Question of Exercise 1

Optics of Class 12

A point source of light is 60 cm from a screen and is kept at the focus of a concave mirror which reflects light on the screen. The focal length of the mirror is 20 cm. The ratio of average intensities of the illumination on the screen when the mirror is present and when the mirror is removed is :


Option 1 36 : 1

Option 2 37 : 1

Option 3 49 : 1

Option 4 10 : 1

Frequently Asked Questions

Calculate the stopping potential Vs2

A: 4.47

B: 3.16

C: 2.76

D: 5.28






Final Answer

Therefore, option A is correct that is 4.47 V.


State and explain Coulombs law in electrostatics



  • Coulomb’s law in electrostatic states that a charge at rest q1 which applies a force F on the other charge q2, which is also at

rest both the charge are separated by the distance r. The force is directly proportional to the product of bothe of these charges

and inversely proportional to the square of the same distance which is separated both the charges

. distance



A cube of side b has a charge q at each of its vertices

Determine the potential and electric field due to this charge array at the centre of the cube.




First make the diagram of the cube of side b, that is shown below

shown below

shown below

shown below

The clear sky appears blue because

A: blue light gets absorbed in the atmosphere.

B: ultraviolet radiations are absorbed in the atmosphere.

C: violet and blue lights get scattered more than lights of all other colours by the atmosphere.

D: light of all other colours is scattered more than the violet and blue colour lights by the atmosphere.




  • Rayleigh scattering of sunlight causes the clear sky to seem blue.
  • The sun's blue light is scattered more by the molecules in the air than it is by red light.
  • The size of the dust particles and the wavelength of the light affect light scattering.
  • The quantity of scattering is inversely proportional to the wavelength multiplied by the fourth power.
  • The sky appears blue because violet and blue sunlight are dispersed more than all other colours by the atmosphere.

Final Answer

The correct option is C i.e violet and blue lights get scattered more than lights of all other colours by the atmosphere.


The image formed by convex lens in a simple microscope is

A: Virtual and erect

B: Real and inverted

C: Of same size as the object

D: None of these




The image formed by the convex lens in a simple microscope is virtual, erect and magnified.

This can be clearly seen with the help of the ray diagram:

Final Answer

The correct answer is A i.e Virtual and erect.


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