Optics

71. If the focal length of a convex lens is 50 cm, which one of the follow

If the focal length of a convex lens is 50 cm, which one of the following is its power?

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+2 dioptre
+0•02 dioptre
-0•5 dioptre
+0•5 dioptre
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UPSC NDA-2 – 2018
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The power of the convex lens with a focal length of 50 cm is +2 dioptre.
– The power of a lens (P) is the reciprocal of its focal length (f).
– The focal length must be expressed in meters when calculating power in dioptres.
– Given focal length f = 50 cm.
– Convert focal length to meters: 50 cm = 0.50 m.
– Power P = 1 / f (in meters).
– P = 1 / 0.50 m = 2 dioptres.
– A convex lens has a positive focal length and therefore a positive power.
– The unit of power of a lens is the dioptre (D), which is equal to m⁻¹.
– Convex lenses are converging lenses and are used to correct hypermetropia (far-sightedness).

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72. The wavelength of X-rays is of the order of

The wavelength of X-rays is of the order of

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1 Å
1 μm
1 mm
1 cm
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UPSC NDA-2 – 2018
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X-rays are a form of electromagnetic radiation with wavelengths ranging approximately from 0.01 to 10 nanometers (nm). An Angstrom (Å) is a unit of length equal to 0.1 nanometers, or 10⁻¹⁰ meters. Therefore, the wavelength of X-rays is of the order of Angstroms (1 Å = 0.1 nm). The other options represent much longer wavelengths: 1 μm (micrometer) = 1000 nm (infrared), 1 mm (millimeter) = 1,000,000 nm (microwave/radio), 1 cm (centimeter) = 10,000,000 nm (radio).
The electromagnetic spectrum covers a wide range of wavelengths and frequencies. Different types of radiation (radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays) are characterized by their distinct wavelength ranges. X-rays are high-energy, short-wavelength radiation.
X-rays are commonly used in medical imaging (radiography) due to their ability to penetrate soft tissues but be absorbed by denser materials like bones. They are also used in crystallography (X-ray diffraction) to determine the atomic and molecular structure of crystals, as the wavelengths of X-rays are comparable to the spacing between atoms in crystal lattices.

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73. Consider the following statements about a microscope and a telescope :

Consider the following statements about a microscope and a telescope :

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  • Both the eyepiece and the objective of a microscope are convex lenses.
  • The focal length of the objective of a telescope is larger than the focal length of its eyepiece.
  • The magnification of a telescope increases with the increase in focal length of its objective.
  • The magnification of a microscope increases with the increase in focal length of its objective.

Which of the statements given above are correct?

1 and 3 only
1 and 4
2, 3 and 4
1, 2 and 3
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UPSC NDA-2 – 2018
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Statements 1, 2, and 3 are correct.
Microscopes and telescopes are optical instruments that use lenses (or mirrors) to produce magnified images. A compound microscope typically uses two convex lenses (objective and eyepiece). A refracting telescope uses an objective lens (usually convex) and an eyepiece lens (convex or concave). The magnification properties depend on the focal lengths and arrangement of the lenses.
1. Both the eyepiece and the objective of a microscope are convex lenses: Correct for a standard compound microscope.
2. The focal length of the objective of a telescope is larger than the focal length of its eyepiece: Correct for a refracting telescope, this arrangement helps achieve significant angular magnification.
3. The magnification of a telescope increases with the increase in focal length of its objective: Correct. For a refracting telescope, angular magnification is approximately the ratio of the objective’s focal length to the eyepiece’s focal length (M ≈ f_objective / f_eyepiece). Increasing f_objective increases magnification.
4. The magnification of a microscope increases with the increase in focal length of its objective: Incorrect. For a compound microscope, the magnification produced by the objective is inversely proportional to its focal length (Magnification_objective ≈ L/f_objective, where L is the distance between lenses). Therefore, increasing the focal length of the objective decreases the magnification.

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74. Consider the following statements about visible light, UV light and X-

Consider the following statements about visible light, UV light and X-rays :

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  • 1. The wavelength of visible light is more than that of X-rays.
  • 2. The energy of X-ray photons is higher than that of UV light photons.
  • 3. The energy of UV light photons is less than that of visible light photons.

Which of the statements given above is/are correct?

1, 2 and 3
1 and 2 only
2 and 3 only
1 only
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UPSC NDA-2 – 2018
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Statements 1 and 2 are correct, while statement 3 is incorrect.
The electromagnetic spectrum is ordered by wavelength and energy/frequency. Wavelength and energy/frequency are inversely related (Energy E = hc/λ, where h is Planck’s constant, c is the speed of light, and λ is wavelength). Lower wavelength corresponds to higher energy/frequency. The order of these radiations by increasing wavelength (decreasing energy/frequency) is X-rays < UV light < Visible light.
1. Wavelength of visible light is more than that of X-rays: Correct, as visible light is further towards the longer wavelength end of the spectrum compared to X-rays.
2. The energy of X-ray photons is higher than that of UV light photons: Correct, as X-rays are to the higher frequency/energy side of UV light in the spectrum.
3. The energy of UV light photons is less than that of visible light photons: Incorrect, as UV light is to the higher frequency/energy side of visible light. UV photons have higher energy than visible light photons.

