Optics

11. With reference to street-lighting, how do sodium lamps differ from LED

With reference to street-lighting, how do sodium lamps differ from LED lamps?

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  • 1. Sodium lamps produce light in 360 degrees but it is not so in the case of LED lamps.
  • 2. As street-lights, sodium lamps have longer life span than LED lamps.
  • 3. The spectrum of visible light from sodium lamps is almost monochromatic while LED lamps offer significant colour advantages in street-lighting.

Select the correct answer using the code given below.

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3 only
2 only
1 and 3 only
1, 2 and 3
This question was previously asked in
UPSC IAS – 2021
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Statement 1 is correct. Traditional sodium lamps (like high-pressure sodium lamps) emit light omnidirectionally (in all directions), requiring reflectors to direct light downwards onto the street. LED lamps, by contrast, are inherently directional light sources, meaning they emit light primarily in a specific direction (forward). This directional nature allows for more efficient targeting of light where needed. Statement 2 is incorrect. LED lamps generally have a much longer lifespan (typically 50,000 to 100,000+ hours) compared to sodium lamps (typically 20,000 to 30,000 hours). Statement 3 is correct. High-pressure sodium lamps produce a predominantly yellow-orange light, resulting in poor color rendering (objects appear unnatural or monochrome). LED lamps can produce white light with various color temperatures and offer significantly better color rendering index (CRI), which improves visibility, recognition of objects and colors, and safety under street lighting.
– Sodium lamps emit light omnidirectionally, while LEDs are directional.
– LEDs have a significantly longer lifespan than sodium lamps.
– Sodium lamps provide poor color rendering (monochromatic light), while LEDs offer much better color rendering.
LED street lights also offer advantages in terms of energy efficiency (lower power consumption), instant on/off capability, dimmability, and environmental benefits (no mercury). These factors have led to a widespread shift from sodium vapor lamps to LED technology for street lighting globally.

12. Rainbow is produced when sunlight falls on drops of rain. Which of the

Rainbow is produced when sunlight falls on drops of rain. Which of the following physical phenomena are responsible for this?

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  • 1. Dispersion
  • 2. Refraction
  • 3. Internal reflection

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1 and 2 only
2 and 3 only
1 and 3 only
1, 2 and 3
This question was previously asked in
UPSC IAS – 2013
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Rainbow is produced when sunlight falls on drops of rain, and the phenomena responsible are dispersion, refraction, and internal reflection.
When sunlight enters a raindrop, it is refracted (bent). As the light enters, it also undergoes dispersion, splitting into its constituent colours because different wavelengths of light are refracted at slightly different angles. The dispersed light then reflects off the back inner surface of the raindrop (internal reflection). Finally, the light is refracted again as it exits the raindrop towards the observer. All three processes are essential for the formation of a rainbow.
Primary rainbows are formed by light undergoing one internal reflection. Secondary rainbows, which are fainter and appear above the primary rainbow with the colours reversed, are formed by light undergoing two internal reflections. In both cases, refraction and dispersion occur at the air-water interfaces.

13. Consider the following phenomena : 1. Size of the sun at dusk 2. Co

Consider the following phenomena :

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  • 1. Size of the sun at dusk
  • 2. Colour of the sun at dawn
  • 3. Moon being visible at dawn
  • 4. Twinkle of stars in the sky
  • 5. Polestar being visible in the sky

Which of the above are optical illusions?

