CBSE Class 10 Science Light – Reflection and Refraction MCQs Set 15

Answer: (b) infinite. A plane mirror has an infinite radius of curvature, so its focal length is infinite.

Teacher's Note:
1. This is why it forms an image of the same size as the object.
2. The image is virtual and as far behind the mirror as the object is in front.

Answer: (c) 20 cm. The image is 10 cm behind the mirror, so the object and image are 10 + 10 = 20 cm apart.

Teacher's Note:
1. Do not stop at the distance from the mirror.
2. The question asks for the object-to-image distance.

Answer: (c) 3. The number of images is 360°/90° − 1 = 3.

Teacher's Note:
1. Smaller angles between the mirrors give more images.
2. This is the principle of a kaleidoscope.

Answer: (d) −6 cm. f = R/2 = 12/2 = 6 cm, and it is negative for a concave mirror, so f = −6 cm.

Teacher's Note:
1. The focal length is half the radius of curvature.
2. Concave mirrors have negative f in the sign convention.

Answer: (c) at the centre of curvature. With u = −20 and f = −10, 1/v = 1/f − 1/u = −1/10 + 1/20 = −1/20, so v = −20 cm, which is the centre of curvature.

Teacher's Note:
1. The image is real, inverted and the same size.
2. This is a standard case: the object at C gives the image at C.

Answer: (b) virtual, erect and diminished. A convex mirror always forms a virtual, erect and smaller image behind the mirror.

Teacher's Note:
1. This gives a wide field of view.
2. It is used as a rear-view mirror in vehicles.

Answer: (d) −0.5. m = −v/u = −(−30)/(−60) = −0.5.

Teacher's Note:
1. The negative sign shows that the image is inverted.
2. The magnitude 0.5 means it is half the size of the object.

Answer: (a) +24 cm. 1/f = 1/v + 1/u = 1/15 − 1/40 = 5/120, so f = +24 cm.

Teacher's Note:
1. A convex mirror has a positive focal length.
2. A negative answer would indicate a concave mirror.

Answer: (c) 12 cm. f = −18 cm and m = 3 gives v = −3u, so 1/v + 1/u = 2/(3u) = −1/18 and u = −12 cm.

Teacher's Note:
1. The object lies between the pole and the focus.
2. Only there does a concave mirror form a magnified virtual image.

Answer: (d) between P and F. The image is erect and enlarged (virtual) only when the object is between the pole and the focus.

Teacher's Note:
1. This is how shaving and dentist's mirrors work.
2. In other positions the image is inverted.

Answer: (a) −30 cm, −2. 1/v = 1/f − 1/u = −1/10 + 1/15 = −1/30, so v = −30 cm and m = −v/u = −(−30)/(−15) = −2.

Teacher's Note:
1. The image is real, inverted and twice as large.
2. It forms beyond C.

Answer: (b) 2 × 10⁸ m/s. v = c/n = (3 × 10⁸)/1.5 = 2 × 10⁸ m/s.

Teacher's Note:
1. Light slows down in an optically denser medium.
2. The refractive index is the ratio c/v.

Answer: (b) 2. n = c/v = (3 × 10⁸)/(1.5 × 10⁸) = 2.

Teacher's Note:
1. Refractive index has no unit.
2. A larger value means a denser medium.

Answer: (a) bends towards the normal. Water is optically denser, so the ray bends towards the normal.

Teacher's Note:
1. The reverse happens when light goes from water to air.
2. A ray along the normal does not bend at all.

Answer: (b) light travels 2.42 times slower in diamond than in air. n = c/v, so the speed of light in diamond is c/2.42.

Teacher's Note:
1. This is a very high refractive index.
2. It is the reason diamonds sparkle.

Answer: (c) 1.21. n = 1.65/1.36 ≈ 1.21.

Teacher's Note:
1. The refractive index of medium 2 with respect to medium 1 is n₂/n₁.
2. Divide the denser one by the other.

Answer: (a) √3 (about 1.73). n = sin 60°/sin 30° = (√3/2)/(1/2) = √3 ≈ 1.73.

Teacher's Note:
1. Use Snell's law n = sin i/sin r.
2. Make sure the calculator is in degree mode.

Answer: (b) at 2F. For an object at 2F, the image is also at 2F on the other side, real, inverted and equal in size.

Teacher's Note:
1. In this case v = u = 2f and m = −1.
2. It is a useful check for ray diagrams.

Answer: (b) virtual, erect and magnified. A convex lens forms a virtual, erect and magnified image on the same side as the object when u is less than f.

Teacher's Note:
1. This is how a magnifying glass works.
2. The image cannot be caught on a screen.

Answer: (a) +60 cm, −2. 1/v = 1/f + 1/u = 1/20 − 1/30 = 1/60, so v = +60 cm and m = v/u = 60/(−30) = −2.

Teacher's Note:
1. The image is real, inverted and twice the size.
2. It forms on the other side of the lens.

Answer: (b) −30 cm. 1/v = 1/15 − 1/10 = −1/30, so v = −30 cm, a virtual image on the same side as the object.

Teacher's Note:
1. Since u is less than f the image is virtual.
2. The magnification is +3.

Answer: (b) +2 D. P = 1/f in metres = 1/0.5 = +2 D.

Teacher's Note:
1. Always convert the focal length to metres.
2. A convex lens has positive power.

Answer: (c) −4 D. f = −0.25 m, so P = 1/(−0.25) = −4 D.

Teacher's Note:
1. A concave lens diverges light, so its power is negative.
2. Spectacles for short sight use such lenses.

Answer: (a) Convex lens of focal length 5 cm. A magnifier is a convex lens of short focal length, which gives larger magnification.

Teacher's Note:
1. A concave lens never gives a magnified image of a real object.
2. A shorter focal length gives a higher power.