31.

A telescope has focal length of objective and eye-piece as 200 cm and 5 cm respectively. What is magnification of telescope?

• 40

• 80

• 50

• 0.01

Hydrogen bomb is based upon

• fission

• fusion

• chemical reaction

• transmutation

Hard X-rays for the study of fractures in bones should have a minimum wavelength of 10' m. The accelerating voltage for electrons in X-ray machine should be

• < 124 kV

• > 124 kV

• between 60 kV and 70 kV

• = 100 kV

In photoelectric effect, the electrons are ejected from metals if the incident light has a certain minimum

• wavelength

• frequency

• amplitude

• angle of incidence

A leaf which contains only green pigments, is illuminated by a laser light of wavelength 6328 pm. It would appear to be

• brown

• black

• red

• green

The operation of a nuclear reactor is said to be critical, if the multiplication factor (k) has a value

• 1

• 1.5

• 2.1

• 2.5

Flash light equipped with a new set of batteries, produces bright white light. As the batteries wear out

• the light intensity gets reduced with no change in its colour

• light colour changes first to yellow and then red with no change in intensity

• it stops working suddenly while giving white light

• colour changes to red and also intensity gets reduced

If alpha, beta and gamma rays carry same momentum, which has the longest wavelength?

• Alpha rays

• Beta rays

• Gamma rays

• None all have same wavelength

Half-lives of two radioactive substances A and B are respectively 20 min and 40 min. Initially the samples of A and B have equal number of nuclei. After 80 min the ratio of remaining number of A and B nuclei is

• 1 : 16

• 4 : 1

• 1 : 4

• 1 : 1

Which of the following is true for rays coming from infinity?

• Two images are formed

• continuous image is formed between focal points of upper and lower lens

• One image is formed

• None of the above