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Q1. Which three lie in the same plane according to first law of refraction?
• a) Incident ray, reflected ray, mirror
• b) Incident ray, refracted ray, normal
• c) Normal, mirror, image
• d) Refracted ray, lens, axis
Answer: b) Incident ray, refracted ray, normal
According to the first law of refraction, the incident ray, the refracted ray, and the normal to the interface of two transparent media at the point of incidence all lie in the same plane. This law describes the geometry of refraction, focusing on the spatial relationship between the rays and the normal, not the physical objects like the mirror or lens.
Q2. The constant ratio in refraction law is between
• a) angle of incidence and normal
• b) sine of angle of incidence and sine of refraction
• c) speed and wavelength
• d) angle of refraction and glass thickness
Answer: b) sine of angle of incidence and sine of refraction
Snell’s law states that for a given pair of media and a specific wavelength of light, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. This constant is the relative refractive index, which fundamentally relates to the ratio of the speeds of light in the two media.
Q3. The law relating sine of angles is called
• a) Newton’s law
• b) Fresnel law
• c) Snell’s law
• d) Einstein’s law
Answer: c) Snell’s law
Snell’s law is the principle that governs the refraction of light. It mathematically relates the angles of incidence and refraction via the refractive indices of the two media. While Willebrord Snell formulated it, the law is also sometimes referred to as the law of sines in the context of optics.
Q4. Refractive index is defined as
• a) bending of light
• b) ratio of speeds of light
• c) reflection angle
• d) color shift of light
Answer: b) ratio of speeds of light
The refractive index of a medium is fundamentally defined as the ratio of the speed of light in a vacuum to the speed of light in that medium (absolute refractive index). For two media, the relative refractive index is the ratio of their respective speeds of light.
Q5. Symbol used for refractive index of medium 2 relative to medium 1 is
• a) n12
• b) n21
• c) nm
• d) R
Answer: b) n21
The symbol n21 denotes the refractive index of medium 2 with respect to medium 1. It is defined as the ratio of the speed of light in medium 1 (v1) to the speed in medium 2 (v2), which equals n2/n1 for absolute indices. The subscript order indicates “with respect to.”
Q6. Refractive index represents change in
• a) wavelength
• b) direction of light
• c) speed of light
• d) brightness
Answer: c) speed of light
At its core, the refractive index is a measure of how much the speed of light is reduced inside a medium compared to vacuum. The bending of light (change in direction) during refraction is a *consequence* of this change in speed when light crosses a boundary at an angle.
Q7. Light travels fastest in
• a) water
• b) glass
• c) vacuum
• d) oil
Answer: c) vacuum
Light achieves its maximum speed in a vacuum, approximately 3 x 10^8 m/s. In any material medium, light interacts with atoms, which causes it to slow down. Therefore, vacuum offers no obstruction, allowing light to travel at its fundamental cosmic speed limit.
Q8. Speed of light in vacuum is
• a) 3×10⁶ m/s
• b) 3×10⁷ m/s
• c) 3×10⁸ m/s
• d) 3×10⁹ m/s
Answer: c) 3×10⁸ m/s
The speed of light in a vacuum, denoted by ‘c’, is a fundamental physical constant. Its exact value is 299,792,458 m/s, which is universally approximated to 3 x 10^8 meters per second for calculations in physics and optics.
Q9. Speed of light in air compared to vacuum is
• a) much more
• b) same
• c) marginally less
• d) zero
Answer: c) marginally less
The speed of light in air is very slightly less than in a vacuum because air, though a sparse medium, has a refractive index just above 1 (about 1.0003). This small difference is often negligible for many calculations but is technically present.
Q10. Speed of light in water is
• a) same as vacuum
• b) slightly more
• c) considerably less
• d) infinite
Answer: c) considerably less
Light slows down significantly in water. With a refractive index of about 1.33, the speed of light in water is roughly 3/4ths of its speed in a vacuum. This substantial reduction is responsible for noticeable effects like bending of light and apparent depth shifts.
