Optics

101. Light waves are incident on an air-glass boundary. Some of the light w

Light waves are incident on an air-glass boundary. Some of the light waves are reflected and some are refracted in the glass. Which one of the following properties is the same for the incident wave and the refracted wave?

[amp_mcq option1=”Speed” option2=”Direction” option3=”Brightness” option4=”Frequency” correct=”option4″]

This question was previously asked in
UPSC NDA-1 – 2021
Download PDFAttempt Online
The correct answer is (D) Frequency.
When light waves pass from one medium to another (like air to glass), their speed and wavelength change, but their frequency remains constant. The frequency of a wave is determined by the source emitting the light and does not change when the medium changes.
Speed of light changes because the refractive index of the medium changes (speed = speed of light in vacuum / refractive index). Wavelength changes proportionally to the speed (speed = frequency * wavelength). Direction changes due to refraction (bending of light), unless the incidence is normal to the surface. Brightness (intensity) changes due to partial reflection at the boundary, absorption, and scattering within the medium.

102. Twinkling of stars is due to

Twinkling of stars is due to

[amp_mcq option1=”particular frequencies of the starlight.” option2=”reflection of starlight from the oceanic surface.” option3=”atmospheric refraction of starlight.” option4=”magnetic field of Earth.” correct=”option3″]

This question was previously asked in
UPSC NDA-1 – 2021
Download PDFAttempt Online
The twinkling of stars is caused by the Earth’s atmosphere. As starlight travels through the atmosphere, it passes through layers of air with varying densities and temperatures. This causes the light to be refracted (bent) multiple times. Since these layers are constantly moving and mixing due to atmospheric turbulence, the path of the starlight changes slightly from moment to moment. This results in fluctuations in the apparent brightness and position of the star, which we perceive as twinkling. This phenomenon is called atmospheric refraction.
Stars appear as point sources of light because they are very distant. The light from a point source is more susceptible to the bending effects of atmospheric turbulence compared to the light from extended sources like planets (which appear as discs). Planets do not typically twinkle for this reason.
Refraction is the bending of light as it passes from one medium to another or through a medium with varying properties. The refractive index of air changes with temperature and pressure, which vary throughout the atmosphere.

103. When light is scattered by a molecule and the frequency of the scatter

When light is scattered by a molecule and the frequency of the scattered light is changed, this phenomenon is called

[amp_mcq option1=”Rayleigh scattering.” option2=”Raman effect.” option3=”Photoelectric effect.” option4=”Rutherford scattering.” correct=”option2″]

This question was previously asked in
UPSC NDA-1 – 2021
Download PDFAttempt Online
The phenomenon where light is scattered by a molecule and the frequency of the scattered light is changed is called B) Raman effect.
– Scattering of light is the process where light is deflected by particles in the medium.
– When the scattering is elastic, the frequency of the scattered light is the same as the incident light (e.g., Rayleigh scattering).
– When the scattering is inelastic, there is an exchange of energy between the light photon and the scattering molecule, resulting in a change in the frequency (and wavelength) of the scattered light.
– The Raman effect is an example of inelastic scattering of light by molecules, where the scattered light can have frequencies shifted up or down from the incident frequency, corresponding to transitions between vibrational or rotational energy levels of the molecule.
– Rayleigh scattering is elastic scattering by particles much smaller than the wavelength of light.
– Photoelectric effect is the emission of electrons from a material when light shines on it.
– Rutherford scattering is the scattering of alpha particles by atomic nuclei, used in the discovery of the atomic nucleus.

104. A glass prism splits white light into different colours. This phenomen

A glass prism splits white light into different colours. This phenomenon is called dispersion of light by prism. Which one of the following statements is correct ?

