Optics

31. Match List I with List II and select the correct answer using the code

Match List I with List II and select the correct answer using the code given below the Lists :

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List I
(Explanation)		 List II
(Term)
A. Colour of an opaque object 1. Fluorescence
B. Colour observed through a coloured glass 2. Reflection
C. Bending of the image of a rod partially dipped in water 3. Transmission
D. Shining observed when one steps on an earthworm 4. Refraction
1 3 4 2
1 4 3 2
2 4 3 1
2 3 4 1
This question was previously asked in
UPSC CAPF – 2014
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Let’s match the explanations with the terms:
A. Colour of an opaque object: Opaque objects get their colour by absorbing certain wavelengths of light and reflecting others. The colour we see is the colour of the light reflected by the object. This matches term 2. Reflection.
B. Colour observed through a coloured glass: Coloured glass allows certain wavelengths of light to pass through while absorbing others. The colour we see is the colour of the light transmitted through the glass. This matches term 3. Transmission.
C. Bending of the image of a rod partially dipped in water: This phenomenon occurs because light changes speed as it passes from one medium (air) to another (water), causing it to bend at the interface. This bending of light is called refraction. This matches term 4. Refraction.
D. Shining observed when one steps on an earthworm: With A, B, and C matched to 2, 3, and 4 respectively, the remaining term for D is 1. Fluorescence is the emission of light by a substance after it has absorbed light. While shining can often be explained by reflection, it is possible that some biological substances exhibit fluorescence. Given the other standard matches, this is the most likely intended pairing in the context of the question’s options.
Matching: A-2, B-3, C-4, D-1. This corresponds to option D.
Understand basic optical phenomena: Reflection (light bouncing off a surface), Transmission (light passing through a substance), Refraction (bending of light as it passes from one medium to another), and Fluorescence (emission of light after absorbing radiation). Apply these concepts to everyday observations.
While “shining” is most commonly associated with reflection, the options force a match with Fluorescence for the last item. It’s possible the question refers to a specific, perhaps less common, property of earthworm mucus or tissues under certain conditions, or it might be a less precise pairing designed within the options provided. However, the matches for A, B, and C are definitive.

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32. The formation of colours in soap bubbles is due to the phenomenon of

The formation of colours in soap bubbles is due to the phenomenon of

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dispersion of light
interference of light
diffraction of light
polarization of light
This question was previously asked in
UPSC CAPF – 2013
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B) interference of light
The iridescent colours observed in soap bubbles are a result of the interference of light waves. When light strikes the thin soap film, it reflects off both the outer and inner surfaces. These two reflected waves interfere with each other. Due to the slight difference in the path length traveled by the waves (determined by the thickness of the film and the angle of incidence) and the phase change upon reflection, certain wavelengths of light are constructively interfered (enhanced), while others are destructively interfered (cancelled). This results in the appearance of vibrant colours, which vary depending on the local thickness of the soap film.
This phenomenon is also observed in other thin films, such as oil slicks on water. The specific colour seen at any point on the bubble depends on the thickness of the film at that point and the angle from which it is viewed. As the soap bubble thins (e.g., due to evaporation), the colours change, often displaying white or black just before bursting when the film becomes extremely thin.

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33. Advanced sunrise and delayed sun-set found in the sky are due to the p

Advanced sunrise and delayed sun-set found in the sky are due to the phenomenon of

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diffraction of sunlight
refraction of sunlight
scattering of sunlight
total internal reflection of sun-light
This question was previously asked in
UPSC CAPF – 2013
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The correct option is B. Advanced sunrise and delayed sunset are due to the phenomenon of refraction of sunlight.
As sunlight enters the Earth’s atmosphere, it passes through layers of air with varying densities. This causes the light rays to bend, or refract. Near the horizon, where sunlight travels through a larger amount of atmosphere, this refraction is significant. The atmosphere bends the light rays downwards, towards the Earth’s surface. This bending makes the Sun appear higher in the sky than its actual position. As a result, we see the Sun before it has actually risen above the horizon (advanced sunrise) and after it has actually set below the horizon (delayed sunset).
Refraction causes the apparent position of the Sun to be about 0.5 degrees higher than its true position when it is near the horizon. This leads to sunrise appearing about two minutes earlier and sunset about two minutes later than they would in the absence of atmospheric refraction. Diffraction is the bending of light around obstacles. Scattering is the redirection of light by particles. Total internal reflection requires specific conditions involving light moving from a denser to a rarer medium.

