# CBSE Delhi-Set-1-2009

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• Q1

The figure shows a plot of three curves a, b, c showing the variation of photocurrent vs collector plate potential for three different intensities I1, I2 and I3  having frequencies   respectively incident on a photosensitive surface.

Point out the two curves for which the incident radiations have same frequency but different intensities.

Marks:1

Curve a, b represents same frequency. The value of stopping potential is same for radiations having same frequency and is independent of the intensity of the incident radiation. Thus, a and b are the two curves for which the incident radiations have same frequency but different intensities.

• Q2

What type of wave front will emerge from a (i) point source, and (ii) distant light source?

Marks:1

(i)     Spherical wavefront will emerge from a point source.
(ii)   Cylindrical wavefront will emerge from the distant light source.

• Q3

Two nuclei have mass numbers in the ratio 1 : 2. What is the ratio of their nuclear densities?

Marks:1

Density of nuclear matter= mass of nucleus / volume of nucleus

=(3m)/4R03
As m and R0 are constants, therefore densityis independent of the mass number of atoms of all elements.
So, nuclear density is same for all nuclei, therefore, 1/2=1

• Q4

If the angle between the pass axis of polarizer and the analyzer is 450, write the ratio of the intensities of original light and the transmitted light after passing through the analyzer.

Marks:1

Using the formula :

I =I0 cos2
Where, I0 = intensity of original light.
I = intensity of transmitted light after passing through the analyzer.

• Q5

You are given following three lenses. Which two lenses will you use as an eyepiece and as an objective to construct an astronomical telescope?

## Lenses

Power (P)

Aperture (A)

L1

3D

8 cm

L2

6D

1 cm

L3

10D

1 cm

Marks:1

Focal length of eyepiece should be smaller than that of objective.
So, power of eyepiece should be greater as compared to an objective.

 L1 3 D 8 cm Objective L3 10D 1 cm Eye piece

We will use L1 lens as an objective and L3 lens as an eyepiece to construct the astronomical telescope.

• Q6

Write the following radiations in ascending order in respect of their frequencies: X-rays, microwaves, UV rays and radio waves?

Marks:1

Radio waves , microwaves, UV rays ,X-rays.

• Q7

Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid. Why?

Marks:1

Magnetic field lines can be entirely confined within the core of toroid because field lines form closed loops, but in case of straight solenoid it is not possible as it will violate Ampere’s law.

• Q8

What is sky wave propagation?

Marks:1

When the radio waves emitted from the transmitting antenna reach the receiving antenna after reflection in the ionosphere then, the wave propagation is known as sky wave propagation.

• Q9

By what percentage will the transmission range of a TV tower be affected when the height of the tower is increased by 21%?

Marks:2

The distance covered by the TV tower of height h is given by:

• Q10

Derive an expression for drift velocity of free electrons in a conductor in terms of relaxation time.

Marks:2

The drift velocity is defined as the average velocity with which the free electrons get drifted towards the positive end of the conductor under the influence of potential difference applied across the conductor. The drift velocity of electrons is of the order of 10-4 ms-1.
The magnitude of electric field is E=V/L.

Where, V=potential difference applied across the end of the conductor.

l = length of the conductor.
Charge on electron is –e, so force experienced by each electron is,

If m is the mass of an electron, the acceleration of each electron is

At any instant of time, the velocity acquired by electron having thermal velocity will be

Similarly, the velocities acquired by other electrons in the conductor will be

The average velocity of all the free electrons in the conductor is the drift velocity of the free electrons.