[amp_mcq option1=”Red light will deviate the most and it is because of the reflection of light.” option2=”Violet light will deviate the most and it is because of the refraction of light.” option3=”Red light will deviate the most and it is because of the refraction of light.” option4=”Violet light will deviate the most and it is because of the reflection of light.” correct=”option2″]

This question was previously asked in
UPSC NDA-1 – 2021
Download PDFAttempt Online
The correct statement is B) Violet light will deviate the most and it is because of the refraction of light.
– Dispersion is the splitting of white light into its constituent colours when it passes through a prism.
– This occurs because the refractive index of the prism material is different for different wavelengths (colours) of light; this phenomenon is called refraction.
– The deviation angle for a given colour depends on the refractive index for that colour.
– Violet light has the shortest wavelength and the highest refractive index in glass, causing it to bend the most.
– Red light has the longest wavelength and the lowest refractive index in glass, causing it to bend the least.
– The order of colours in the spectrum from least deviated to most deviated is Red, Orange, Yellow, Green, Blue, Indigo, Violet (VIBGYOR in reverse).
– Dispersion is a phenomenon of refraction, not reflection. Reflection is the bouncing back of light from a surface.

105. Which one of the following statements is * not * correct for light ray

Which one of the following statements is *not* correct for light rays ?

[amp_mcq option1=”Light travels at different speeds in different media.” option2=”Light travels at almost 300 million metres per second in air.” option3=”Light speeds down as it leaves a water surface and enters the air.” option4=”Light speeds up as it leaves a glass surface and enters the air.” correct=”option3″]

This question was previously asked in
UPSC NDA-1 – 2021
Download PDFAttempt Online
The correct statement that is *not* correct is C) Light speeds down as it leaves a water surface and enters the air.
– The speed of light changes as it passes from one medium to another.
– Light travels fastest in a vacuum (approximately 3 x 10⁸ m/s).
– The speed of light is slower in optically denser media (like water or glass) than in optically less dense media (like air or vacuum).
– A) is correct: The speed of light depends on the refractive index of the medium.
– B) is correct: The speed of light in air is very close to its speed in vacuum.
– C) is incorrect: Water is optically denser than air. When light moves from a denser medium (water) to a less dense medium (air), its speed *increases*.
– D) is correct: Glass is optically denser than air. When light moves from a denser medium (glass) to a less dense medium (air), its speed *increases*.

106. According to the New Cartesian Sign Convention, which one of the follo

According to the New Cartesian Sign Convention, which one of the following is correct in respect of the formula $\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$, where symbols have their usual meanings?

[amp_mcq option1=”It applies only to spherical mirrors.” option2=”It applies only to spherical lenses.” option3=”It applies to spherical mirrors as well as spherical lenses.” option4=”It is an invalid formula.” correct=”option3″]

This question was previously asked in
UPSC NDA-1 – 2021
Download PDFAttempt Online
The formula $\frac{1}{f} = \frac{1}{v} + \frac{1}{u}$ is the standard equation that applies to both spherical mirrors and spherical lenses when used with the New Cartesian Sign Convention.
For spherical mirrors, this is known as the mirror formula. For spherical lenses, it is known as the thin lens formula. The New Cartesian Sign Convention provides a consistent set of rules for assigning positive or negative signs to object distance ($u$), image distance ($v$), and focal length ($f$), ensuring that the formula holds true for both real and virtual images/objects and for both converging and diverging mirrors/lenses.
While the formula form is the same, the application of the sign convention and the meaning of $f$ (positive for converging, negative for diverging) correctly distinguish between types of mirrors and lenses. Other related formulas, like the magnification formula ($m = \frac{h’}{h} = -\frac{v}{u}$ for mirrors and $m = \frac{h’}{h} = \frac{v}{u}$ for lenses, with the sign convention applied), are also used in conjunction with these fundamental formulas.

107. Which of the following are the primary colours of light ?

Which of the following are the primary colours of light ?