Q11. If light goes from medium 1 to 2, refractive index is
• a) v2/v1
• b) v1/v2
• c) v1+v2
• d) v2−v1
Answer: b) v1/v2
The refractive index of medium 2 with respect to medium 1 (n21) is defined as the ratio of the speed in the first medium (v1) to the speed in the second medium (v2). This ratio determines how much the light ray will bend when entering the second medium.
Q12. Refractive index depends on
• a) shape
• b) color
• c) speed in medium
• d) size
Answer: c) speed in medium
The defining parameter for the refractive index of a material is the speed at which light propagates through it. It is an intrinsic property of the material composition and its interaction with light, independent of the shape or size of the sample.
Q13. Angle of incidence is represented by
• a) r
• b) i
• c) n
• d) v
Answer: b) i
In optics, the angle of incidence is universally denoted by the symbol ‘i’. It is the angle between the incident ray and the normal drawn at the point of incidence on the interface separating two media.
Q14. Angle of refraction is denoted by
• a) r
• b) i
• c) n
• d) c
Answer: a) r
The angle of refraction, which is the angle between the refracted ray and the normal, is conventionally represented by the symbol ‘r’. This notation is standard in Snell’s law: n1 sin i = n2 sin r.
Q15. Refractive index of water is
• a) 1.00
• b) 1.25
• c) 1.33
• d) 1.50
Answer: c) 1.33
The absolute refractive index of pure water is approximately 1.33 for yellow light. This means light travels about 1.33 times slower in water than in a vacuum. This value is crucial for understanding phenomena like apparent depth and the focusing of light in aquatic environments.
Q16. Refractive index indicates
• a) mass density
• b) optical density
• c) thickness
• d) brightness
Answer: b) optical density
Refractive index is a measure of a medium’s optical density, which refers to its ability to slow down light. A higher optical density (higher refractive index) means light travels slower, causing more bending. It is not directly related to mass density.
Q17. When light enters another medium obliquely, it
• a) stops
• b) bends
• c) reflects
• d) vanishes
Answer: b) bends
When light strikes the boundary between two different media at an angle (obliquely), its speed changes, causing it to change direction or bend. This phenomenon is called refraction. Some light is also reflected, but the question refers to the transmitted ray’s behavior.
Q18. Refractive index of medium 1 w.r.t 2 is
• a) n21
• b) n12
• c) nm
• d) R
Answer: b) n12
The refractive index of medium 1 with respect to medium 2 is denoted as n12. It is the reciprocal of n21 (n12 = 1/n21). It represents the factor by which the speed of light changes when going from medium 2 to medium 1.
Q19. Absolute refractive index is taken with respect to
• a) water
• b) glass
• c) air/vacuum
• d) oil
Answer: c) air/vacuum
Absolute refractive index is defined relative to a vacuum, as it is the standard reference where light travels fastest. For practical purposes, the refractive index relative to air is often used as an approximation since the values are very close.
Q20. Symbol for absolute refractive index is
• a) n21
• b) nm
• c) n12
• d) r
Answer: b) nm
The absolute refractive index of a specific medium is typically denoted by ‘n’ with a subscript identifying the medium, like n_water or n_glass. Simply ‘n’ or n_m (where m stands for medium) is commonly used to represent it in formulas.
Q21. Formula for absolute refractive index is
• a) v/c
• b) c/v
• c) v1/v2
• d) v2/v1
Answer: b) c/v
The absolute refractive index (n) of a medium is calculated by dividing the speed of light in a vacuum (c) by the speed of light in that medium (v). Therefore, n = c / v. A higher ‘n’ means a lower speed ‘v’ in the medium.
Q22. Refraction occurs due to change in
• a) direction only
• b) wavelength only
• c) speed only
• d) speed and direction
Answer: d) speed and direction
Refraction is fundamentally caused by a change in the speed of light as it passes from one medium to another. If the light ray enters obliquely, this change in speed invariably results in a change in its direction of propagation.
Q23. Refractive index is a
• a) vector
• b) scalar
• c) force
• d) energy
Answer: b) scalar
Refractive index is a scalar quantity because it has only magnitude and no specific direction. It is a dimensionless number that characterizes a material’s optical property, unlike a vector which requires both magnitude and direction.
