﻿ A telescope has focal length of objective and eye-piece as 2

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# 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

A.

40

In relaxed eye state of telescope or when final image is formed at infinity, the magnification of telescope is given by

where fo = focal length of objective = 200 cm
fe = focal length of eye-piece = 5 cm

hence

= 40

Note:- The aperture of the objective lens is kept large so that more and more rays coming from the heavenly body may enter the telescope and a bright image is formed by the objective lens. The aperture of the eye lens is kept comparatively small so that all the rays may enter the eye.

32.

Hydrogen bomb is based upon

• fission

• fusion

• chemical reaction

• transmutation

33.

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

34.

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

• wavelength

• frequency

• amplitude

• angle of incidence

35.

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

36.

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

37.

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

38.

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

39.

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

40.

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