51.

The light reflected by a plane mirrormay form a real image

• if the rays incident on the mirror are converging

• if the rays incident on the mirror are diverging

• under no circumstances

• if the object is placed very close to the mirror

A convex is made up of three different materials as shown in the figure. For a point object places on its axis, the number of images formed are

• 5

• 1

• 3

• 4

Light appears to travel in straight lines because

• light consists of very small particles

• the frequency of light is very small

• the velocity of light is different for different colours

• the wavelength of light is very small

In Young's double slit experiment, the central bright fringe can be identified

• as it is narrower than other bright fringes

• by using white light instead of monochromatic light

• as it has a greater intensity than other bright fringes

• as it is wider than other bright fringes

An object is placed 12 cm to the left of a converging lens of focal length 8 cm. Another converging lens of 6 cm focal length is placed at a distance of 30 cm to the right of the first lens. The second lens will produce

• a virtual enlarged image

• no image

• a real inverted image

• a real enlarged image

Whenever a hydrogen atom emits a photon in the Balmer series, it

• may emit another photon in the Paschen series

• need not emit any more photon

• may emit another photon in the Balmer series

• must emit another photon in the Lyman series

The SI unit of radioactivity is

• Rutherford

• Roentgen

• Becqueral

• Curie

To increase both the resolving power and magnifying power of a telescope

• the focal length of the objective has to be increased

• both the focal length and aperture of the objective has to be increased

• the wavelength of light has to be decreased

• the aperture of the objective has to be increased

In the Bohr model of the hydrogen atom, let R, V and E represent the radius of the orbit, the speed of electron and the total energy of the electron respectively. Which of the following quantity is proportional to the quantum number n ?

• $\frac{\mathrm{E}}{\mathrm{V}}$

• $\frac{\mathrm{R}}{\mathrm{E}}$

• VR

• RE

In a sample of radioactive material, what percentage of the initial number of active nuclei will decay during one mean life ?

• 63 %

• 69.3 %

• 37 %

• 50 %