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1, 2 and 3
3, 4 and 5
1, 2 and 4
2, 3 and 5
This question was previously asked in
UPSC IAS – 2013
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The size of the sun at dusk, the colour of the sun at dawn, and the twinkle of stars in the sky are considered optical phenomena or illusions.
1. The apparent increase in the size of the sun (or moon) near the horizon (dusk) is an optical illusion known as the Moon illusion. 2. The reddish/orange colour of the sun at dawn is due to atmospheric scattering (Rayleigh scattering), where shorter wavelengths (blue/green) are scattered away more than longer wavelengths (red/orange) when light travels a longer path through the atmosphere. This phenomenon leads to a perceived colour change. 4. The twinkling of stars is caused by atmospheric refraction due to turbulence in the air, which makes the light from distant stars appear to fluctuate in brightness and position.
Phenomena 3 (Moon being visible at dawn) and 5 (Polestar being visible in the sky) are real astronomical occurrences related to the celestial positions of these bodies and are not optical illusions in the sense of being perceptual distortions or atmospheric effects causing altered appearance. Therefore, 1, 2, and 4 fit the description of optical illusions or related atmospheric optical phenomena causing altered visual perception.

14. Which one among the following has largest energy per photon ?

Which one among the following has largest energy per photon ?

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X-ray
Ultra-violet ray
Visible-ray
Infra-red ray
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UPSC CAPF – 2024
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The energy of a photon is directly proportional to its frequency. In the electromagnetic spectrum, energy increases as frequency increases and wavelength decreases. Among the given options, X-rays have the highest frequency (and shortest wavelength), followed by Ultra-violet rays, Visible rays, and Infra-red rays.
– The relationship between photon energy (E), Planck’s constant (h), frequency (f), speed of light (c), and wavelength (λ) is given by E = hf = hc/λ.
– Higher frequency or shorter wavelength corresponds to higher photon energy.
– The order of electromagnetic radiation in increasing frequency (and thus increasing energy per photon) is: Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma ray.
Visible light occupies a small portion of the spectrum. Infrared is below visible light in frequency, and Ultraviolet, X-rays, and Gamma rays are above visible light in frequency. X-rays are high-energy electromagnetic radiation capable of penetrating various materials, widely used in medical imaging and security screening.

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15. The magnitude of focal length of a concave lens is 2 m. What is the po

The magnitude of focal length of a concave lens is 2 m. What is the power of the lens?

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+0.5 dioptre
-0.5 dioptre
+2.0 dioptre
-1.0 dioptre
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UPSC CAPF – 2022
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The correct answer is B, -0.5 dioptre.
The power (P) of a lens is defined as the reciprocal of its focal length (f), when the focal length is measured in meters. The unit of power is the dioptre (D), where 1 D = 1 m⁻¹.
The formula is P = 1/f.
For a concave lens, the focal length is conventionally considered negative.
Given the magnitude of the focal length of the concave lens is 2 m, the focal length f = -2 m.
Power P = 1 / (-2 m) = -0.5 m⁻¹ = -0.5 dioptre (D).
Convex lenses have positive focal lengths and positive power, converging light rays. Concave lenses have negative focal lengths and negative power, diverging light rays. The power of a lens indicates its ability to converge or diverge light; a higher magnitude of power corresponds to a shorter focal length and stronger bending of light.

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16. The optical phenomenon responsible for the blue colour of sky is

The optical phenomenon responsible for the blue colour of sky is

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dispersion
reflection
refraction
scattering
This question was previously asked in
UPSC CAPF – 2022
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The correct answer is D, scattering.
The blue colour of the sky is primarily caused by the scattering of sunlight by molecules and tiny particles in the Earth’s atmosphere.
This phenomenon is explained by Rayleigh scattering, which states that the amount of scattering is inversely proportional to the fourth power of the wavelength. Shorter wavelengths (like blue and violet) are scattered much more effectively than longer wavelengths (like red and orange).
When sunlight enters the atmosphere, the blue light is scattered in all directions by the atmospheric particles, making the sky appear blue. Our eyes are more sensitive to blue light than violet, contributing to the perception of a blue sky.
Dispersion is the splitting of white light into its constituent colours due to different wavelengths having different refractive indices in a medium (like in a prism or rainbow). Reflection is the bouncing of light off a surface. Refraction is the bending of light as it passes from one medium to another. While refraction plays a role in atmospheric phenomena like mirages, it’s scattering that causes the blue sky.

