Download App

Photometry — Physics STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 SciencePhysicsPhotometry3 Marks
Question
A point source emitting light uniformly in all directions is placed 60cm above a table-top. The illuminance at a point on the table-top, directly below the source, is 15 lux. Find the illuminance at a point on the table-top 80cm away from the first point.
✓

Answer


given that $\text{E}_\text{a}=15\text{ lux}=\frac{\text{l}_0}{60^2}$
$\Rightarrow\text{l}_0=15\times(0.6)^2=5.4\text{ candela}$
 So, $\text{E}_\text{B}=\frac{\text{l}_0\cos\theta}{(\text{OB})^2}=\frac{5.4\times\Big(\frac{3}{5}\Big)}{1^2}=3.24\text{ lux}$

Need a full question paper?

Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.

Start Generating Free

Explore more

More "3 Marks Question" from Photometry→Photometry — all question types→Physics STD 12 Science question papers→Rajasthan Board STD 12 Science question papers→Rajasthan Board question paper generator→

Similar questions

An electric bulb, when connected across a power supply of 220V, consumes a power of 60W. If the supply drops to 180V, what will be the power consumed? If the supply is suddenly increased to 240V, what will be the power consumed?
→
An object P is focused by a microscope M. A glass slab of thickness 2.1cm is introduced between P and M. If the refractive index of the slab is 1.5, by what distance should the microscope be shifted to focus the object again?
→
Find the equivalent resistance between the terminals $A$ and $B$ in the network shown in Figure. Given each resistor $R$ is $10 \Omega$.
Image
→
  1. Derive the relation between the decay constant and half life of a radioactive substance.
  2. A radioactive element reduces to 25% of its initial mass in 1000 years. Find its half life.
→
Radiation of wavelength 5000 Å falls on a metal of work function 1.9 eV. Calculate the energy of emitted photoelectrons and stopping potential.
→
It is found that photosynthesis starts in certain plants when exposed to the sunlight but it does not start if the plant is exposed only to infrared light. Explain.
→
Four identical rods $AB, CD, CF$ and $DE$ are joined as shown in figure. The length, cross$-$sectional area and thermal conductivity of each rod are $l, A$ and $K$ respectively. The ends $A, E$ and $F$ are maintained at temperatures $T_1, T_2$ and $T_3$ respectively. Assuming no loss of heat to the atmosphere, find the temperature at $B.$​​​​​​​
→
A circular coil of $200$ turns and radius $10 \ cm$ is placed in a uniform magnetic field of $0.5 T,$ normal to the plane of the coil. If the current in the coil is $3.0 A,$ calculate the
  1. Total torque on the coil.
  2. Total force on the coil.
  3. Average force on each electron in the coil, due to the magnetic field.
Assume the area of cross$-$section of the wire to be $10–5m^2$ and the free electron density is $10^{29}/m^3.$
→
  1. Consider an arbitrary electrostatic field configuration. A small test charge is placed at a null point (i.e., where E = 0) of the configuration. Show that the equilibrium of the test charge is necessarily unstable.
  2. Verify this result for the simple configuration of two charges of the same magnitude and sign placed a certain distance apart.
→
Explain Electric Flux.
→