The minimum magnifying power of an astronomical telescope is 40. If the length of the telescope is 205 cm, then the focal length of its field lens is:

1. 5 cm

2. 200 cm

3. 40 cm

4. 140 cm

Subtopic:  Telescope |
 66%
From NCERT
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A boy with defective eye-sight cannot see things beyond 50 cm. The corrective lens required has the power:

1. +1 D

2. +2 D

3. -1 D

4. -2 D

Subtopic:  Human Eye |
 69%
From NCERT
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The focal lengths of the objective and eyepiece of a terrestrial telescope are 50 cm and 10 cm respectively. The magnifying power of the telescope when the final image is formed at infinity is

(1) 3.75

(2) -2.5

(3) -4.25

(4) +5

Subtopic:  Telescope |
 87%
From NCERT
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A convex mirror of focal length \(20\) cm forms an image that is quarter of the object's size. The distance of the object from the mirror is:
1. \(50\) cm
2. \(-60\) cm
3. \(-40\) cm
4. \(-70\) cm

Subtopic:  Reflection at Spherical Surface |
 84%
From NCERT
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The focal length of a thin convex lens for red and blue light is 0.5m and 0.484m.  The dispersive power of the lens is 

1. 0.984

2. 0.032

3. 0.484

4. 0.434

Subtopic:  Lenses |
 69%
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What is the length of the optical path of two media in contact having thickness 2 cm and 3 cm and refractive indices 1.5 and 1.7 respectively?

(1) 4.5 cm

(2) 8.1 cm

(3) 6.2 cm

(4) 9.4 cm

Subtopic:  Refraction at Plane Surface |
 77%
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Two thin convex lenses of focal length 20cm and 5cm are placed 10cm apart. The power of the combination of lenses is 

1. 10D

2. 15D

3. 20D

4. 25D

Subtopic:  Lenses |
 71%
From NCERT
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A converging beam of light forms a point image on a screen of an object. A plane glass plate of thickness 2 cm and a refractive index 32 is introduced in the path of the beam. The screen should be shifted with respect to its initial position to obtain point image on the screen will be

(1) 3 cm

(2) 1.5 cm

(3) 0.67 cm

(4) 1.33 cm

Subtopic:  Refraction at Plane Surface |
 70%
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Select the incorrect option

(1) A plane mirror always forms a virtual image

(2) The virtual image formed by a concave mirror is always enlarged

(3) The critical angle is always defined in the denser medium

(4) A glass slab cannot deviate the light

Subtopic:  Reflection at Plane Surface |
 52%
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Select the correct option.

(1) The velocity of light is constant in all media

(2) A convex lens always forms a real image

(3) If the aperture of the lens is halved then image size will be halved

(4) The ray optics is applicable when the size of the obstacle is more than the wavelength of light

Subtopic:  Lenses |
 69%
From NCERT
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