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17. Which one of the following materials *cannot* be used to make a convex

Which one of the following materials *cannot* be used to make a convex lens?

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Aluminium
Glass
Sapphire
Water
This question was previously asked in
UPSC CAPF – 2022
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The correct answer is A, Aluminium.
A lens works by refracting (bending) light as it passes through the material. For a material to be used to make a lens, it must be transparent or at least translucent to the type of light intended to be used (e.g., visible light for optical lenses).
Glass, Sapphire, and Water are all transparent or translucent materials that can refract light, making them suitable for manufacturing lenses.
Aluminium is a metal that is opaque to visible light. Light does not pass through it but is reflected or absorbed. Therefore, Aluminium cannot be used to make a convex lens for visible light.
Lenses are typically made from materials like glass, plastic, quartz, sapphire, fluorite, or even liquids like water. The choice of material depends on the desired optical properties (like refractive index, dispersion), cost, durability, and the specific wavelength range of light for which the lens is intended.

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18. Which one among the following is not a luminous object?

Which one among the following is not a luminous object?

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Sun
Proxima Centauri
Jupiter
Alpha Centauri
This question was previously asked in
UPSC CAPF – 2021
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Luminous objects are those that produce and emit their own light. Non-luminous objects do not produce light but are visible because they reflect light from a luminous source.
The Sun, Proxima Centauri, and Alpha Centauri are all stars. Stars are massive, luminous celestial bodies that emit light through nuclear fusion. Jupiter is a planet, which is a large celestial body orbiting a star. Planets are non-luminous; they are visible because they reflect the light from their star (in Jupiter’s case, the Sun).
Examples of luminous objects include stars, lamps, and candles. Examples of non-luminous objects include planets, moons, tables, and chairs.

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19. The following figure shows the image of a clock in a plane mirror : [I

The following figure shows the image of a clock in a plane mirror :
[Image of a clock face with hands]
Which one of the following is the correct time?

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2:35
3:45
9:15
9:25
This question was previously asked in
UPSC CAPF – 2021
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A plane mirror produces a laterally inverted image. To find the actual time from a mirror image of a clock, one can imagine flipping the clock face horizontally or subtract the mirror time from 12:00.
Observing the image provided, the minute hand points exactly at the ‘7’, indicating 35 minutes. The hour hand is positioned between ‘2’ and ‘3’, closer to ‘2’. This position of the hour hand, coupled with the minute hand at 7 (35 minutes past the hour), suggests the time shown in the mirror image is 2:35.
If the mirror image shows 2:35, the actual time can be calculated as 12:00 – 2:35. 12:00 is equivalent to 11 hours and 60 minutes. So, (11 hours – 2 hours) and (60 minutes – 35 minutes) gives 9 hours and 25 minutes. Thus, the correct time is 9:25. This matches option D.

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20. When a white light beam is made to fall on a hollow prism filled with

When a white light beam is made to fall on a hollow prism filled with water, it breaks into seven constituent colours. Which of the following colours suffers the maximum angle of deviation?

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Red
Violet
Green
All suffer by equal amount of deviation
This question was previously asked in
UPSC CAPF – 2021
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When a beam of white light passes through a prism, it is dispersed into its seven constituent colours due to the difference in the refractive index of the prism material for different wavelengths of light. The extent to which a colour is deviated depends on the refractive index for that colour; a higher refractive index leads to a greater deviation.
The refractive index of a material is highest for shorter wavelengths (like violet light) and lowest for longer wavelengths (like red light). Consequently, violet light undergoes the maximum deviation, while red light undergoes the minimum deviation.
This phenomenon of splitting white light into its constituent colours is called dispersion. The band of seven colours produced is called the spectrum. The acronym VIBGYOR represents the order of colours in the spectrum from maximum deviation (Violet) to minimum deviation (Red). A hollow prism filled with water still functions as a prism made of water, exhibiting the same dispersion property.

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