Q24. Snell’s law is valid for
• a) given color & media
• b) all colors equally
• c) vacuum only
• d) water only
Answer: a) given color & media
Snell’s law holds true for a specific pair of media and for a specific wavelength (color) of light. This is because the refractive index of a material varies slightly with wavelength, a phenomenon known as dispersion.
Q25. Refractive index changes when
• a) medium changes
• b) object moves
• c) angle changes
• d) thickness changes
Answer: a) medium changes
The refractive index is an intrinsic property of a material medium. It is constant for a given substance under specific conditions (like temperature and wavelength) and does not change with the angle of incidence or the geometry of the sample.
Q26. Normal is drawn
• a) parallel
• b) perpendicular
• c) slanted
• d) curved
Answer: b) perpendicular
In optics, the normal is an imaginary line drawn perpendicular (at a 90-degree angle) to the surface at the exact point where the light ray strikes (the point of incidence). All angles of incidence and refraction are measured from this normal.
Q27. No refraction when incidence is
• a) oblique
• b) large
• c) normal
• d) small
Answer: c) normal
When light strikes the interface along the normal (i.e., at 0° angle of incidence), it passes straight through without bending. Although the speed still changes, the direction does not alter because the ray is perpendicular to the boundary.
Q28. sin i / sin r is
• a) random
• b) constant
• c) zero
• d) infinite
Answer: b) constant
For two given transparent media and light of a specific color, the ratio sin i / sin r is a constant, as stated by Snell’s law. This constant is the relative refractive index (n21). It is not random but a fixed property of the media pair.
Q29. Refraction proves light changes
• a) color
• b) speed
• c) shape
• d) size
Answer: b) speed
The phenomenon of refraction, where light bends at an interface, provides direct evidence that the speed of light changes when it moves between different media. The bending is a direct consequence of this speed change.
Q30. Higher refractive index indicates
• a) higher speed
• b) lower speed
• c) same speed
• d) random speed
Answer: b) lower speed
Since refractive index n = c / v, a higher value of ‘n’ means the denominator ‘v’ (speed in the medium) must be smaller. Therefore, a material with a higher refractive index slows down light more significantly.
Q31. When light slows, refractive index
• a) falls
• b) rises
• c) vanishes
• d) reverses
Answer: b) rises
Refractive index and speed are inversely related. If light slows down more in a medium (v decreases), the value of n = c / v increases. A “denser” optical medium has a higher refractive index and a lower speed of light.
Q32. Refraction happens at
• a) center
• b) mirror
• c) boundary
• d) source
Answer: c) boundary
Refraction is a boundary phenomenon. It occurs precisely at the interface or boundary separating two transparent media with different optical densities. The change in speed and subsequent bending happens as light crosses this boundary.
Q33. Refractive index of vacuum is
• a) 0
• b) 1
• c) 1.33
• d) infinity
Answer: b) 1
By definition, the refractive index of a vacuum is exactly 1. This serves as the reference point because the speed of light in a vacuum (c) divided by itself equals 1. Air has a refractive index very close to 1 (approximately 1.0003).
Q34. Glass has higher refractive index than
• a) air
• b) diamond
• c) water
• d) plastic
Answer: a) air
Typical glass (n ≈ 1.5) has a higher refractive index than air (n ≈ 1). However, diamond (n ≈ 2.42) has a much higher index than glass. Among the options, only air is consistently less optically dense than common glass.
Q35. Snell’s law relates
• a) angles
• b) sines
• c) speeds
• d) distances
Answer: b) sines
Snell’s law directly relates the sines of the angles of incidence and refraction to the refractive indices: n1 sin i = n2 sin r. While the angles themselves are involved, the law is specifically an equation relating their sine values.
Q36. Refraction occurs in
• a) mirrors
• b) transparent media
• c) metals
• d) vacuum only
Answer: b) transparent media
Refraction requires light to pass from one medium into another. This can only happen in transparent or translucent media that allow light transmission. Mirrors cause reflection, and metals are generally opaque, preventing refraction.
Q37. Maximum slowing of light occurs in
• a) air
• b) water
• c) glass
• d) vacuum
Answer: c) glass
Among the given options, glass (with a refractive index of about 1.5) slows light more than air (~1) or water (~1.33). While diamond would slow it even more, glass causes the maximum slowing from the choices provided.
