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Q1. A lens produces magnification m = +2. This means the image is:
Positive magnification means the image is virtual and erect (upright). A magnification of +2 means the image is twice the size of the object and formed on the same side as the object. Negative magnification indicates a real and inverted image.
Q2. If the distance between lens and image of the Sun is measured, it gives:
When sunlight (parallel rays) is focused by a convex lens, the image of the Sun is formed at the focus. The distance between the lens and this sharp image gives the focal length of the lens.
Q3. If a lens has power –1 D, its focal length is:
Power of a lens is given by P = 1/f (where f is in meters). If P = –1 D, then f = 1/P = 1/(–1) = –1 m. The negative sign indicates it is a concave lens.
Q4. Lenses follow sign conventions similar to:
Lenses follow the same Cartesian sign convention as spherical mirrors. Distances measured in the direction of incident light are positive, and those measured opposite are negative. The optical centre is taken as the origin.
Q5. What happens if you look at the Sun directly or through a lens?
Looking at the Sun directly or through a lens can cause permanent damage to your eyes. A convex lens concentrates sunlight onto the retina, which can burn it and cause blindness. Never look at the Sun through a lens.
Q6. The value of magnification m > 1 means:
Magnification (m) is the ratio of the height of the image to the height of the object. If m > 1, the image is larger than the object (enlarged). If m < 1, the image is smaller (diminished).
Q7. The plane passing through the principal focus is called the:
The focal plane is the plane perpendicular to the principal axis that passes through the principal focus. Rays coming from infinity converge on this plane. It is an important concept in lens optics.
Q8. The symbol F represents:
The symbol F represents the principal focus of a lens or mirror. It is the point on the principal axis where parallel rays converge (for convex lens) or appear to diverge from (for concave lens).
Q9. Power of a concave lens is:
A concave lens has a negative focal length, so its power (P = 1/f) is also negative. This indicates that the lens diverges light rays. Convex lenses have positive power.
Q10. A concave lens always forms an image that is:
A concave lens always forms a virtual, erect, and diminished image regardless of the object’s position. The image is formed on the same side as the object and cannot be obtained on a screen.
Q11. A virtual image formed by a lens is always:
A virtual image formed by a lens is always on the same side as the object. It is erect and cannot be obtained on a screen. A real image is formed on the opposite side of the lens.
Q12. The focal length of a concave lens is:
According to the sign convention, the focal length of a concave lens is negative because its focus is virtual and lies on the same side as the incident light. A convex lens has a positive focal length.
Q13. A lens with high power bends rays:
Power is a measure of how strongly a lens converges or diverges light. A lens with high power has a short focal length and bends light rays strongly. A low power lens bends rays slightly.
Q14. A lens with power –2.5 D has focal length:
Power P = 1/f, so f = 1/P = 1/(–2.5) = –0.40 m. The negative sign indicates it is a concave lens. The focal length is 40 cm on the same side as the object.
Q15. Optical instruments such as microscopes use many lenses to:
Microscopes and other optical instruments use multiple lenses in combination to achieve high magnification and improve image sharpness. Each lens contributes to the overall magnification and helps correct aberrations.
Q16. For a concave lens, image distance is always:
For a concave lens, the image is always virtual and formed on the same side as the object. According to the sign convention, image distance (v) for a virtual image is negative.
Q17. For a concave lens, parallel rays after refraction:
Parallel rays incident on a concave lens diverge after refraction. They appear to come from a point (the principal focus) on the same side as the incident light. This is why concave lenses are called diverging lenses.
Q18. The magnification produced by a lens is the ratio of:
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 can also be calculated as m = v/u for lenses.
Q19. Magnification is also given by the ratio:
For a lens, magnification is also given by m = v/u, where v is the image distance and u is the object distance (with proper sign convention). This formula is useful when heights are not known.
Q20. Increasing the power of a convex lens means:
Power of a lens is P = 1/f. Increasing power means decreasing focal length. A lens with shorter focal length bends light more strongly and has higher power.
Q21. The focal length of a convex lens is considered:
According to the sign convention, a convex lens has a positive focal length because its principal focus is on the opposite side of the lens (real focus). A concave lens has a negative focal length.
Q22. A real image formed by a lens is always:
A real image formed by a lens is always inverted (upside down). It is formed on the opposite side of the lens and can be obtained on a screen. Virtual images are always erect.
Q23. If magnification m = –0.33, the negative sign indicates:
A negative magnification indicates that the image is inverted. For a lens, negative magnification means the image is real and inverted. The magnitude 0.33 means the image is diminished.
Q24. Power of a convex lens is:
A convex lens converges light and has a positive focal length. Since power P = 1/f, a positive focal length gives positive power. Convex lenses are used to correct hypermetropia.
Q25. In the example, a concave lens with f = –15 cm forms an image at –10 cm. The object is placed at:
Using the lens formula 1/f = 1/v – 1/u: 1/(–15) = 1/(–10) – 1/u → –1/15 = –1/10 – 1/u → 1/u = –1/10 + 1/15 = (–3 + 2)/30 = –1/30 → u = –30 cm.
Q26. 1 dioptre equals the power of a lens with focal length:
One dioptre (1 D) is the power of a lens with a focal length of 1 meter. The formula is P = 1/f, where f is in meters. So, 1 D = 1/1 m = 1 m.