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34. Diamond Ring, God’s Eye and Baily’s Beads are the parts of which one a

Diamond Ring, God’s Eye and Baily’s Beads are the parts of which one among the following natural phenomena?

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Solar eclipse
Aurora
Lightning
Solar storm
This question was previously asked in
UPSC CAPF – 2013
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The correct option is A. Diamond Ring, God’s Eye, and Baily’s Beads are phenomena observed during a solar eclipse.
During a total solar eclipse, as the Moon nearly completely covers the Sun, the last few drops of sunlight streaming through the valleys on the Moon’s limb create points of light known as Baily’s Beads. Just before totality, when only one bright spot remains, it creates a dazzling effect resembling a diamond ring on the Sun’s corona. ‘God’s Eye’ is sometimes used informally to describe the overall appearance of the corona during a total solar eclipse.
These phenomena are transient and depend on the alignment of the Sun, Moon, and Earth during an eclipse. They are celebrated sights for observers of total solar eclipses. Auroras are atmospheric light displays caused by charged particles from the Sun interacting with Earth’s magnetic field. Lightning is an electrical discharge during thunderstorms. Solar storms are eruptions of energy from the Sun.

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35. Spectacles used for viewing 3-Dimensional films have :

Spectacles used for viewing 3-Dimensional films have :

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convex lens.
polaroids.
concave lens.
bifocal lens.
This question was previously asked in
UPSC CAPF – 2010
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Spectacles used for viewing 3-Dimensional films often use polaroids (polarizing filters).
– One common method for displaying 3D films uses polarized light. Two images are projected simultaneously, each polarized differently (e.g., linearly polarized at 90 degrees to each other, or circularly polarized in opposite directions).
– The 3D glasses contain polarizing filters (polaroids) that match the polarization of the projected images. The filter over the left eye allows only the image intended for the left eye to pass through, and the filter over the right eye allows only the image intended for the right eye.
– The brain then combines these two slightly different images to create the perception of depth.
Older 3D systems used colored filters (anaglyph glasses, typically red and cyan). Modern active 3D systems use glasses with electronic shutters (LCD lenses) that rapidly open and close in sync with the display showing alternating images for the left and right eyes. The question likely refers to passive polarization glasses commonly used in cinemas or with some 3D TVs.

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36. Which one of the following lenses would you prefer to use while readin

Which one of the following lenses would you prefer to use while reading very small letters printed on a label ?

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Convex lens of large focal length
Concave lens of large focal length
Convex lens of small focal length
Concave lens of small focal length
This question was previously asked in
UPSC CAPF – 2009
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A Convex lens of small focal length is preferred for reading very small letters printed on a label.
To read small letters, a magnifying glass is used. A magnifying glass is a convex lens. A convex lens forms a magnified, virtual, and erect image when the object is placed within its focal length. The magnification power of a simple magnifying glass is given by M = 1 + (D/f), where D is the least distance of distinct vision (approx. 25 cm) and f is the focal length of the lens. To achieve higher magnification, the focal length (f) must be smaller.
A concave lens is a diverging lens and produces reduced or same-size images (virtual and erect or real and inverted), not magnified images for viewing small objects directly. A convex lens of large focal length will produce less magnification compared to one with a small focal length.

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37. Which one among the following is the correct focal length of a combina

Which one among the following is the correct focal length of a combination of lenses of power 2·5 D and –2·0 D ?

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+0·5 m
–0·5 m
+2·0 m
–2·0 m
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UPSC NDA-2 – 2024
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For thin lenses in contact, the total power of the combination (P_total) is the algebraic sum of the individual powers (P₁, P₂). Given P₁ = 2.5 D and P₂ = -2.0 D, the total power is P_total = P₁ + P₂ = 2.5 D + (-2.0 D) = 0.5 D. The focal length (f) of a lens or lens combination is the reciprocal of its power (P), with power in dioptres (D) and focal length in meters (m). So, f_total = 1 / P_total = 1 / 0.5 D = 1 / (0.5 m⁻¹) = 2 m. The focal length is positive, indicating a converging lens combination.
Power of a lens (P) is the reciprocal of its focal length (f) in meters (P = 1/f). The power of a combination of thin lenses in contact is the sum of their individual powers. Positive power indicates a converging lens, and negative power indicates a diverging lens.
The unit of power is the dioptre (D), which is equal to m⁻¹. A combination of a converging lens (+P) and a diverging lens (-P’) can result in a net converging or diverging combination depending on the relative magnitudes of their powers.

