Physics Important Questions Class 12 UP Board

Explanation: This question asks how the direction of a Light ray changes when it moves from a medium where it travels slower to one where it travels faster. Refraction occurs due to a change in speed of Light between two media of different optical densities. The refractive index represents how much a medium slows down Light compared to vacuum. When Light crosses the boundary between media, its speed changes, causing a change in direction unless it strikes normally.

To analyze this, consider that Light travels slower in a denser medium and faster in a rarer medium. At the boundary, the component of velocity parallel to the surface remains unchanged, but the perpendicular component changes due to speed variation. According to Snell’s law, the ratio of sine of angles depends on refractive indices. As Light enters a medium where it speeds up, its path adjusts accordingly. The bending depends on relative speeds and angles of incidence.

Think of a person running from mud onto a smooth road—the sudden increase in speed alters their path direction. Similarly, Light adjusts its path due to speed change at the interface.

In summary, the direction change depends on the transition between media and the associated variation in Light speed governed by refractive indices.

Explanation: This question explores whether all colours of light behave identically while traveling through the same medium. Light is made up of different wavelengths corresponding to different colours. The refractive index of a medium is not constant for all wavelengths; instead, it varies slightly depending on the wavelength, a phenomenon known as dispersion.

Because refractive index varies, the speed of light in a medium also varies for different colours. The relationship is given by v = c/n, where c is the speed of light in vacuum and n is the refractive index. Since n depends on wavelength, each colour travels at a slightly different speed. This is why white light splits into a Spectrum when passing through a prism.

For instance, when sunlight enters a glass prism, shorter wavelengths behave differently compared to longer wavelengths, leading to separation. This variation in speed is subtle but significant enough to produce visible effects.

In summary, the propagation of light in a medium depends on wavelength, leading to differences in speed among various colours due to dispersion.

Explanation: This question focuses on why the apparent size of the Sun looks closest to its true value when it is directly overhead at noon. As sunlight travels through Earth’s Atmosphere, it undergoes refraction due to changes in air density. The extent of refraction depends on the angle at which light enters the Atmosphere, which varies throughout the day.

When the Sun is near the horizon, its rays pass through a thicker layer of Atmosphere, experiencing greater bending and distortion. This affects both its apparent position and shape. At noon, the Sun is nearly overhead, and its rays enter the Atmosphere almost perpendicularly. In this case, the path through the Atmosphere is shortest, and bending is minimal.

This situation can be compared to looking at an object through a thick versus thin layer of glass—the thicker layer distorts more. Similarly, reduced atmospheric thickness at noon minimizes distortion.

In summary, the apparent size is least affected when atmospheric refraction is minimal due to near-vertical entry of sunlight at noon.

Explanation: This question examines how the spatial extent of a light wavefront changes when entering a medium where light slows down. A wavefront represents points in space where the wave has the same phase. The spacing between successive wavefronts is directly related to the wavelength of light in that medium.

When light enters a denser medium, its speed decreases while frequency remains constant. Since wavelength is given by λ = v/f, a decrease in velocity leads to a decrease in wavelength. As a result, the distance between successive wavefronts becomes smaller, effectively reducing their width.

Imagine compressing a spring: when you push it, the spacing between coils reduces. Similarly, wavefronts get closer together when light slows down in a denser medium.

In summary, the change in wave speed directly affects wavelength, causing the wavefront spacing to adjust accordingly in the new medium.

Explanation: This question explores how refractive indices for different colours compare in a medium like air. The refractive index depends on how much a medium slows down light, and it slightly varies with wavelength due to dispersion. However, the magnitude of this variation depends on the medium itself.

In air, the refractive index is very close to 1 for all visible wavelengths. Although there is a slight dependence on wavelength, the variation is extremely small compared to denser media like glass. As a result, differences in refractive index for colours such as orange and green are negligible in practical terms.

This can be likened to driving on a nearly frictionless surface—different vehicles may behave slightly differently, but the effect is hardly noticeable.

In summary, while dispersion exists, its effect in air is minimal, making refractive index values for different colours nearly identical.

Explanation: This question deals with how refractive indices for different colours compare in a medium like glass, where dispersion is significant. In such media, shorter wavelengths interact more strongly with the material, leading to a higher refractive index compared to longer wavelengths.

As a result, colours like green and orange do not experience identical slowing. Since refractive index depends on wavelength, the ratio between them reflects this variation. The difference is more noticeable in glass than in air, which is why prisms can separate white light into its component colours.

A useful analogy is vehicles moving through a rough surface—lighter vehicles may slow down differently than heavier ones, leading to variations in speed.

In summary, dispersion in glass causes measurable differences in refractive indices for different colours, leading to a ratio that reflects wavelength dependence.

Explanation: This question investigates which colour experiences the greatest optical density in a material like quartz. The refractive index of a medium depends on how strongly it interacts with light of different wavelengths. Generally, shorter wavelengths interact more strongly with the Atomic Structure of the medium.

Because of this interaction, shorter wavelength light slows down more compared to longer wavelengths. Since refractive index is inversely related to speed in the medium, a greater slowing implies a higher refractive index. This principle is consistent across many transparent materials.

An everyday example is how blue or violet light bends more sharply than red light when passing through a prism, indicating stronger interaction.

In summary, the refractive index varies with wavelength, with shorter wavelengths typically experiencing greater optical density in transparent materials.

Explanation: This question explores multiple factors affecting visibility in water, even when it appears clear. Light traveling through water undergoes changes due to differences in refractive index, leading to bending of rays. Additionally, even clear water can cause slight scattering due to microscopic particles or fluctuations in density.

The human eye is adapted for focusing light in air. When submerged or viewing through water, the focusing mechanism changes because the refractive indices of surrounding media differ. This affects image formation on the retina, reducing clarity. Moreover, reduced speed of light in water alters the path of rays, further influencing how images are perceived.

This is similar to looking at objects through a slightly uneven glass surface—everything is visible but lacks sharpness.

In summary, a combination of scattering, altered ray paths, and changes in eye focusing contribute to reduced visual clarity in water.