Physics: Optics and Sound: UPSC Previous Year Questions (Science and Technology)

Explanation

Option (c) is correct: Night vision apparatus detects infrared waves emitted by objects as heat. Warmer objects emit more infrared radiation which is invisible to the human eye but can be captured and converted into a visible image by night vision devices. Infrared waves are electromagnetic waves with wavelengths longer than visible light but shorter than microwaves, typically used in thermal imaging, night vision, and remote sensing. Options (a) and (b) are incorrect: Microwaves are used in cooking and radar systems and radio waves are used for communication in broadcasting, mobile phones, and satellite transmissions.

Explanation

Statement 1 is incorrect: The frequency of sound produced by a flute is inversely proportional to its length. A shorter flute produces sound waves of higher frequency (higher pitch), while a longer flute produces sound waves of lower frequency (lower pitch).

Statement 2 is incorrect: Sound (or seismic waves) in rocks travels in both longitudinal (P-waves) and transverse (S-waves) forms. Longitudinal waves compress and expand the material, while transverse waves move the material perpendicular to the direction of propagation of the wave.

Explanation

The diffusion of light in the atmosphere occurs mainly due to dust particles and aerosols, which scatter sunlight in all directions. This scattering, known as Mie scattering, happens when particles like dust or water droplets are comparable in size to the wavelength of light, scattering all wavelengths almost equally. It causes the sky to appear uniformly bright and ensures visibility even in shadows. Other aerosols like Water Vapors, Smoke, etc can also contribute to the diffusion effect. However, dust particles are the most prominent cause in this context.

Explanation

Option (c) is incorrect: Humidity (water vapor) actually increases the speed of sound in air. This is because water molecules are lighter than nitrogen and oxygen molecules, reducing the overall density of the air. The speed of sound in humid air is higher than in dry air at the same temperature. Options (a), (b) and (d) are correct:

The velocity of sound in air increases with temperature because higher temperatures increase the energy of air molecules, allowing sound to travel faster. The velocity of sound in air is independent of pressure at constant temperature since both pressure and density change proportionally, leaving the speed unchanged. The velocity of sound is unaffected by amplitude and frequency as it depends only on the medium>s properties like temperature, density, and composition.

Explanation

Statement 1 is incorrect: According to Rayleigh scattering, light with shorter wavelengths (e.g., blue light) is scattered more than light with longer wavelengths (e.g., red light).

Statement 2 is incorrect: The refractive index of water is approximately 1.33, so the speed of light in water is about 1/1.33 0.75° the speed in a vacuum.

Statement 3 is correct: Radio waves are a type of electromagnetic radiation generated when electrical charges oscillate in an antenna. These oscillations create alternating electric and magnetic fields that propagate as waves. This principle is the basis of wireless communication technologies like radio, TV, and mobile networks.

Statement 4 is correct: The Doppler effect is used in radar guns to measure the speed of moving vehicles by analyzing the frequency shift of reflected radio waves.

Explanation

A mirage is primarily caused by terrestrial heating and the refraction of light. Mirage causes deceptive appearance of a distant object or objects caused by light ray bending (refraction) in different density layers of air. Terrestrial Heating: The ground is intensely heated by the sun (such as a hot road or desert). This heats the air near the surface, making it less dense than the cooler air above. The temperature gradient creates a gradient in the refractive index of the air. Refraction of Light: Light from the sky or distant objects passes through the layers of air with varying temperatures and densities. The light bends (refracts) as it moves from cooler, denser air to hotter, less dense air. This bending creates the illusion of water or a mirrored surface on the ground.

Explanation

When light waves pass from air to glass, the wavelength and velocity are affected. Velocity decreases because the speed of light in glass is less than in air due to the higher refractive index of glass. Wavelength also decreases because it is directly proportional to the velocity of light in the medium (=v/f, where ‘f’ is the frequency). Frequency remains unchanged because it is determined by the source of the light and does not depend on the medium.

Explanation

Decibels (dB) are units used to measure the intensity of sound. The decibel scale is logarithmic, meaning each 10 dB increase represents a tenfold increase in sound intensity.

Option (d) is correct: A noise level of 100 decibels corresponds to Noise from a machine shop. A sound at 100 decibels is quite loud. It is typical of sounds like those found in a machine shop or at a loud concert. For reference: 0 decibels is the threshold of hearing (just audible sound). 30-40 decibels corresponds to a quiet room. 60-70 decibels corresponds to ordinary conversation. 100 decibels is in the range of very loud noise, such as machinery or a chainsaw. Directions: The following items consist of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer these items using the code given below. (2000)

Explanation

In the case of a compact disc (CD), when sunlight or any other light source hits its surface, the following optical phenomena occur. Diffraction: The CD’s surface contains tiny, closely spaced grooves that act like a diffraction grating. This causes the light to spread out into different colors, creating the rainbow effect that is visible. Refraction: Light also passes through the transparent plastic layer of the CD, and as it enters and exits this layer, it bends due to the change in the refractive index between the plastic and air. Transmission: Some light also passes through the CD material, further contributing to the observed effects. Together, these phenomena--refraction, diffraction, and transmission--combine to produce the beautiful rainbow-like colors seen when a CD is viewed in sunlight.

Explanation

When light passes from one medium to another, its speed and direction change due to differences in the refractive indices of the two media. This phenomenon is known as refraction.

Statement 1 is correct: In a denser medium (with a higher refractive index), light travels slower. When light moves into a less dense medium (with a lower refractive index), it speeds up. As a result, the light bends away from the normal (the imaginary line perpendicular to the surface at the point of incidence). The object appears to be closer from the surface than it actually is, as seen in the case of a coin in water when viewed from above.

Statement 2 is correct: If a person is underwater looking at a coin above the surface, the coin appears higher than its actual position, not lower. This is because light bends away from the normal when moving from water (denser) to air (rarer).