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Q1. In the sign convention for spherical mirrors, distances are measured from the __________.
According to the New Cartesian Sign Convention, all distances are measured from the pole (P) of the spherical mirror. The pole is the center of the reflecting surface. Distances are measured along the principal axis, with the sign convention determining whether they are positive or negative.
Q2. According to the sign convention, distances measured in the direction of the incident light are taken as __________.
In the New Cartesian Sign Convention, distances measured in the direction of the incident light are taken as positive. Distances measured opposite to the direction of incident light are taken as negative. This convention is used consistently for both mirrors and lenses.
Q3. For a spherical mirror, the focal length (f) and object distance (u) are both negative. What type of mirror is it?
For a concave mirror, the focus lies in front of the mirror (on the same side as the incident light). Therefore, the focal length (f) is negative. The object distance (u) is also negative because the object is placed in front of the mirror. For convex mirrors, the focal length is positive.
Q4. The mirror formula is __________.
The mirror formula is 1/v + 1/u = 1/f, where u is the object distance, v is the image distance, and f is the focal length. This formula applies to both concave and convex mirrors when used with the proper sign convention.
Q5. Magnification (m) for a mirror is the ratio of the height of the image to the __________.
Magnification (m) is defined as the ratio of the height of the image (hᵢ) to the height of the object (h₀). So, m = hᵢ/h₀. It tells us how many times the image is enlarged or diminished compared to the object.
Q6. Magnification can also be calculated using the formula m = __________.
Magnification for a mirror is also given by m = -v/u, where v is the image distance and u is the object distance. The negative sign is included because of the sign convention, and it helps determine whether the image is real/inverted or virtual/erect.
Q7. A positive magnification value indicates that the image is __________.
A positive magnification means the image is erect (upright) and virtual. This happens when the image is formed behind the mirror. A negative magnification means the image is real and inverted.
Q8. The bending of light as it passes from one medium to another is called __________.
Refraction is the bending of light when it passes obliquely from one transparent medium to another. This bending occurs because the speed of light changes as it enters a different medium. Reflection is bouncing back, while dispersion is splitting of light into colors.
Q9. The Refractive Index is the ratio of the speed of light in vacuum to the speed of light in __________.
The absolute refractive index (n) of a medium is the ratio of the speed of light in a vacuum (c) to the speed of light in that medium (v). So, n = c/v. It compares the speed in vacuum to the speed in the given medium.
Q10. Which medium has the highest refractive index among the given options?
Diamond has the highest refractive index among common materials, with n ≈ 2.42. This means light slows down significantly in diamond, which is why diamonds sparkle. Water has n ≈ 1.33, glass has n ≈ 1.50-1.60, and alcohol has n ≈ 1.36.
Q11. Which medium has the lowest refractive index among the given common materials?
Air has the lowest refractive index (approximately 1.0003) among common materials. Water has n ≈ 1.33, ice has n ≈ 1.31, and kerosene has n ≈ 1.44. Vacuum has the lowest possible refractive index of exactly 1.00.
Q12. A transparent material with a curved surface, used to converge or diverge light, is called a __________.
A lens is a transparent material (like glass or plastic) with one or two curved surfaces. It is designed to converge or diverge light rays. A convex lens converges light, while a concave lens diverges light. Mirrors reflect light, while lenses refract light.
Q13. A lens that is thicker in the middle and thinner at the edges is a __________ lens.
A convex lens is thicker in the middle and thinner at the edges. It converges parallel rays of light to a point (focus). It is also called a converging lens. A concave lens is thinner in the middle and thicker at the edges.
Q14. For a convex lens, a real and inverted image is formed when the object is placed __________.
A convex lens forms a real and inverted image only when the object is placed beyond the focus (F). If the object is between the lens and the focus, the image is virtual, erect, and magnified. At the focus, the image is formed at infinity.
Q15. The formula that relates object distance (u), image distance (v), and focal length (f) for a lens is __________.
The lens formula is 1/v – 1/u = 1/f, where u is the object distance, v is the image distance, and f is the focal length. This formula is used with the sign convention for lenses and applies to both convex and concave lenses.
Q16. The power of a lens is defined as the reciprocal of its __________.
The power of a lens (P) is the reciprocal of its focal length (f) measured in meters. So, P = 1/f. A lens with a shorter focal length has more power, meaning it bends light more strongly. The unit of power is the dioptre.
Q17. The unit of power of a lens is __________.
