Q23 of 26 Page 1

Meeta’s father was driving her to the school. At the traffic signal she noticed that each traffic light was made of many tiny lights instead of a single bulb. When Meeta asked this question to her father, he explained the reason for this.

Answer the following questions based on above information:


(i) What were the values displayed by Meeta and her father?


(ii) What answer did Meeta’s father give?


(iii) What are the tiny lights in traffic signals called and how do these operate?


(i) The value displayed by Meeta and her father


1. Meeta: Curious and Volatile Mind


2. Meeta’s Father: Is elder that is why possess wisdom and patience


(ii)


The answer given by Meeta’s father was advantages of LED over single bulb.


Advantages of LED


1. Power consumption is less compared to a light bulbs


2. The cost of LED is 75% less as compared to the light bulbs


3. The LED work independent of each other meaning if one is not working it will not hinder the working of the others.


(iii)


The tiny light that Meeta’s father is talking about is a LED, that work on the de-excitation of the electron in a forward biased semiconductor when charge is passed through it.


More from this chapter

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21

(i) State Bohr’s quantization condition for defining stationary orbits. How does de Broglie hypothesis explain the stationary orbits?


(ii) Find the relation between the three wavelengths λ1, λ2 and λ3 from the energy level diagram shown below.



 22

Draw a schematic ray diagram of reflecting telescope showing how rays coming from a distant object are received at the eye-piece. Write its two important advantages over a refracting telescope.

 24

(i) An a.c. source of voltage V = V0 sin ωt is connected to a series combination of L, C and R. Use the phasor diagram to obtain expressions for impedance of the circuit and phase angle between voltage and current. Find the condition when current will be in phase with the voltage. What is the circuit in this condition called?

(ii) In a series LR circuit XL = R and power factor of the circuit is P1. When capacitor with capacitance C such that XL = XC is put in series, the power factor becomes P2. Calculate P1/P2.


OR


(i) Write the function of a transformer. State its principle of working with the help of a diagram. Mention various energy losses in this device.


(ii) The primary coil of an ideal step up transformer has 100 turns and transformation ratio is also 100. The input voltage and power are respectively 220 V and 1100 W. Calculate


(a) Number of turns in secondary


(b) Current in primary


(c) Voltage across secondary


(d) Current in secondary


(e) power in secondary


 25

(i) In Young’s double slit experiment, deduce the condition for (a) constructive, and (b) destructive interference at a point on the screen. Draw a graph showing variation of intensity in the interference pattern against position ‘x’ on the screen.

(ii) Compare the interference pattern observed in Young’s double slit experiment with single slit diffraction pattern, pointing out three distinguishing features.


OR


(i) Plot a graph to show variation of the angle of deviation as a function of angle of incidence for light passing through a prism. Derive an expression for refractive index of the prism in terms of angle of minimum deviation and angle of prism.


(ii) What is dispersion of light? What is its cause?


(iii) A ray of light incident normally on one face of a right isosceles prism is totally reflected as shown in fig. What must be the minimum value of refractive index of glass? Give relevant calculations.