Optics

21. Which one of the following statements with regard to a short-sighted p

Which one of the following statements with regard to a short-sighted person is correct?

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A short-sighted person sees the near objects blurred but distant objects clearly and a convex lens is used to correct this defect.
A short-sighted person sees the near objects clearly but distant objects blurred and a concave lens is used to correct this defect.
A short-sighted person sees the near objects blurred but distant objects clearly and a concave lens is used to correct this defect.
A short-sighted person sees the near objects clearly but distant objects blurred and a convex lens is used to correct this defect.
This question was previously asked in
UPSC CAPF – 2021
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Short-sightedness is medically known as Myopia.
A person with myopia has difficulty seeing distant objects clearly, while they can see near objects clearly. This occurs because the eye converges distant light rays too strongly, causing the image to form in front of the retina instead of on the retina.
To correct myopia, a diverging lens (concave lens) is used. A concave lens spreads out the light rays slightly before they enter the eye, reducing the overall convergence and allowing the image of distant objects to be focused correctly on the retina.
Let’s examine the options:
A) States near objects are blurred and distant objects are clear, which is the opposite of myopia (this describes hyperopia or long-sightedness).
B) States near objects are clear and distant objects are blurred, which correctly describes myopia, and also correctly states that a concave lens is used for correction.
C) States near objects are blurred and distant objects are clear (hyperopia), and suggests a concave lens (corrects myopia). Incorrect combination.
D) States near objects are clear and distant objects are blurred (myopia), but suggests a convex lens (corrects hyperopia). Incorrect lens.
– Understand the definition and symptoms of short-sightedness (Myopia).
– Know the type of lens (converging/convex or diverging/concave) used to correct Myopia.
– Myopia: Near objects clear, distant objects blurred. Image forms in front of retina. Corrected by concave lens.
– Hyperopia: Near objects blurred, distant objects clear. Image forms behind retina. Corrected by convex lens.
The power of a lens used to correct myopia is negative, which corresponds to a concave lens. The power needed depends on the severity of the myopia. Myopia can be caused by an eyeball that is too long or a cornea/lens that is too curved.

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22. An object is placed 10 cm in front of a lens. The image formed is real

An object is placed 10 cm in front of a lens. The image formed is real, inverted and of same size as the object. What is the focal length and nature of the lens?

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5 cm, converging
10 cm, diverging
20 cm, converging
20 cm, diverging
This question was previously asked in
UPSC CAPF – 2021
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An object is placed 10 cm in front of a lens. So, object distance u = 10 cm.
The image formed is real, inverted, and of the same size as the object.
A real and inverted image is formed by a converging lens (convex lens). Diverging lenses (concave lenses) always produce virtual, erect, and diminished images.
For a lens, the magnification (m) is given by the ratio of image distance (v) to object distance (u), with a sign convention: m = v/u for erect images and m = -v/u for inverted images.
Since the image is inverted, m = -v/u.
The image is of the same size as the object, so the magnitude of magnification |m| = 1.
Thus, m = -1.
-v/u = -1 => v = u.
Given u = 10 cm, so v = 10 cm.
Using the lens formula (thin lens equation): 1/f = 1/v – 1/u.
Applying the sign convention: object distance u is typically taken as negative when placed in front of the lens, so u = -10 cm. The image is real and formed on the opposite side of the lens from the object, so image distance v is positive, v = +10 cm.
1/f = 1/(+10 cm) – 1/(-10 cm)
1/f = 1/10 + 1/10
1/f = 2/10
1/f = 1/5
f = 5 cm.
The focal length is positive (f > 0), which confirms that the lens is a converging lens (convex lens).
Alternatively, for a converging lens, a real image of the same size as the object is formed only when the object is placed at a distance of 2f from the lens, and the image is also formed at 2f on the other side. So, u = 2f.
Given u = 10 cm, 10 cm = 2f => f = 5 cm.
The nature of the lens is converging.
– Identify the nature of the lens based on the image characteristics (real, inverted implies converging).
– Use the magnification information (same size implies |m|=1) and image type (inverted implies m=-1) to relate object and image distances (v=u).
– Apply the lens formula (1/f = 1/v – 1/u) with appropriate sign conventions or use the special case rule for an image of the same size formed by a converging lens (object at 2f).
– Determine the sign of the focal length to confirm the nature of the lens (positive f for converging).
For a converging lens (convex lens):
– Object at infinity: real, inverted, point image at F.
– Object beyond 2F: real, inverted, diminished image between F and 2F.
– Object at 2F: real, inverted, same size image at 2F.
– Object between F and 2F: real, inverted, magnified image beyond 2F.
– Object at F: real, inverted, image at infinity.
– Object between the lens and F: virtual, erect, magnified image on the same side as the object.
The condition “real, inverted, and of same size” is unique to the object being placed at 2F for a converging lens.