The power of a lens is measured in dioptres (D). One dioptre is the power of a lens with a focal length of 1 meter. So, P = 1/f (in meters). A convex lens has positive power, and a concave lens has negative power.
Q18. A convex lens has a __________ focal length and __________ power.
A convex lens converges light and has a real focus on the opposite side of the lens. According to the sign convention, its focal length is positive. Since power P = 1/f, a positive focal length gives positive power.
Q19. A concave lens always forms a __________ image.
A concave lens always forms a virtual, erect, and diminished image, regardless of where the object is placed. The image is formed on the same side as the object and cannot be obtained on a screen. This is why concave lenses are used to correct myopia.
Q20. Magnification for a lens is given by the formula m = __________.
Magnification for a lens is given by m = v/u (without the negative sign that is used for mirrors). It is the ratio of the image distance (v) to the object distance (u). A positive m means the image is erect and virtual; a negative m means the image is inverted and real.
Q21. If the magnification produced by a lens is +2, the image is __________.
A positive magnification means the image is erect (upright) and virtual. A magnification of +2 means the image is twice the size of the object and is formed on the same side as the object. Negative magnification would indicate a real and inverted image.
Q22. Which type of lens is used to correct myopia (short-sightedness)?
Myopia (short-sightedness) occurs when the eye cannot see distant objects clearly because the image forms in front of the retina. A concave lens is used to correct this by diverging the light rays before they enter the eye, moving the image back onto the retina.
Q23. The refractive index of water is approximately __________.
The refractive index of water is approximately 1.33. This means light travels about 1.33 times slower in water than in a vacuum. This is why a straw appears bent when placed in a glass of water—the light bends as it passes from water to air.
Q24. Light travels fastest in which medium?
Light travels fastest in a vacuum, at approximately 3 × 10⁸ m/s. In any material medium like water, glass, or diamond, light slows down. Vacuum has the lowest refractive index (1.00), meaning no slowing of light occurs.
Q25. According to the sign convention for lenses, the focal length of a convex lens is __________.
According to the Cartesian sign convention for lenses, the focal length of a convex lens is positive because its focus lies on the opposite side of the incident light (real focus). For a concave lens, the focal length is negative because its focus is virtual and on the same side as the object.
Q26. A lens with a power of +4.0 D is a __________ lens.
A positive power indicates a convex lens because convex lenses have positive focal lengths and positive power. A power of +4.0 D means the lens has a focal length of f = 1/P = 1/4 = 0.25 m (25 cm), which is a converging lens.
Q27. If the image distance (v) is positive for a lens, the image is formed on the __________ side.
For lenses, a positive image distance (v) means the image is formed on the opposite side of the lens from the object. This indicates a real image. If v is negative, the image is formed on the same side as the object, indicating a virtual image.
Q28. Which lens has a negative focal length?
A concave lens has a negative focal length because its focus is virtual and lies on the same side as the object. A convex lens has a positive focal length because its focus is real and lies on the opposite side.
Q29. When an object is placed at 2F (twice the focal length) of a convex lens, the image is formed __________.
When an object is placed at 2F (twice the focal length) of a convex lens, the image is formed at 2F on the other side of the lens. The image is real, inverted, and of the same size as the object. This is similar to the centre of curvature case in mirrors.
Q30. The refractive index of a medium depends on the __________ of light.
Refractive index depends on the wavelength (color) of light. Different colors have different speeds in the same medium, so they have different refractive indices. This is why dispersion occurs—a prism separates white light into its constituent colors.
Q31. Which of the following has a refractive index greater than 1.5?
Dense flint glass has a refractive index of about 1.62-1.65, which is greater than 1.5. Water has n ≈ 1.33, crown glass has n ≈ 1.52 (slightly greater than 1.5), and alcohol has n ≈ 1.36. Dense flint glass is used in lenses for its high refractive index.
Q32. Snell’s Law relates the angles of incidence and refraction with the __________.
Snell’s law states that n₁ sin i = n₂ sin r, where n₁ and n₂ are the refractive indices of the two media. So, it relates the angles of incidence and refraction to the refractive indices of the media, not to distances or focal lengths.
Q33. A lens forms an image that is four times the size of the object. The magnification is __________.
Magnification (m) is the ratio of image size to object size. If the image is four times the size, |m| = 4. The sign depends on whether the image is real/inverted (negative m) or virtual/erect (positive m). Without more information, the magnitude is 4.
