Optics

1. The image formed by a plane mirror is :

The image formed by a plane mirror is :

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always virtual and erect.
always virtual but erect or inverted upon the size of the object.
never virtual but always erect.
always real and erect.
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UPSC CISF-AC-EXE – 2022
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A plane mirror is a flat reflective surface. The image formed by a plane mirror has specific characteristics:
1. **Virtual:** The image is formed where the light rays appear to diverge from, but do not actually intersect. It cannot be projected onto a screen.
2. **Erect:** The image is upright, meaning it is oriented the same way as the object (top is top, bottom is bottom).
3. **Laterally Inverted:** The image is flipped left-to-right.
4. **Same size:** The size of the image is equal to the size of the object.
5. **Same distance:** The image is located behind the mirror at the same distance as the object is in front of the mirror.

Based on these properties, the image formed by a plane mirror is always virtual and always erect.
Option A states “always virtual and erect,” which matches the known properties.
Option B is incorrect because the image is always erect, not dependent on object size.
Option C is incorrect because the image is always virtual.
Option D is incorrect because the image is always virtual, not real.

– A plane mirror produces a virtual image.
– A plane mirror produces an erect image.
– The image is also laterally inverted and the same size as the object.
Real images are formed when light rays actually converge at a point and can be projected onto a screen (e.g., image formed by a projector lens). Virtual images cannot be projected (e.g., image in a plane mirror or through a magnifying glass). Erect means the image orientation is the same as the object; inverted means it is upside down.

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2. Which one of the following materials cannot be used to make a lens ?

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

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Water
Glass
Plastic
Clay
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UPSC CISF-AC-EXE – 2022
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A lens is an optical device that transmits and refracts light, causing the convergence or divergence of light rays. For a material to be used to make a lens, it must be transparent or at least translucent enough to allow light to pass through it and must have a refractive index different from the surrounding medium (usually air) so that refraction occurs.
Let’s examine the given options:
A) Water: Water is transparent and has a refractive index. Lenses can be made from water (e.g., water-filled lenses) or exist naturally (e.g., a droplet of water can act as a lens).
B) Glass: Glass is a common, transparent material with a suitable refractive index used extensively for making lenses.
C) Plastic: Many types of plastic are transparent and are widely used for making lenses, especially in eyeglasses, contact lenses, and camera lenses.
D) Clay: Clay is typically opaque (does not allow light to pass through) and therefore cannot be used to make a functional lens that refracts light in the required manner. While it can be molded, its opaqueness makes it unsuitable for transmitting images.
– A lens works by refracting light, which requires the material to be transparent or translucent.
– The material must have a different refractive index from the surrounding medium.
– Opaque materials cannot be used to make standard lenses.
Materials like glass and plastic are commonly used because they are transparent, can be precisely shaped, and have stable optical properties. Water’s shape changes easily, making it less practical for stable lenses unless contained. Clay’s opaqueness fundamentally prevents it from being used for light transmission and refraction in the way a lens operates.

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3. The blue colour of the sky and the reddening of the Sun at sunrise and

The blue colour of the sky and the reddening of the Sun at sunrise and sunset are caused due to the phenomenon of :