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23. Which one of the following statements on photoelectric effect is NOT c

Which one of the following statements on photoelectric effect is NOT correct ?

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”Albert
”For
”At
”The
This question was previously asked in
UPSC CAPF – 2020
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The statement “At a frequency v > vā‚€, the kinetic energy of ejected electrons does not change on increasing the frequency of incident light” is NOT correct.
According to Einstein’s explanation of the photoelectric effect, the maximum kinetic energy (KE_max) of the ejected electrons is given by KE_max = h(v – vā‚€), where h is Planck’s constant, v is the frequency of incident light, and vā‚€ is the threshold frequency. This equation shows that for v > vā‚€, the maximum kinetic energy is directly proportional to the frequency of the incident light; increasing v leads to increased KE_max.
Statement A is correct; Einstein received the Nobel Prize in Physics in 1921 for his explanation of the photoelectric effect. Statement B is correct; a minimum frequency (threshold frequency) is required for electron emission. Statement D is correct; the number of emitted electrons is proportional to the intensity of the incident light (for v > vā‚€), as higher intensity means more photons striking the surface.

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24. When a beam of white light is allowed to fall on a prism, the maximum

When a beam of white light is allowed to fall on a prism, the maximum deviation of the emerging light occurs for

red light
violet light
yellow light
blue light
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UPSC CAPF – 2019
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The correct answer is B) violet light.
When a beam of white light passes through a prism, it undergoes dispersion, splitting into its constituent colours (spectrum: Violet, Indigo, Blue, Green, Yellow, Orange, Red – VIBGYOR). This happens because the refractive index of the prism material is different for different wavelengths (colours) of light.

* Refraction is the bending of light as it passes from one medium to another. The amount of bending depends on the refractive index of the medium and the angle of incidence.
* For visible light passing through a prism, the refractive index of the prism material is higher for shorter wavelengths and lower for longer wavelengths.
* Violet light has the shortest wavelength among the colours in the visible spectrum.
* Red light has the longest wavelength among the colours in the visible spectrum.
* Since the refractive index is highest for violet light, it is bent the most (deviated the maximum) when passing through the prism.
* Conversely, since the refractive index is lowest for red light, it is bent the least (deviated the minimum).

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The order of deviation from maximum to minimum is typically Violet > Indigo > Blue > Green > Yellow > Orange > Red.

This phenomenon of dispersion is described by Cauchy’s equation, which relates the refractive index of a material to the wavelength of light. This effect is responsible for the formation of rainbows, where raindrops act like tiny prisms, dispersing sunlight.

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25. Which one of the following is NOT a luminous object ?

Which one of the following is NOT a luminous object ?

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Sun
Electric lamp
Candle
Moon
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UPSC CAPF – 2019
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The correct answer is D) Moon.
Objects can be classified based on whether they produce their own light or merely reflect light from another source.

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* **Luminous Objects:** These objects emit their own light.
* A) Sun: The Sun is a star that produces light and heat through nuclear fusion. It is a luminous object.
* B) Electric lamp: An electric lamp (like an incandescent bulb, fluorescent tube, or LED lamp) produces light when electricity is passed through it. It is a luminous object.
* C) Candle: A burning candle produces light through the process of combustion. It is a luminous object.
* **Non-luminous Objects:** These objects do not produce their own light but become visible because they reflect light from a luminous source.
* D) Moon: The Moon does not generate its own light. We see the Moon because it reflects sunlight. Therefore, the Moon is a non-luminous object.

Most objects we see around us on Earth, such as furniture, books, trees, etc., are non-luminous. They are visible because they reflect ambient light from sources like the Sun or artificial lamps.

26. Brilliance of diamond is due to

Brilliance of diamond is due to

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interference of light
diffraction of light
polarization of light
total internal reflection of light
This question was previously asked in
UPSC CAPF – 2019
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The extraordinary brilliance and sparkle of a cut diamond are primarily due to its high refractive index and the specific angles at which its facets are cut, which together facilitate total internal reflection of light within the stone. Light entering the diamond undergoes multiple reflections before exiting, creating the sparkling effect.
– Total Internal Reflection (TIR) occurs when light travels from a denser medium to a less dense medium at an angle greater than the critical angle.
– Diamond has a very high refractive index (around 2.42), resulting in a small critical angle (around 24.4 degrees).
– The precise cutting of diamond facets is designed to maximise TIR, ensuring that a large amount of light is reflected back out through the top of the stone, enhancing its brilliance (white light reflection) and fire (dispersion of light into spectral colours).