Q27. A lens with power +2.0 D is:
Positive power indicates a convex lens because convex lenses have positive focal lengths and positive power. A power of +2.0 D means the lens has a focal length of f = 1/2 = 0.5 m.
Q28. In the lens formula, u is:
In the lens formula 1/f = 1/v – 1/u, u represents the object distance (distance between the object and the optical centre). v represents the image distance, and f is the focal length.
Q29. SI unit of power of a lens is:
The SI unit of power of a lens is the dioptre (D). One dioptre is the power of a lens with a focal length of 1 meter. Candela is for luminous intensity, watt is for power (energy), and lumen is for luminous flux.
Q30. In the lens formula, v represents:
In the lens formula 1/f = 1/v – 1/u, v represents the image distance (distance between the image and the optical centre). u is object distance, and f is focal length.
Q31. Parallel rays falling on a convex lens:
Parallel rays falling on a convex lens converge to a point called the principal focus. This is why convex lenses are called converging lenses. The distance from the lens to this point is the focal length.
Q32. A convex lens of focal length +10 cm forms an image of an object placed at:
Using the lens formula with f = +10 cm and u = –20 cm (object distance is negative): 1/v = 1/f + 1/u = 1/10 + 1/(–20) = 2/20 – 1/20 = 1/20 → v = +20 cm. The image is real and formed at 20 cm on the opposite side.
Q33. The ability of a lens to converge or diverge light rays depends on its:
The ability of a lens to converge or diverge light is determined by its focal length. A shorter focal length means stronger convergence (for convex) or stronger divergence (for concave). This ability is measured as the power of the lens.
Q34. The point at which rays parallel to the principal axis converge after passing through a convex lens is called:
The principal focus (F) of a convex lens is the point on the principal axis where parallel rays converge after passing through the lens. The distance between the optical centre and the focus is the focal length.
Q35. A lens of focal length 0.5 m has power:
Power P = 1/f = 1/0.5 = +2 D. The positive sign indicates it is a convex lens. A lens with a shorter focal length has higher power and bends light more strongly.
Q36. A lens has how many principal foci?
A lens has two principal foci—one on each side of the lens. These are called the first focus (F₁) and the second focus (F₂). This is because light can enter from either side of the lens.
Q37. A convex lens of short focal length bends light rays:
A convex lens with a short focal length has high power and bends light rays through larger angles. This means it converges light more strongly. Such lenses are used in magnifying glasses and microscopes.
Q38. A concave lens always forms images that are:
A concave lens always forms virtual, erect, and diminished images. The image is always smaller than the object regardless of where the object is placed. This is why concave lenses are used to correct myopia.
Q39. The bright spot formed on paper by a convex lens when focusing sunlight is actually:
When a convex lens focuses sunlight on paper, the bright spot is a real image of the Sun formed at the focus. This is a real image because the light rays actually meet at that point and can burn the paper.
Q40. The distance between the optical centre and the principal focus is called:
Focal length (f) is the distance between the optical centre of the lens and its principal focus. It determines the power of the lens. The aperture is the diameter of the lens, and magnification is the ratio of image size to object size.
Q41. When a convex lens focuses sunlight on paper, what is formed on the paper?
The image formed on paper when a convex lens focuses sunlight is a real image. It can be obtained on a screen (paper) and is inverted. The heat from this concentrated light can burn the paper.
Q42. Opticians prefer using power instead of focal length because:
When multiple lenses are placed together, their powers simply add up (P = P₁ + P₂ + …). This makes it easier for opticians to calculate the combined power of a lens system. Focal lengths are more difficult to add directly.
Q43. The lens formula is a relationship between:
The lens formula is 1/f = 1/v – 1/u, which relates object distance (u), image distance (v), and focal length (f). It does not include height (h), which is used in magnification calculations.
Q44. Rays coming from the Sun are considered:
The Sun is very far away, so the rays of light coming from it are considered parallel to each other. This is why a convex lens can focus sunlight to a point (the focus) and form a real image of the Sun.
Q45. The formula for net power of lenses in contact is:
When lenses are placed in contact, their powers add algebraically. So, P = P₁ + P₂ + P₃ + … This is why opticians prefer using power—it makes calculations of combined lens systems simple.
Q46. Why does the paper begin to burn when sunlight is focused on it using a convex lens?
The convex lens converges sunlight to a small point (the focus). This concentration of light energy produces intense heat at that point, which can burn the paper. This demonstrates the focusing power of convex lenses.
Q47. All sign convention measurements for lenses are taken from the:
For lenses, all distances are measured from the optical centre (the centre point of the lens). For mirrors, distances are measured from the pole. The optical centre is the origin in the sign convention for lenses.
Q48. A combination of two lenses of +1 D and –2 D has net power:
When lenses are in contact, their powers add: P = P₁ + P₂ = (+1) + (–2) = –1 D. The negative sign indicates that the combination behaves like a concave lens with a focal length of –1 m.
Q49. A concave lens of very short focal length has:
A concave lens with a very short focal length has high power (negative) and diverges light rays strongly. It spreads out light more than a lens with a longer focal length.
Q50. Power of a lens is defined as:
The power of a lens is defined as the reciprocal of its focal length measured in meters. So, P = 1/f. The unit of power is the dioptre (D). A lens with a shorter focal length has higher power.