Light — Reflection and Refraction — Questions & Answers

Sample questions from Light — Reflection and Refraction

1. Q1. A convex lens produces a virtual image with magnification +1.9 for an object at 15 cm. The focal length is approximately:

   • A.29.6 cm
   • B.31.7 cm✓
   • C.33.8 cm
   • D.35.9 cm
   Solution

   m=v/u=1.9, u=-15 cm: v=1.9×(-15)=-28.5 cm. Using 1/f=1/v-1/u=1/(-28.5)-1/(-15)=-0.0351+0.0667=0.0316, so f≈31.7 cm.

2. Q2. The magnification produced by a plane mirror is:

   • A.+1✓
   • B.−1
   • C.0
   • D.infinite
   Solution

   A plane mirror forms a virtual, erect image of the same size, so m = +1 (positive sign indicates an erect virtual image).

3. Q3. A concave mirror forms a real image 4 times the linear size of the object, when the object is 20 cm from the mirror. The mirror's focal length is:

   • A.12 cm
   • B.16 cm✓
   • C.20 cm
   • D.24 cm
   Solution

   m = -v/u = -4, u = -20: v = -4 × (-20)... using m=-v/u: -4 = -v/(-20) → v = -80 cm. 1/f = 1/v + 1/u = -1/80 - 1/20 = -1/80 - 4/80 = -5/80, so f = -16 cm.

4. Q4. A student argues that a convex mirror can never form a real image under any circumstances, even with a virtual object. Evaluate this claim.

   • A.True; convex mirrors always diverge reflected rays regardless of incoming ray convergence
   • B.False; a sufficiently convergent virtual object (converging beam) can allow a convex mirror to form a real image✓
   • C.True; only concave mirrors can form real images
   • D.False; convex mirrors form real images for all objects beyond C
   Solution

   If incoming light is already converging strongly enough toward a point behind the convex mirror (a virtual object) before reflecting, the mirror can still bring the rays together to form a real image in front of it, contrary to the case with an ordinary real object.

5. Q5. In the given figure, a ray of light enters a rectangular glass slab at an angle of incidence of 45°. The angle of emergence at the opposite face is:

   • A.0°
   • B.30°
   • C.45°✓
   • D.90°
   Solution

   For a slab with parallel faces the angle of emergence equals the angle of incidence, so it is 45°.

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