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38. Which one among the following is correct for a person suffering from m

Which one among the following is correct for a person suffering from myopia ?

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The person can see near objects clearly
The person can see distant objects clearly
The person cannot distinguish colours
The person can neither see near objects nor distant objects clearly
This question was previously asked in
UPSC NDA-2 – 2024
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Myopia, or nearsightedness, is a common refractive error where light focuses in front of the retina instead of on it. This results in distant objects appearing blurred, while near objects can be seen clearly.
A person with myopia has difficulty seeing objects that are far away. The eye might be longer than normal, or the cornea might be too curved, causing light to converge too strongly.
Myopia can be corrected using diverging lenses (concave lenses), which cause the light rays to diverge slightly before entering the eye, thereby shifting the focal point back onto the retina. Hyperopia (farsightedness) is the opposite condition, where near objects are blurred. Astigmatism causes blurred vision at all distances due to an irregularly shaped cornea or lens.

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39. A point object is placed at the centre of curvature of a spherical con

A point object is placed at the centre of curvature of a spherical concave mirror. Which one among the following would be the correct location of image formed ?

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At infinity
At the centre of curvature
At the focal point
Between the focal point and the centre of curvature
This question was previously asked in
UPSC NDA-2 – 2024
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For a spherical concave mirror, when an object is placed at the centre of curvature (C), the image formed is also located at the centre of curvature (C).
When the object is at the centre of curvature of a concave mirror, the image formed is real, inverted, and has the same size as the object. Ray diagrams show that rays from the object passing through the focal point become parallel after reflection, and rays hitting the mirror perpendicularly pass through the centre of curvature after reflection. These reflected rays intersect at the centre of curvature.
The relationship between object distance (u), image distance (v), and focal length (f) for a spherical mirror is given by the mirror formula: 1/f = 1/u + 1/v. For a concave mirror, f is positive. The centre of curvature (C) is located at a distance 2f from the pole of the mirror. If u = 2f (object at C), then 1/f = 1/(2f) + 1/v. Solving for 1/v gives 1/v = 1/f – 1/(2f) = 2/(2f) – 1/(2f) = 1/(2f). Thus, v = 2f, meaning the image is also formed at the centre of curvature.

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40. Which one among the following figures correctly represents the ray dia

Which one among the following figures correctly represents the ray diagram ? (Consider the lens to be thin)

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Figure (a)
Figure (b)
Figure (c)
Figure (d)
This question was previously asked in
UPSC NDA-2 – 2024
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Assuming Figure (a) correctly depicts a standard ray diagram for a thin lens following the rules of refraction, option A is the correct answer.
– Ray diagrams for lenses use standard principal rays to trace the path of light and locate the image formed by the lens. The rules for these rays are based on the lens’s focal points and optical center.
– For a convex (converging) lens: (1) A ray parallel to the principal axis passes through the focal point on the other side after refraction. (2) A ray passing through the optical center goes straight through without deviation. (3) A ray passing through the focal point on the object side becomes parallel to the principal axis after refraction.
– For a concave (diverging) lens: (1) A ray parallel to the principal axis appears to diverge from the focal point on the same side after refraction. (2) A ray passing through the optical center goes straight through without deviation. (3) A ray directed towards the focal point on the other side becomes parallel to the principal axis after refraction.
– A correct ray diagram will accurately show the refraction of light rays according to these rules and the formation of the image where the refracted rays (or their extensions) intersect. Without the actual figures, it is impossible to verify which diagram is correct; this explanation assumes diagram (a) follows the correct physics principles for the lens depicted.
Ray diagrams are a useful tool in geometric optics for visualizing the formation of images by lenses and mirrors and understanding the properties (real/virtual, inverted/erect, magnified/diminished) of the image. Accuracy in drawing the rays according to the rules is crucial.

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