[amp_mcq option1=”Yellow, Red and Green” option2=”Blue, Red and Green” option3=”Violet, Red and Yellow” option4=”Indigo, Violet and Green” correct=”option2″]

This question was previously asked in
UPSC NDA-1 – 2021
Download PDFAttempt Online
The primary colours of light are Blue, Red, and Green.
Primary colours of light are those colours that cannot be produced by mixing other colours of light, and from which a wide range of other colours can be created by mixing. In additive colour mixing (mixing of light), the primary colours are Red, Green, and Blue (RGB). This is the basis for how colours are displayed on screens (TVs, monitors, phones).
The combination of the three primary colours of light in equal proportions results in white light. Combining any two primary colours produces a secondary colour: Red + Green = Yellow, Red + Blue = Magenta, Green + Blue = Cyan. These secondary colours of light are the primary colours of pigment (in subtractive mixing).

108. Which one of the following colours may be obtained by combining green

Which one of the following colours may be obtained by combining green and red colours ?

[amp_mcq option1=”Blue” option2=”Magenta” option3=”Pink” option4=”Yellow” correct=”option4″]

This question was previously asked in
UPSC NDA-1 – 2021
Download PDFAttempt Online
By combining green and red colours of light, the colour yellow is obtained.
This refers to additive colour mixing, which is relevant for light. The primary colours of light are Red, Green, and Blue (RGB). When these primary colours are mixed in different proportions, they produce other colours. Combining two primary colours results in a secondary colour: Red + Green = Yellow, Red + Blue = Magenta, Green + Blue = Cyan.
Subtractive colour mixing, relevant for pigments and dyes, works differently. The primary colours in subtractive mixing are Cyan, Magenta, and Yellow (CMY). Combining these primaries removes wavelengths of light. For example, mixing yellow and magenta paints typically results in red.

109. The image we see in plane mirror is

The image we see in plane mirror is

[amp_mcq option1=”real and thus can be photographed.” option2=”virtual and nearer than the object.” option3=”virtual and is laterally inverted.” option4=”real but cannot be photographed.” correct=”option3″]

This question was previously asked in
UPSC NDA-1 – 2021
Download PDFAttempt Online
The image we see in a plane mirror is virtual and is laterally inverted.
A virtual image is one where the light rays appear to diverge from the image location but do not actually pass through it. Virtual images cannot be projected onto a screen. A plane mirror always forms a virtual image. Lateral inversion means that the image is reversed from left to right relative to the object. The image formed by a plane mirror is also upright and the same size as the object, and located at the same distance behind the mirror as the object is in front.
Real images are formed when light rays actually converge at the image location, and they can be projected onto a screen (and thus photographed directly). Examples include images formed by converging lenses in cameras or projectors, or by concave mirrors when the object is placed outside the focal point.

110. When a beam of white light passes through a glass prism, the colour of

When a beam of white light passes through a glass prism, the colour of light beam that deviates the least is

[amp_mcq option1=”Blue” option2=”Red” option3=”Green” option4=”Violet” correct=”option2″]

This question was previously asked in
UPSC NDA-1 – 2019
Download PDFAttempt Online
The correct option is B) Red. When white light passes through a glass prism, red light deviates the least.
– When white light passes through a prism, it undergoes dispersion, splitting into its constituent colors because the refractive index of the prism material is different for different wavelengths (colors) of light.
– This phenomenon is called dispersion. The spectrum of white light in order of increasing wavelength (and decreasing frequency) is Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).
– The deviation experienced by light passing through a prism is related to the refractive index of the material for that color. Generally, refractive index decreases with increasing wavelength.
– Longer wavelengths (like red) are refracted less, meaning they deviate less from their original path.
– Shorter wavelengths (like violet) are refracted more, meaning they deviate the most.
– Therefore, red light deviates the least, and violet light deviates the most.
The angle of deviation (δ) for a prism is related to the angle of incidence, angle of the prism (A), and the refractive index (μ) of the prism material by complex formulas, but generally, for a given prism angle and angle of incidence, a higher refractive index leads to a greater angle of deviation. Since μ is higher for shorter wavelengths (violet) and lower for longer wavelengths (red), violet deviates more than red. This is why the colors are separated when white light passes through a prism.
Page 1Page 2Page 3Page 4Page 5