A screen is placed a distance 40cm away from an illuminated object. A converging lens is placed between the source and the screen and it is attempted to form the image of the source on the screen. If no position could be found, the focal length of the lens:
Explanation:
$\text{v}=(40-4)$
$\frac{1}{\text{f}}=\frac{1}{40-4}-\frac{1}{(-\text{u})}$
$\frac{\text{df}}{\text{du}}=0$ for f minimum.
$\frac{\text{df}}{\text{du}}=1-\frac{\text{u}}{20}=0$
$\text{u}=20$
$\text{f}_{\text{min}}=10\text{cm}$
Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.
The binding energy of the innermost electron in tungsten is 40 keV. To produce characteristic X-rays using a tungsten target in an X-rays tube the potential difference V between the cathode and the anti-cathode should be
|
(a) V< 40 kV |
(b) V ≤ 40 kV |
(c) V > 40 kV |
(d) V >/< 40 kV |
For the circuit shown, which of the following statements is true
|
(a) With |
|
(b) With |
|
(c) With |
|
(d) With |
A narrow electron beam passes undeviated through an electric field E = 3 
and an overlapping magnetic field B = 2 
. If electric field and magnetic field are mutually perpendicular. The speed of the electrons is
|
(a) 60 m/s |
(b) 10.3 |
(c) 1.5 |
(d) 0.67 |
In order to coincide the parabolas formed by singly ionised ions in one spectrograph and doubly ionized ions in the other Thomson’s mass spectrograph, the electric fields and magnetic fields are kept in the ratios 1 : 2 and 3 : 2 respectively. Then the ratio of masses of the ions is
|
(a) 3 : 4 |
(b) 1 : 3 |
(c) 9 : 4 |
(d) None of these |
A sample of radioactive element has a mass of 10 gm at an instant t = 0.The approximate mass of this element in the sample after two mean lives is
|
(a) 2.50 gm |
(b) 3.70 gm |
(c) 6.30 gm |
(d) 1.35 gm |
Focal length of a convex lens of refractive index 1.5 is 2 cm. Focal length of lens when immersed in a liquid of refractive index of 1.25 will be
|
(a) 10 cm |
(b) 2.5 cm |
(c) 5 cm |
(d) 7.5 cm |
