Intensity

Q. Give the SI unit and dimension of work, energy and power.

Q. About 5% of the power of a 100 W light bulb is converted to visible radiation. What is the average intensity of visible radiation (a) at a distance of 1m from the bulb? (b) at a distance of 10 m? Assume that the radiation is emitted isotropically and neglect reflection.

Q.

A sphere of radius 1.00 cm is placed in the path of a parallel beam of light of large aperture. The intensity of the light is 0.50 W $c{m}^{-2}$. If the sphere completely absorbs the radiation falling on it, find the force exerted by the light beam on the sphere.

Q.

A bulb of power 100 W that radiates light isotropically(equally in all directions) has dimensions so small compared to the distances in question that the source can be assumed to be a point source.

Find the intensity 10 m away from it?

1. 1/π W/m²

2. 1/(4π) W/m²

3. None of these

4. 2/π W/m²

Q.

A Light source that radiates 10 Joules of energy per second is kept at the centre of a hemisphere. How much energy will the hemisphere receive?

Energy (in Joules) =

Q. Draw a schematic diagram of a reflecting telescope (Cassegrain). Write its two advantages over a refracting telescope.

Q. If a light of intensity $60\text{W}$ falls normally on an area of $1{\text{m}}^2$. If the reflectivity of the surface is $75\%$, find the force experienced by the surface.

1. $3.5\times {10}^{-10}\text{N}$
2. $3.5\times {10}^{-7}\text{N}$
3. $3.5\mu \text{N}$
4. $3.5\text{mN}$

Q. At a given place where acceleration due to gravity is $gm/s^2$, a sphere of lead of density $dkg/m^3$ is gently released in a column of liquid of density $\rho kg/m^3$. If $d>\rho$, the sphere will :

1. Fall vertically with an acceleration $g\left(\frac{d-\rho }{d}\right)$
2. Fall vertically with an acceleration $gm/s^2$
3. Fall vertically with an acceleration $g\left(\frac{\rho }{d}\right)$
4. Fall vertically with no acceleration

Q. Which planet was discovered by NASA's MAVEN to have a twisted magnetic tail?

1. Venus
2. Saturn
3. Mars
4. Neptune

Q.

2 planets 'Xargonia' and 'Olfuba' have taken keen interest in our star 'The Sun'. They are observing our sun from the observatories built on their planet.

Xargonia is 1 Light year away and the aperture area of its telescope is $1m^2$, while Olfuba is 2 Light years away and the aperture area of its telescope is $4m^2$.

Find out whose telescope receives more energy? (Assume the Sun radiates E joules of energy per sec.)

1. Data insufficient

2. Olfuba

3. Both receive equal

4. Xargonia

Q. The unit of poynting vector is

1. Watt
2. Joule
3. $\frac{Watt}{m^2}$
4. $\frac{joule}{m^2}$

Q. If $2.5\times {10}^{-6}\text{N}$ average force is exerted by a light wave on a non-reflecting surface of area $30{\text{cm}}^2$ during $40$ minutes of time span, the energy flux of light just before it falls on the surface is ______ ${\text{W/cm}}^2$.

$($Round off to the nearest integer$)$
$($Assume complete absorption and normal incidence conditions are there$)$

Q. For the propagation of light wave, medium is required. This is according to

1. Newton's theory
2. Planck's theory
3. Maxwell's theory
4. Huygen's theory

Q. The capacitance of spherical conductor of radius r is proportional to :

1. $1/r$
2. $r$
3. $1/r^2$
4. $r^2$

Q.

A point source radiates energy at the rate of 10 Joules per second, or 10 Watts. As shown in the diagram, two spherical shells of radii $r_{1}and{r}_2(r_1<r_2)$ receive energies $E_{1}and{E}_2$ per second, respectively. Which of the following option(s) is/are true?

1. E₁ > E₂

2. E₁ = E₂

3. E₁ < E₂

4. Energy from a point source spreads out isotropically in all directions.

Q.

A Light source that radiates E Joules of energy per secondis kept at the centre of a hemisphere. How much energy will the hemisphere receive?

Energy = __

Q.

A point source radiates energy at the rate of 10 Joules per second, or 10 Watts. As shown in the diagram, two spherical shells of radii $r_{1}and{r}_2(r_1<r_2)$ receive energies $E_{1}and{E}_2$ per second, respectively. Which of the following option(s) is/are true?

1. $E_1>E_2$

2. $E_1=E_2$

3. $E_1<E_2$

4. Energy from a point source spreads out isotropically in all directions.

Q. expalin three types of wave front

Q. A surface at temperature $T_o$ $K$ receives power $P$ by radiation from a small sphere at temperature $T<T_o$ and at a distance $d$. If both $T$ and $d$ are doubled, the power received by the surface will become

1. $P$
2. $16P$
3. $4P$
4. $2P$

Q. Four lenses of focal length 15 cm, 20 cm, 150 cm and 250 cm are available for making an astronomical telescope. To produce the largest magnification, what should the focal length of the eyepiece be?

1. $+15cm$
2. $+150cm$
3. $+250cm$
4. $+20cm$

Q.

2 planets 'Xargonia' and 'Olfuba' have taken keen interest in our star 'The Sun'. They are observing our sun from the observatories built on their planet.

Xargonia is 1 Light year away and the aperture area of its telescope is $1m^2$, while Olfuba is 2 Light years away and the aperture area of its telescope is $4m^2$.

Find out whose telescope receives more energy? (Assume the Sun radiates E joules of energy per sec.)

1. Xargonia

2. Both receive equal

3. Olfuba

4. Data insufficient

Q.

Which of these light sources will have a cylindrical wave front?

1. Point source

2. Plane source

3. Spherical source

4. Infinite Line source

Q. Complete the following statement. The diffraction of a wave through a single opening produces

1. an angle of incidence
2. an increase in speed of the wave
3. refraction
4. a semicircular wave pattern
5. a decreases in speed of the wave

Q.

Intensity is:

1. Energy density in a given volume

2. Power density in a given volume

3. Energy per unit area

4. Power per unit area

Q. An air wedge is formed between two glass plates separated at one edge by a very fine wire of circular cross section as shown in Figure. When the wedge is illuminated from above by 600-nm light and viewed from above, 30 dark fringes are observed. Calculate the diameter d of the wire.

Q. The distance of a galaxy from Earth is of the order of ${10}^{25}$m. Calculate the order of magnitude of the time taken by light to reach us from the galaxy.

Q.

2 planets 'Xargonia' and 'Olfuba' have taken keen interest in our star 'The Sun'. They are observing our sun from the observatories built on their planet.

Xargonia is 1 Light year away and the aperture area of its telescope is $1m^2$, while Olfuba is 2 Light years away and the aperture area of its telescope is $4m^2$.

Find out whose telescope receives more energy? (Assume the Sun radiates E joules of energy per sec.)

1. Xargonia

2. Olfuba

3. Both receive equal

4. Data insufficient

Q. The gravitational field of a sphere of mass $M$ and radius $R$ at a distance $2R$ from the centre of the sphere is ............... if a cavity of radius $R/3$ is made at the periphery as shown in figure.

1. $\frac{11GM}{48R^2}$
2. none of these
3. $\frac{7GM}{48R^2}$
4. $\frac{9GM}{48R^2}$

Q. An optical device used by watch repairers is __________.

Q. An object O is stuck on the surface of a transparent solid sphere of radius 20 cm. The refractive index of the sphere such that rays from the object after refraction from the opposite side emerge as a parallel beam is

1. $\sqrt{3}$
2. $1.5$
3. $\sqrt{2}$
4. $2$