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dispersion of light.
reflection and refraction of light.
aberration of light.
scattering of light.
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UPSC CISF-AC-EXE – 2022
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The blue colour of the sky and the reddening of the Sun at sunrise and sunset are classic examples of the phenomenon of scattering of light by particles in the atmosphere.
When sunlight enters the Earth’s atmosphere, it interacts with the gas molecules (mainly Nitrogen and Oxygen) and tiny particles present. This interaction causes the light to be scattered in various directions. This process is called scattering.
Rayleigh scattering explains why the sky is blue. It states that the intensity of scattered light is inversely proportional to the fourth power of the wavelength. Shorter wavelengths, like blue and violet light, are scattered much more effectively than longer wavelengths, like red and orange light. When we look at the sky during the day, we see the scattered blue light from all directions.
At sunrise and sunset, the sunlight travels a much longer path through the atmosphere to reach our eyes. During this long journey, most of the shorter wavelength blue and green light is scattered away by the atmospheric particles. The longer wavelength light, such as red and orange, which is scattered less, is left to reach our eyes, making the Sun and the sky around it appear reddish or orange.
– The colour of the sky and the Sun’s colour at sunrise/sunset are due to the interaction of sunlight with atmospheric particles.
– This interaction involves scattering of light.
– Shorter wavelengths (blue) are scattered more effectively than longer wavelengths (red) by atmospheric gases (Rayleigh scattering).
– The path length of sunlight through the atmosphere affects which colours are predominantly seen.
Dispersion of light is the splitting of white light into its constituent colours (spectrum) due to the dependence of refractive index on wavelength (e.g., prism). Reflection is the bouncing of light off a surface. Refraction is the bending of light as it passes from one medium to another. Aberration refers to defects in image formation by optical systems. None of these fully explain the blue sky or red sunrise/sunset phenomenon, which is primarily a scattering effect.

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4. What is the time difference between the actual sunset and apparent sun

What is the time difference between the actual sunset and apparent sunset ?

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About half a minute
About one minute
About two minutes
About twenty minutes
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The time difference between actual sunset and apparent sunset is caused by atmospheric refraction. Sunlight bends as it passes through the Earth’s atmosphere because the density of the atmosphere increases towards the Earth’s surface.
This refraction causes the sun’s rays to bend downwards, making the sun appear higher in the sky than its actual position. Near the horizon, this effect is most pronounced. As a result, the sun appears to rise earlier in the morning (apparent sunrise before actual sunrise) and set later in the evening (apparent sunset after actual sunset).
The apparent flattening of the sun’s disc near the horizon is also a result of atmospheric refraction. The time difference between actual and apparent sunrise or sunset is typically about 2 minutes. This phenomenon effectively increases the duration of daylight by about 4 minutes each day (2 minutes in the morning and 2 minutes in the evening).

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5. Which one among the following is a non-luminous object ?

Which one among the following is a non-luminous object ?

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Sun
Candle
LED bulb
Moon
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A non-luminous object is an object that does not produce its own light. It is visible because it reflects light from a luminous source. The Moon does not produce its own light; it reflects sunlight.
Luminous objects emit light (e.g., Sun, candle flame, light bulb).
Non-luminous objects reflect light (e.g., Moon, planets, furniture, books).
The apparent brightness of non-luminous objects depends on the intensity of the incident light and their reflective properties.

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6. Light waves are

Light waves are

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progressive in nature.
mechanical in nature.
longitudinal in nature.
transverse in nature.
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Light waves are electromagnetic waves, which means they are fluctuations of electric and magnetic fields. Electromagnetic waves, including light, are characterized by their transverse nature, where the oscillations of the fields are perpendicular to the direction of wave propagation.
Transverse waves have oscillations perpendicular to the direction of energy transfer (propagation).
Longitudinal waves have oscillations parallel to the direction of energy transfer (propagation), like sound waves in air.
Light is non-mechanical; it does not require a medium to travel.
The electromagnetic spectrum includes various types of waves like radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays, all of which are transverse electromagnetic waves.

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7. An object is kept at infinity from the position of a concave (spherica

An object is kept at infinity from the position of a concave (spherical) mirror. Which one is *not* true about the image of the object?

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Position of image is at the focus of the mirror
Size of image is the same as that of the object
Image is real
Image is inverted
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When an object is placed at infinity from a concave spherical mirror, the light rays from the object are considered to be parallel to the principal axis. After reflection from the concave mirror, these parallel rays converge at the principal focus (F) of the mirror. The image formed at the focus is real, inverted (relative to the infinitely distant object), and highly diminished (essentially a point image).
For an object at infinity from a concave mirror, the image is formed at the focus, is real, inverted, and highly diminished (point-sized).
Option A states the position of the image is at the focus, which is true. Option C states the image is real, which is true for converging rays formed on the same side as the object. Option D states the image is inverted, which is true for real images formed by a concave mirror (even though a point image doesn’t visually appear inverted). Option B states the size of the image is the same as that of the object, which is false; the image is highly diminished. Therefore, the statement that is *not* true is B.

