Refraction / Optics - Properties of Light : 1
Q: What is the unit of angular displacement ?
Ans: Radian (rad)
Explanation: Angular displacement measures rotation. One radian is the angle subtended at the center of a circle by an arc equal to the radius.
Q: Which lens has negative focal length ?
Ans: Concave lens
Explanation: Concave lenses diverge rays. Their focal point is virtual, on the same side as the object. Hence, focal length is negative.
Q: In which medium does light travel fastest ?
Ans: Vacuum
Explanation: Light moves fastest in vacuum because there are no particles to slow it down. Denser media like water or glass reduce its speed.
Q: Which lens is preferred for reading small letters on white paper ?
Ans: Convex lens of 5 cm focal length
Explanation: A short focal length convex lens magnifies small letters, making reading easier, especially for near-sighted vision correction.
Q: A convex lens has focal length 20 cm. What is its power ?
Ans: 5 D
Explanation: Power (P) = 100 / f (in cm). So, P = 100/20 = 5 dioptres. Convex lens power is positive.
Q: Which material cannot be used to make lenses ?
Ans: Clay
Explanation: Lenses must be transparent. Clay is opaque, so it cannot refract light to form images.
Q: How many laws of refraction are there ?
Ans: Two
Explanation: 1) Incident ray, refracted ray, and normal lie in the same plane. 2) nā sin i = nā sin r (Snell’s law).
Q: Which lens produces only virtual images ?
Ans: Concave lens
Explanation: Concave lenses always diverge rays, forming virtual, upright, and diminished images regardless of object distance.
Q: How is a convex lens shaped ?
Ans: Thicker in the middle than at edges
Explanation: Convex lenses converge light rays to a focus. Their middle is thicker to bend rays inward, forming real or virtual images.
Q: Where should an object be placed in front of a convex lens to get an image of the same size ?
Ans: At the center of curvature
Explanation: Placing an object at twice the focal length (2f) produces a real, inverted image equal in size to the object.
Q: Where is a real, inverted, diminished image formed by a convex lens ?
Ans: Beyond infinity (very far away)
Explanation: When an object is very close to the focus, the image forms far away, inverted and diminished.
Q: Which object can form a real image ?
Ans: Convex lens
Explanation: Convex lenses converge rays, forming real, inverted images on the opposite side of the lens when the object is beyond the focus.
Q: What is the purpose of photography ?
Ans: Convex lens
Explanation: Cameras use convex lenses to converge light rays and form real, focused images on the film or sensor.
Q: In which medium is light slowest ?
Ans: Glass (or according to options)
Explanation: Denser media like glass slow down light more due to interactions with particles, compared to air or vacuum.
Q: Where should an object be placed in front of a concave lens to get an image of the same size ?
Ans: At the center of curvature
Explanation: Placing the object at the center of curvature of a concave mirror/lens produces an image of equal size.
Q: Write the law of refraction.
Ans: n = sin i / sin r
Explanation: Refractive index measures bending of light. i = angle of incidence, r = angle of refraction. It shows optical density of the medium.
Q: Where should an object be placed in front of a convex mirror to get image equal to object ?
Ans: At the center of curvature
Explanation: For mirrors/lenses, placing an object at the center of curvature gives an image equal in size, upright or inverted depending on mirror type.
Q: Power of a concave lens with 25 cm focal length ?
Ans: 4 D
Explanation: Power P = 100 / f (cm). So, P = 100 / 25 = 4 D. Concave lens power is negative by convention.
Q: Refractive index of water ?
Ans: 1.33
Explanation: Light bends when entering water. The refractive index n = 1.33 indicates light travels slower in water than in air.
Q: Which lens forms both real and virtual images ?
Ans: Convex lens
Explanation: Convex lenses produce real images when object is beyond focus, virtual images when object is within focus.
Q: A lens with 20 cm focal length, what type ?
Ans: Convex lens
Explanation: Positive focal length indicates a converging lens (convex).
Q: Power of concave lens with 1 m focal length ?
Ans: 1 D
Explanation: P = 100 / f (cm) → f = 100 cm → P = 1 D, negative for concave lens.
Q: In an experiment with a concave mirror forming an image on a screen, what distances need to be measured ?
Ans: Between mirror & screen and mirror & object
Explanation: Measuring these distances helps determine focal length using mirror formula 1/f = 1/v + 1/u.
Q: Material through which light passes easily ?
Ans: Transparent
Explanation: Transparent materials allow light to pass with minimal scattering, e.g., glass, water.
Q: Which mirror can produce a real image ?
Ans: Only concave mirror
Explanation: Concave mirrors converge rays to form real, inverted images on a screen.
Q: Which mirror can form virtual images ?
Ans: All mirrors
Explanation: Plane, concave, and convex mirrors can form virtual, upright images depending on object position.
Q: Which mirror produces smaller, virtual images ?
Ans: Convex mirror
Explanation: Convex mirrors diverge rays, producing diminished, upright, virtual images.
Q: Focal length of concave mirror is related to its radius of curvature as ?
Ans: Half
Explanation: f = R/2 for spherical mirrors (concave or convex).
Q: Which mirror is in front of a motor vehicle driver ?
Ans: Convex mirror
Explanation: Convex mirrors provide a wide view of the road, upright, and diminished images, improving safety.
Q: Image formed by convex mirror ?
Ans: Virtual and upright
Explanation: Convex mirrors diverge light rays, forming virtual, upright images, never real.
Q: Equation f = R/2 is true for which mirrors ?
Ans: Both concave and convex mirrors
Explanation: For any spherical mirror, focal length = half of radius of curvature.
Q: Bending of light when it passes from one medium to another ?
Ans: Refraction
Explanation: Light changes speed when moving between media, bending at the interface—this is called refraction.
Q: When light passes from rarer to denser medium ?
Ans: Bends toward the normal
Explanation: Speed decreases; ray bends toward the perpendicular (normal) at the interface.
Q: When light hits the boundary between two media ?
Ans: Partially reflected
Explanation: Some light is reflected back, some refracted—explains reflection and partial transparency.
Q: Why does the bottom of a water bucket appear shallower ?
Ans: Due to refraction
Explanation: Light bends at water-air interface, making objects appear nearer to the surface than they are.
Q: When light passes from denser to rarer medium ?
Ans: Bends away from normal
Explanation: Light speeds up, bending away from the perpendicular at the interface.
Q: How often does the human heart beat ?
Ans: 1/16 s
Explanation: Each heartbeat (cardiac cycle) takes approximately 0.0625 seconds (assuming standard textbook value).
Q: Where is an image formed in the human eye ?
Ans: Retina
Explanation: Lens of the eye focuses light onto the retina, forming a clear image for the brain to interpret.
Q: Where is maximum refraction in the human eye ?
Ans: Outer surface of cornea
Explanation: Cornea bends light most sharply due to higher curvature and refractive index, focusing light onto the retina.