Two thin lenses of focal lengths f₁ and f₂ are in contact and coaxial. The power of the combination is

A boy is trying to start a fire by focusing sunlight on a piece of paper using an equiconvex lens of focal length 10 cm. The diameter of the sun is 1.39 $$ \times $$ 10⁹ m and its mean distance from the earth is 1.5 $$ \times $$ 10¹¹ m. What is the diameter of the sun's image on the paper ?

A particle of mass 1 mg has the same wavelength as an electron moving with a velocity of 3 $$ \times $$ 10⁶ m s^$$-$$1. The velocity of the particle is

The work function of a surface of a photosensitive material is 6.2 eV. The wavelength of the incident radiation for which the stopping potential is 5 V lies in the