Q34. The power of a lens with a focal length of 0.5 meters is __________.
Power of a lens P = 1/f (in meters). Here, f = 0.5 m, so P = 1/0.5 = 2 D. A power of 2 dioptres means the lens converges (or diverges) light strongly.
Q35. A concave lens is also called a __________ lens.
A concave lens is called a diverging lens because it spreads out (diverges) parallel rays of light. The rays appear to come from a point (focus) on the same side as the object. In contrast, a convex lens is a converging lens.
Q36. For a lens, if the object distance (u) is negative, it means the object is __________.
According to the sign convention for lenses, object distance (u) is always negative because the object is placed on the same side as the incident light (the left side). This is a standard convention, regardless of whether the object is real or virtual.
Q37. Which of these materials is likely to bend light the most?
Diamond has the highest refractive index among the options, so it bends light the most. The higher the refractive index, the more light slows down and bends when entering the material. This is why diamonds have such brilliant sparkle.
Q38. The image formed by a convex lens is virtual and erect when the object is placed __________.
A convex lens forms a virtual, erect, and magnified image only when the object is placed between the lens and its focus (F). This is the principle used in a magnifying glass. The image is formed on the same side as the object.
Q39. Two lenses of power +2.0 D and -0.5 D are placed in contact. The combined power is __________.
When lenses are placed in contact, their powers add algebraically. So, P_total = P₁ + P₂ = (+2.0) + (-0.5) = +1.5 D. This is a convex lens combination with a net positive power.
Q40. The principal focus of a concave lens is __________.
The principal focus of a concave lens is virtual. This means the light rays after refraction do not actually meet at the focus; they only appear to diverge from it. The focus is located on the same side as the object.
Q41. The lens formula, 1/v – 1/u = 1/f, is applicable to __________.
The lens formula 1/v – 1/u = 1/f applies to both convex and concave lenses. It is used with the proper sign convention for each type of lens. The same formula is not used for mirrors (which use 1/v + 1/u = 1/f).
Q42. If the magnification is less than 1 (|m| < 1), the image is __________.
If the magnitude of magnification is less than 1 (|m| < 1), the image is smaller than the object, meaning it is diminished. If |m| = 1, the image is the same size, and if |m| > 1, the image is enlarged.
Q43. Which lens is used as a simple microscope (magnifying glass)?
A simple microscope (magnifying glass) uses a convex lens with a short focal length. When an object is placed between the lens and its focus, it forms a virtual, erect, and magnified image. A short focal length gives higher magnification.
Q44. The speed of light in a medium with refractive index 1.5 is __________.
The speed of light in a medium is given by v = c/n. Here, c = 3 × 10⁸ m/s and n = 1.5. So, v = 3 × 10⁸ / 1.5 = 2 × 10⁸ m/s. Light slows down to two-thirds of its speed in vacuum in this medium.
Q45. A real image formed by a lens can be obtained on a __________.
A real image is formed when light rays actually converge at a point. Such an image can be obtained on a screen. Virtual images are formed where rays only appear to meet and cannot be captured on a screen.
Q46. The refractive index of a medium is always __________.
The absolute refractive index of a medium is always greater than or equal to 1. It equals 1 only for a vacuum (or approximately for air). For all other media, n > 1 because light always travels slower in a material medium than in a vacuum.
Q47. When light enters a denser medium from a rarer medium, it bends __________ the normal.
When light travels from a rarer medium (like air) to a denser medium (like water or glass), it slows down and bends towards the normal. The angle of refraction (r) is smaller than the angle of incidence (i). This is a fundamental rule of refraction.
Q48. A lens of power -2.0 D is a __________ lens of focal length __________ m.
A negative power indicates a concave lens. P = 1/f, so f = 1/P = 1/(-2.0) = -0.5 m. The focal length is -0.5 m (negative for concave lens). So it’s a concave lens with a focal length of 0.5 m.
Q49. Which of these statements about magnification is true?
Magnification for a lens is given by m = v/u (with sign convention). It is a dimensionless ratio (no unit). It can be positive or negative, and can be greater than, equal to, or less than 1, depending on the image size.
Q50. For a convex lens, if the object is placed at the focus (F), the image is formed at __________.
When an object is placed at the focus (F) of a convex lens, the refracted rays emerge parallel to each other. These parallel rays meet only at infinity. This principle is used in projectors and spotlights where a strong beam of light is needed.