Refraction / Optics - Properties of Light.
Q: What type of wave is light ?
Ans: Transverse wave
Explanation: Light has oscillating electric and magnetic fields perpendicular to its motion. Unlike sound (longitudinal), light moves side-to-side and up-down while traveling forward.
Q: How does light travel ?
Ans: In a straight line
Explanation: In a uniform medium, light moves straight. This explains shadows and pinhole camera images, until reflection, refraction, or obstacles alter its path.
Q: Where does light travel fastest ?
Ans: In vacuum
Explanation: Vacuum has no particles to slow light, so it travels fastest here. Air, water, or glass slow light due to interactions with molecules.
Q: What is the speed of light in vacuum ?
Ans: 3 × 10⁸ m/s
Explanation: This constant, “c,” is the fastest in nature and used in physics calculations, including optics, relativity, and electromagnetic waves.
Q: How many laws of reflection are there ?
Ans: Two
Explanation: First, incident ray, reflected ray, and normal lie in the same plane. Second, the angle of incidence equals the angle of reflection.
Q: What are the laws of reflection ?
Ans: Involving incident ray, reflected ray, and normal
Explanation: The incident ray hits a surface, the reflected ray bounces off, and the normal is perpendicular at the point of incidence, defining the reflection angle.
Q: What kind of image does a plane mirror form ?
Ans: Virtual
Explanation: Plane mirrors produce upright images behind the mirror. These cannot be projected on a screen, but appear equal in size and laterally inverted.
Q: Which mirror gives a straight image at any distance ?
Ans: Plane or convex
Explanation: Plane mirrors reflect exact size images. Convex mirrors reflect smaller, upright images. Both keep images straight, making them useful for safety and daily life.
Q: What is the focal length of a plane mirror ?
Ans: Infinity
Explanation: Plane mirrors do not converge or diverge light rays. Since rays never meet, the focal point is considered at infinity, forming virtual images at the same distance as the object.
Q: Where to place an object in a concave mirror for a real, inverted, same-size image ?
Ans: At the center of curvature
Explanation: Rays reflect and meet at the center of curvature, creating a real, inverted, and same-size image. Used in experiments and certain optical devices.
Q: What kind of image does a convex mirror form ?
Ans: Upright, diminished, virtual
Explanation: Convex mirrors diverge light rays, producing small, upright images behind the mirror. These images are virtual and cannot be projected on a screen, making them safe for vehicles.
Q: What is the mirror formula ?
Ans: 1/f = 1/v + 1/u
Explanation: This formula relates the focal length (f), object distance (u), and image distance (v) for spherical mirrors, helping to calculate image position and size.
Q: Which mirror is used to make a solar furnace ?
Ans: Concave mirror
Explanation: Concave mirrors converge sunlight to a single point, producing intense heat, which is used in solar cookers and solar furnaces.
Q: What is the lens formula ?
Ans: 1/f = 1/v - 1/u
Explanation: This formula relates focal length (f), image distance (v), and object distance (u) for lenses, allowing calculation of image properties for converging or diverging lenses.
Q: Which mirror is used inside cars ?
Ans: Convex mirror
Explanation: Convex mirrors give a wider field of view and small, upright images, helping drivers see vehicles behind without blind spots.
Q: Which mirror is used as a side mirror in vehicles ?
Ans: Convex mirror
Explanation: Side mirrors are convex to provide a wider view of approaching traffic, reducing accidents and improving safety.
Q: What kind of image is formed by mirrors under NEP guidelines ?
Ans: Diminished
Explanation: Convex mirrors or concave mirrors for certain distances produce smaller, upright images. This ensures safety and clear visibility in educational or practical setups.
Q: Which mirror has a negative focal length ?
Ans: Convex mirror
Explanation: Convex mirrors diverge rays; hence their focal point is virtual and behind the mirror. By convention, this focal length is considered negative.
Q: If magnification is negative, what is the nature of the image ?
Ans: Real and inverted
Explanation: Negative magnification occurs in concave mirrors or converging lenses when the image forms on the opposite side of the object, inverted relative to the object.