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75. The full form of LED is

The full form of LED is

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Light Emitting Diode
Light Emitting Device
Light Enhancing Device
Light Enhancing Diode
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LED stands for Light Emitting Diode. It is a semiconductor device that emits light when an electric current is passed through it.
This is a straightforward question testing basic knowledge of common electronic component abbreviations.
LEDs are a type of diode, which is a two-terminal electronic component that conducts current primarily in one direction. When forward biased, electrons and holes recombine in the semiconductor material, releasing energy in the form of photons (light).

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76. Which one of the following statements is not correct ?

Which one of the following statements is not correct ?

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Human eye is a refracting system containing a diverging lens.
The retina of the human eye contains millions of light sensitive cells, called rods and cones, which convert the light into electrical messages.
Every image that is focused on the retina is upside down.
We need both eyes to judge the relative positions of objects accurately.
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UPSC NDA-2 – 2017
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The correct answer is A.
Statement A is incorrect because the human eye contains a converging lens (convex lens), not a diverging lens, which focuses light onto the retina.
Statement B is correct as the retina has photoreceptor cells (rods and cones) that convert light into electrical signals. Statement C is correct; the image formed on the retina is real and inverted. Statement D is correct; binocular vision allows for depth perception and accurate judgment of relative positions.

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77. A rainbow is produced due to which one of the following phenomena?

A rainbow is produced due to which one of the following phenomena?

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Dispersion of light
Interference of light
Diffraction of light
Scattering of light by atmospheric dust
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UPSC NDA-2 – 2017
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The correct answer is Dispersion of light.
A rainbow is formed when sunlight passes through raindrops, which act like tiny prisms. White light is dispersed (split) into its constituent colours (spectrum) as it is refracted upon entering and leaving the raindrop, and also undergoing internal reflection. The dispersion of light is the phenomenon responsible for separating the colours.
Refraction and internal reflection are also essential processes in rainbow formation, allowing the dispersed light to reach the observer’s eyes at specific angles for each colour. However, the appearance of the band of colours itself is due to dispersion, the phenomenon by which the angle of refraction of light depends on its wavelength. Interference and diffraction relate to wave properties that are not the primary cause of the colour separation in a rainbow. Scattering by atmospheric dust contributes to phenomena like the blue colour of the sky or the reddish sunset, but not the spectrum seen in a rainbow formed by raindrops.

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78. Concave mirror is used in headlights of vehicles, because it

Concave mirror is used in headlights of vehicles, because it

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focuses light from the bulb onto nearby vehicles
sends parallel rays
fits well into the shape of the headlight
is cheaper than other mirrors
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UPSC NDA-2 – 2017
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A concave mirror is used in vehicle headlights because, when the light source (bulb) is placed at or near its focal point, it reflects the light rays as a nearly parallel beam. This parallel beam effectively projects the light forward, illuminating the road ahead.
Concave mirrors are used as reflectors in headlights to create a focused, parallel beam of light by placing the light source at or near the mirror’s focal point.
Other options are incorrect: A concave mirror reflects light forward, not onto nearby vehicles in a focused way; while shape fitting and cost are practical considerations, they are not the primary *optical* reason for choosing a concave mirror. Parabolic reflectors, a specific type of concave mirror, are commonly used in headlights to produce a highly parallel beam.

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79. The mirrors used as rear-view mirrors in vehicles are

The mirrors used as rear-view mirrors in vehicles are

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concave
convex
cylindrical
plane
This question was previously asked in
UPSC NDA-2 – 2017
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Rear-view mirrors in vehicles are typically convex mirrors. Convex mirrors form virtual, erect, and diminished images of objects. The principal advantage of a convex mirror for this purpose is that it provides a wider field of view compared to a plane mirror of the same size. This allows the driver to see a larger area behind the vehicle, enhancing safety. While the image is smaller (diminished), this is an acceptable trade-off for the increased field of vision.
– Convex mirrors produce virtual, erect, and diminished images.
– Convex mirrors provide a wider field of view.
Concave mirrors can produce real or virtual, inverted or erect, magnified or diminished images depending on the object’s position. They are not suitable for providing a consistent wide-angle view of distant objects. Plane mirrors produce virtual, erect, and same-sized images, but their field of view is limited compared to convex mirrors of the same size.

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80. Electron emission from a metallic surface by application of light is k

Electron emission from a metallic surface by application of light is known as

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Thermionic emission
Photoelectric emission
High field emission
Autoelectronic emission
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UPSC NDA-2 – 2017
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Electron emission from a metallic surface caused by the application of light is known as the photoelectric effect, and the emission itself is called photoelectric emission. When light of sufficient frequency strikes a metal surface, it can impart enough energy to electrons to overcome the work function (binding energy) and be ejected from the surface.
– Photoelectric emission is electron emission triggered by light.
– This phenomenon is the basis of the photoelectric effect.
Thermionic emission is electron emission due to heat. High field emission (or field emission or autoelectronic emission) is electron emission caused by a strong electric field. These are different mechanisms for electron emission from surfaces.

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