Photo cell is a device which converts light energy into electrical energy. It works on the principle of photo electric effect. When light is incident on photosensitive materials, their electric properties will get affected, based on which Photo cells are classified into three types. They are
(i) Photo emissive cell
(ii) Photo voltaic cell
(iii) Photo conductive cell

Robots are automated machines which have many mechanical parts like wheels, arms, fingers, legs etc. Therefore a strong material is needed. For a given size, steel has several times stronger.

(i) To move the hole in a given direction, the valence electrons move in the opposite direction.
(ii) Electrons flow in a N-type semiconductor is similar to electrons moving in a metallic wire.
(iii) The N- type dopant atoms will yield electrons available for conduction.

RADAR stands for RAdio Detection And Ranging.

A terrestrial telescope has an additional erecting lens to make the final image erect.

Isotopes are atoms of the same element having same atomic number Z, but different mass number A.
(Ex: Hydrogen, \(_{ 1 }^{ 1 }{ H }\) ((hydrogen), \(_{ 1 }^{ 2 }{ H }\) (deuterium),and \(_{ 1 }^{ 3 }{ H }\) (tritium)) 

Wave theory of light:
(i) According to Huygen's wave theory, light is propagated in the form of longitudinal waves through an invisible elastic medium called ether, which pervades all space.
(ii) Later Fresnel and Young suggested that light waves are transverse. Wave theory could satisfactorily explain reflection, refraction, interference diffraction and polarisation.
(iii) According to this theory, the velocity of light in a denser medium is lesser than that in a rarer medium.

The principle of reversibility states that light will follow exactly the same path if its direction of travel is reversed.

Coulomb's inverse square law states that the force of attraction or repulsion between two magnetic poles is directly proportional to the product of their pole strengths and inversely proportional to the square of the distance between them.
\(\vec { F } =k \frac { { q }_{ m_{A} }{ q }_{ m_{B} } }{ { r }^{ 2 } } \hat { r } \)

Resonant frequency is the frequency of the applied alternating source at which the current in the circuit reaches its maximum value.

Even for a conductor in the form of a sheet or plate, an emf is induced when magnetic flux linked with it changes. But the difference is that there is no definite loop or path for induced current to flow away. As a result, the induced currents flow in concentric circular paths. As these electric currents resemble eddies of water, these are known as Eddy currents. They are also called Foucault currents.

\(\oint _l\vec{B} \cdot \overrightarrow{d l}=\mu_{o} i_{\text {c }}+\mu_{o} \varepsilon_{o} \frac{d}{d t} \oint _s \vec{E} \cdot \overrightarrow{d A}\)

Electrostatic potential energy is the work done to assemble the charges at the given locations brought from infinity. If q₁ and q₂ are the charges to be assembled with a separation of distance r, the electric potential energy is, \(U=\frac{1}{4\piε_o}\frac{q_1q_2}{r}\)

It there are more than two charges, the total force acting on a given charge is equal to the vector sum of forces exerted on it by all the other charges.
Consider a system of n charges namely q₁, q₂, q₃ ...q_n. The force on q₁ exerted by the charge q₂ is \(\overrightarrow{F_{12}}=k \frac{q_{1} q_{2}}{r_{21}^{2}} \hat{r}_{21}\) .
The force on q₁ exerted by the charge q₃ is \(\overrightarrow{F_{13}}=k \frac{q_{1} q_{3}}{r_{31}^{2}} \hat{r}_{31}\)
By continuing this, the total force acting on the charge q₁ due to all other charges is given by
\( \vec{F}_{1}^{\text { tot }}=\overrightarrow{F_{12}}+\overrightarrow{F_{13}}+\overrightarrow{F_{14}}+\ldots+\vec{F}_{1 n} \)
\(\vec{F}_{1} ^{\text { tot }}=k\left\{\frac{q_{1} q_{2}}{r_{21}^{2}} \hat{r}_{21}+\frac{q_{1} q_{3}}{r_{31}^{2}} \hat{r}_{31}+\frac{q_{1} q_{4}}{r_{41}^{2}} \hat{r}_{41}+\ldots+\frac{q_{1} q_{n}}{r_{n 1}^{2}} \hat{r}_{n 1}\right\}\) 

Peltier discovered that, when an electric current is passed through a circuit of a thermocouple heat is evolved at one junction and absorbed at the other junction. This is known as Peltier effect.

Electric power:
(i) The electric power P is the rate at which the electrical potential energy is delivered.
(ii) The electric power P is the rate at which the work is done.
\( P= \frac{d U}{d t} (or)= \frac{d W}{d t} (or)=VI(or)\frac{V^2}{R}\)
Unit: watt (W)
Electric energy:
(i) The electric energy is the product of power (P) and duration of the time (t) when electric energy is delivered.
(ii) E = Pt
Unit: watt-hour (Wh)

(i) The image formed by a plane mirror is virtual, erect, and laterally inverted.
(ii) The size of the image is equal to the size of the object.
(iii) The image distance behind the mirror is equal to the object distance in front of the mirror.
(iv) If an object is placed between two plane mirrors inclined at an angle 0, then the number or
images n formed is given in Table.