Q: Which mirror does a dentist use to see enlarged teeth ?
Ans: Concave mirror
Explanation: Concave mirrors converge rays to produce a magnified, upright image of small objects like teeth, allowing dentists to examine details clearly.
Q: What is the radius of curvature of a spherical mirror ?
Ans: Diameter of the spherical mirror
Explanation: The radius of curvature (R) is the distance from the mirror’s surface to the center of the sphere it is part of. For a full sphere, R = diameter/2.
Q: Which mirror is used in headlights ?
Ans: Concave mirror
Explanation: Concave mirrors converge light rays to produce a parallel, intense beam, illuminating roads effectively at night.
Q: What kind of image is formed by a car mirror ?
Ans: Upright
Explanation: Car mirrors (convex) produce upright images, which are easier to see and safer for driving, providing a wide field of view.
Q: What is the relation between focal length and radius of curvature ?
Ans: f = R/2
Explanation: For spherical mirrors, the focal length is half the radius of curvature. Concave or convex mirrors follow this simple relation for image calculations.
Q: Where to place an object in a concave mirror for virtual, upright, enlarged image ?
Ans: Between pole and focus
Explanation: Placing the object between the mirror’s surface (pole) and its focus produces an upright, virtual, and magnified image, used in shaving and makeup mirrors.
Q: Which mirror always gives upright image of distant objects ?
Ans: Convex mirror
Explanation: Convex mirrors diverge rays so distant objects always produce small, upright, virtual images. This is useful for vehicle side mirrors.
Q: What is the focal length of a concave lens ?
Ans: Negative
Explanation: Concave lenses diverge rays, so the focal point is virtual, located on the same side as the object. By convention, focal length is negative.
Q: Which object produces a smaller image ?
Ans: Convex mirror
Explanation: Convex mirrors diverge light rays, producing diminished, upright images regardless of object size, useful for rear-view mirrors.
Q: How does light travel in milk ?
Ans: In a straight, orderly path
Explanation: Light in milk scatters slightly, but generally follows straight paths. This explains phenomena like the Tyndall effect where particles scatter light.
Q: Does the speed of light vary in different media ?
Ans: Yes
Explanation: Light slows down when entering denser media (water, glass) due to interaction with particles. Maximum speed is in vacuum; minimum in dense materials.
Q: Why does an object appear shallower in water ?
Ans: Due to refraction of light
Explanation: Light bends when entering from water to air, making objects appear closer to the surface than they actually are.
Q: What is the unit of magnification ?
Ans: Dimensionless
Explanation: Magnification is a ratio of image size to object size, so it has no units; it can be positive (upright) or negative (inverted).
Q: How many principal foci does a lens have ?
Ans: Two
Explanation: Each lens has a focus on both sides where parallel rays converge (convex) or appear to diverge (concave). These are called principal foci.
Q: Magnification of a concave lens
Ans: m = -v/u
Explanation: Concave lenses produce virtual, diminished images. Magnification formula (negative) indicates the image is upright relative to the object.
Q: How is refractive index defined ?
Ans: Refractive index (n) = sin i / sin r
Explanation: Refractive index measures how much light bends entering a medium. i = angle of incidence, r = angle of refraction. Higher n = more bending.
Q: Which material has the highest refractive index ?
Ans: Diamond
Explanation: The refractive index measures how much light bends when entering a medium from another.
- i = angle of incidence (in first medium, usually air)
- r = angle of refraction (in the second medium)
It shows the optical “density” of a medium. Higher n → light bends more. For example, diamond has n ≈ 2.42, glass ≈ 1.5, water ≈ 1.33.
Q: Which lens is converging ?
A: Convex lens
Explanation: Convex lenses converge parallel light rays to a focal point, producing real or virtual images depending on object distance.
Q: Which lens is diverging ?
A: Concave lens
Explanation: Concave lenses diverge parallel rays outward. The rays appear to come from a virtual focus; they always produce virtual, upright, and smaller images.