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27. The term LASER stands for

The term LASER stands for

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Light Amplification by Stimulated Emission of Radiation
Light Amplification by Spontaneous Emission of Radiation
Light Amplification by Stimulated Emission of Rays
Light Amplification by Stimulated Energy of Radiation
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UPSC CAPF – 2018
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The term LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.
LASER is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The full form reflects the fundamental principle behind its operation.
The first working laser was demonstrated in 1960 by Theodore Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow, building on the principles established by Albert Einstein regarding stimulated emission.

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28. Which of the following statements about optical microscope is/are corr

Which of the following statements about optical microscope is/are correct?

  • 1. Both the eyepiece and objective of a microscope are convex lenses.
  • 2. The magnification of a microscope increases with increase in focal length of the objective.
  • 3. The magnification of a microscope depends upon the length of the microscope tube.
  • 4. The eyepiece of a microscope is a concave lens.

Select the correct answer using the code given below.

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1 and 3
3 only
3 and 4
1, 2 and 4
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UPSC CAPF – 2018
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The correct answer is A) 1 and 3.
Statement 1 is correct: In a standard compound optical microscope, both the objective lens (or lens system) and the eyepiece lens (or lens system) act as convex lenses to produce magnification. The objective creates a magnified real intermediate image, and the eyepiece magnifies this intermediate image to form a virtual final image.
Statement 2 is incorrect: The magnification of the objective lens is inversely proportional to its focal length. A shorter focal length objective provides higher magnification (and typically lower depth of field and working distance).
Statement 3 is correct: The overall magnification of a compound microscope is approximately the product of the magnification of the objective and the magnification of the eyepiece. The magnification provided by the objective is related to the distance between the objective and the intermediate image (which is determined by the microscope’s tube length) and the focal length of the objective. Thus, the length of the microscope tube affects the magnification.
Statement 4 is incorrect: As mentioned in statement 1, the eyepiece acts as a magnifying glass, which is achieved using a convex lens or a combination of lenses that function as a convex lens.
The total magnification of a compound microscope is typically calculated as M_total = M_objective Ɨ M_eyepiece. The objective magnification is often given by M_objective ā‰ˆ L / f_objective, where L is the mechanical tube length and f_objective is the focal length of the objective. This confirms that tube length (L) affects the magnification.

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29. The optical phenomenon that is responsible for the propagation of ligh

The optical phenomenon that is responsible for the propagation of light signal through an optical fibre is

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interference
scattering
total internal reflection
refraction
This question was previously asked in
UPSC CAPF – 2018
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The correct answer is C) total internal reflection.
Optical fibres work by guiding light along a core made of a material with a higher refractive index, surrounded by a cladding material with a lower refractive index. Light rays entering the core at appropriate angles strike the interface between the core and the cladding at an angle greater than the critical angle. When this condition is met, the light is completely reflected back into the core, a phenomenon called total internal reflection (TIR). This process repeats along the length of the fibre, allowing the light signal to propagate with minimal loss.
Refraction is the bending of light as it passes from one medium to another with a different refractive index, which is involved when light *enters* the fibre but not for its propagation *within* the fibre. Interference and scattering are optical phenomena but are not the primary principle responsible for guiding light in optical fibres; scattering, in fact, is a cause of signal loss.

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30. Consider the following statements: Statement-I: Occasionally we see

Consider the following statements:

  • Statement-I: Occasionally we see two rainbows together, the primary rainbow with another less intense secondary rainbow about 10 degrees away
  • Statement-II: The secondary rainbow appears due to refraction after a reflection of sunlight from the water droplets

Which one of the following is correct in respect of the above statements and the code?

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Both the statements are individually true and Statement II is the correct explanation of Statement I
Both the statements are individually true but Statement II is NOT the correct explanation of Statement I
Statement I is true but Statement II is false
Statement I is false but Statement II is true
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UPSC CAPF – 2017
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Statement I is true; it is possible to see a secondary rainbow outside the primary one. The secondary rainbow is less intense and typically appears about 10 degrees wider than the primary bow, with the colours reversed. Statement II is false; the secondary rainbow is formed by sunlight undergoing *two* internal reflections inside the water droplets before refracting out. The primary rainbow is formed by a single internal reflection.
A secondary rainbow results from two internal reflections of light within raindrops, not one reflection followed by refraction.
The order of colours in the primary rainbow (red on the outside, violet on the inside) is reversed in the secondary rainbow (violet on the outside, red on the inside) due to the extra internal reflection.

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