Q38. Symbol for refractive index
• a) f
• b) m
• c) n
• d) R
Answer: c) n
The standard and almost universal symbol for refractive index is the lowercase letter ‘n’. For example, in Snell’s law (n1 sin θ1 = n2 sin θ2) and in the formula for absolute refractive index (n = c/v).
Q39. Refractive index increases when speed
• a) increases
• b) decreases
• c) same
• d) random
Answer: b) decreases
The relationship is inverse: n = c / v. If the speed of light in the medium (v) decreases, the value of the refractive index (n) increases. They cannot increase together; one goes up as the other goes down.
Q40. Refractive index depends on color because of
• a) reflection
• b) dispersion
• c) absorption
• d) expansion
Answer: b) dispersion
The variation of refractive index with the color (wavelength) of light is called dispersion. Shorter wavelengths (violet/blue) are slowed more and bent more (higher n) than longer wavelengths (red). This is what causes a prism to separate white light into a spectrum.
Q41. Law of refraction states ratio is constant for
• a) all light
• b) white light only
• c) given color
• d) night light
Answer: c) given color
Snell’s law, and the constant ratio of sines, holds for monochromatic light (light of a single specific color or wavelength). For white light, which contains many colors, each color has its own slightly different constant due to dispersion.
Q42. Refractive index compares speed in
• a) two media
• b) two mirrors
• c) same medium
• d) vacuum only
Answer: a) two media
The relative refractive index (n21) is a comparison of the speed of light in two different media (v1/v2). The absolute refractive index compares the speed in a medium to that in a vacuum, which is also a comparison between two “media.”
Q43. If medium 2 is denser, refracted ray bends
• a) towards normal
• b) away from normal
• c) straight
• d) back
Answer: a) towards normal
When light travels from a rarer (lower n) to a denser (higher n) medium, it slows down. This causes the ray to bend towards the normal. The greater the difference in refractive indices, the more pronounced the bending towards the normal.
Q44. Absolute refractive index compares medium with
• a) water
• b) glass
• c) air/vacuum
• d) oil
Answer: c) air/vacuum
The definition of absolute refractive index uses a vacuum as the universal reference. For most practical calculations, air is used as the reference because its refractive index is so close to 1, making the values nearly identical to the absolute index.
Q45. Refractive index unit is
• a) m/s
• b) meter
• c) none
• d) second
Answer: c) none
Refractive index is a dimensionless quantity. It is a pure ratio of two speeds (c/v) or two sines (sin i/sin r). Since both the numerator and denominator have the same units, they cancel out, leaving no physical unit.
Q46. Smaller wavelength means
• a) lower refractive index
• b) higher refractive index
• c) no effect
• d) infinite index
Answer: b) higher refractive index
Due to dispersion, a light wave with a smaller wavelength (like blue light) experiences a higher refractive index in a given medium compared to a larger wavelength (like red light). This is why blue light bends more in a prism.
Q47. Light entering denser medium becomes
• a) faster
• b) slower
• c) same
• d) zero
Answer: b) slower
A denser optical medium has a higher refractive index, which directly implies a lower speed of light. Therefore, when light enters a denser medium from a rarer one, its speed always decreases.
Q48. Refraction shows light does not travel
• a) in vacuum
• b) in air only
• c) same in all
• d) in lanes
Answer: c) same in all
The very fact that refraction occurs—light bending at an interface—demonstrates that light does not travel at the same speed in all materials. If it did, there would be no change in speed and hence no bending.
Q49. Medium with lowest refractive index is
• a) glass
• b) water
• c) air
• d) diamond
Answer: c) air
Among the listed options, air has the refractive index closest to 1 (approximately 1.0003), which is the lowest. Water (~1.33), glass (~1.5), and diamond (~2.42) all have progressively higher indices, meaning they are more optically dense.
Q50. Highest refractive index among given is
• a) air
• b) water
• c) glass
• d) diamond
Answer: d) diamond
Diamond has one of the highest known refractive indices for natural materials, approximately 2.42. This exceptionally high value is responsible for diamond’s brilliant sparkle, as it causes extensive bending and internal reflection of light.