u)=(1/f)
v- image distance
u- object distance
f – focal length
4. Magnification, m = h’/h
h – object height
h’ – image height
5. Power of a lens, P= (1/f)
f – focal length

### ELECTRICITY

1) Electric current is expressed in terms of the amount of charge flowing through a particular area in unit time.
2) It is the rate of flow of electric charge.
3) Current is denoted by letter I.
4) Direction of current is opposite to the direction of flow of electrons.
5) The SI unit of current is amperes.
6) Ampere is denoted by letter A.
7) To generate current flow in an electric circuit we use battery, which creates a potential difference across its terminals.
8) Potential difference is often denoted by letter V.
9) SI unit of potential difference is Volts.
10) Ohm’s law states that the potential difference across the resistor is directly proportional to the current flowing through it.
V= I×R
11) Resistance is a property of a material to obstruct the flow of electrons.
12) Resistance is denoted by R.
13) Practically every object in the universe is a resistor.
14) Resistance is also related to the length, material of the conductor and cross-sectional area of the conductor as the formula below shows :
R = p × (l/A)
Where p- density of the material
l- length of the material
A- cross-sectional area
15) A conductor is material that offers no resistance to the flow of electrons though them.
16) An insulator is a material that does not allow the flow of electrons through them.
17) A good conductor of electricity is a poor insulator and ever insulator is a poor conductor of electricity.
18) The equivalent resistance
a) in a series connection is :
R = R1+ R2+ R3+ R4+……+Rn
b) in a parallel connection is :
1/R= (1/R1)+(1/R2)+(1/R3)+(1/R4)+……+(1/Rn)
if ‘n’ number of resistors are connected.
19) The unit of power is Watt. It is denoted by W.