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8. Tyndall effect appears due to which one of the following properties of

Tyndall effect appears due to which one of the following properties of light?

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Reflection of light
Diffraction of light
Polarization of light
Scattering of light
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The Tyndall effect is the phenomenon where the path of a beam of light becomes visible as it passes through a colloidal dispersion or a fine suspension. This effect occurs because the larger particles in the colloid or suspension scatter the light in all directions when it strikes them.
Tyndall effect is the scattering of light by particles in a colloid or a very fine suspension.
Reflection occurs when light bounces off a surface. Diffraction is the bending of light waves as they pass around the edge of an obstacle or through a narrow slit. Polarization is the restriction of the vibration of light waves to a single plane. Scattering is the process by which light is deflected in various directions as it interacts with a medium or particles within it, which is the principle behind the Tyndall effect.

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9. Stars twinkle in the sky at night because

Stars twinkle in the sky at night because

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refractive index of the atmosphere changes due to the change of temperature
stars emit light in the form of pulses
of interference of light coming from different stars
of diffraction of light
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Stars twinkle in the sky at night because refractive index of the atmosphere changes due to the change of temperature.
Twinkling of stars (scintillation) is caused by atmospheric refraction.
– Light from distant stars travels through the Earth’s atmosphere before reaching our eyes.
– The atmosphere is not uniform; it consists of layers with varying temperatures and densities.
– Variations in temperature and density cause variations in the refractive index of the air.
– As light from a star passes through these turbulent layers with changing refractive index, it undergoes continuous refraction in random directions.
– This causes fluctuations in the apparent position and brightness of the star as seen from Earth. These rapid fluctuations are perceived as twinkling.
– Planets, being much closer, appear as extended sources of light rather than point sources. The light from different parts of a planet’s disc undergoes similar but independent variations, which average out, so planets do not twinkle noticeably.

Option A correctly identifies the cause: changes in atmospheric refractive index due to temperature variations (and hence density variations) lead to varying refraction of starlight.
Option B is incorrect; stars emit light continuously.
Option C is incorrect; twinkling is an effect on light from a single star due to atmospheric effects, not interference from different stars.
Option D is incorrect; while diffraction occurs, twinkling is primarily an effect of refraction due to atmospheric turbulence.

Atmospheric refraction is also responsible for phenomena like the apparent flattening of the sun at sunrise/sunset and the fact that we can see the sun just before it rises and just after it sets. The degree of twinkling is affected by atmospheric conditions (turbulence).

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10. The minimum length of a plane mirror to see your full length image is

The minimum length of a plane mirror to see your full length image is

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one quarter of your height
one-third of your height
half of your height
equal to your height
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UPSC CISF-AC-EXE – 2019
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The minimum length of a plane mirror to see your full length image is half of your height.
This is a fundamental principle of reflection from a plane mirror. To see the full image of an object in a plane mirror, the minimum vertical extent of the mirror required is half the vertical extent of the object.

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Consider a person standing in front of a mirror. Light rays from the top of their head reflect off the top part of the mirror and enter their eyes. Light rays from their feet reflect off the bottom part of the mirror and enter their eyes.
– The angle of incidence equals the angle of reflection.
– The law of reflection dictates that to see the top of your head, the top edge of the mirror must be halfway between the top of your head and your eye level.
– Similarly, to see your feet, the bottom edge of the mirror must be halfway between your feet and your eye level.
– The distance between these two points on the mirror is the minimum required length. This distance is (1/2 * distance from head to eye) + (1/2 * distance from eye to feet). Since (distance from head to eye) + (distance from eye to feet) equals the total height of the person, the required mirror length is half the person’s height.

The distance of the person from the mirror does not affect the *minimum length* required, although it does affect the *field of view*.

This principle is utilized in everyday life, for instance, when installing full-length mirrors. The mirror doesn’t need to be as tall as the person.

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