Reflection, Refraction, Lenses and Optical Phenomena

Attempt the quiz above first, then review every question with its correct answer and a short explanation below.

1. Q1.The speed of light in vacuum is about:

   • A.3 × 10^8 m/s✓ Correct Answer
   • B.3 × 10^6 m/s
   • C.3 × 10^10 m/s
   • D.340 m/s

   Explanation: This is the maximum speed in nature.

2. Q2.The image formed by a plane mirror is:

   • A.real and magnified
   • B.virtual and of the same size✓ Correct Answer
   • C.real and diminished
   • D.virtual and magnified

   Explanation: It is formed as far behind the mirror as the object is in front.

3. Q3.Which mirror is used in vehicle headlights?

   • A.convex mirror
   • B.concave mirror✓ Correct Answer
   • C.plane mirror
   • D.cylindrical mirror

   Explanation: A bulb at the focus gives a parallel beam.

4. Q4.Which mirror is used as a rear-view mirror in vehicles?

   • A.concave mirror
   • B.convex mirror✓ Correct Answer
   • C.plane mirror
   • D.parabolic mirror

   Explanation: It gives a wider field of view with an erect image.

5. Q5.The rainbow is formed due to the:

   • A.dispersion of sunlight by raindrops✓ Correct Answer
   • B.diffraction of sound
   • C.polarisation of light
   • D.interference of radio waves

   Explanation: Refraction, internal reflection and dispersion split white light into colours.

6. Q6.Myopia (short-sightedness) is corrected using a:

   • A.convex lens
   • B.concave lens✓ Correct Answer
   • C.plane mirror
   • D.prism

   Explanation: A concave lens diverges rays so the image falls on the retina.

7. Q7.According to the law of reflection:

   • A.the angle of incidence is double the angle of reflection
   • B.the angles add to 180 degrees
   • C.the angle of incidence equals the angle of reflection✓ Correct Answer
   • D.there is no relation between the angles

   Explanation: The incident ray, reflected ray and normal lie in one plane.

8. Q8.Hypermetropia (long-sightedness) is corrected using a:

   • A.concave lens
   • B.cylindrical mirror
   • C.convex lens✓ Correct Answer
   • D.plane glass

   Explanation: A convex lens converges rays before they reach the eye.

9. Q9.Snell's law of refraction relates:

   • A.speed to frequency
   • B.mass to charge
   • C.wavelength to temperature
   • D.sin i / sin r to the refractive index✓ Correct Answer

   Explanation: n = sin i / sin r for a ray passing from air into the medium.

10. Q10.Optical fibres work on the principle of:

    • A.total internal reflection✓ Correct Answer
    • B.diffraction
    • C.polarisation
    • D.interference

    Explanation: Light stays trapped in the core by repeated total reflections.

11. Q11.The sky appears blue because of the:

    • A.reflection from the sea
    • B.absorption by clouds
    • C.emission by the Sun in blue only
    • D.scattering of light by air molecules✓ Correct Answer

    Explanation: Shorter blue wavelengths scatter more than red.

12. Q12.The Sun appears red at sunrise and sunset because:

    • A.the Sun becomes cooler
    • B.the Sun emits only red then
    • C.blue light is scattered away over the long path through the atmosphere✓ Correct Answer
    • D.clouds absorb all other colours

    Explanation: Red, the least scattered colour, reaches the eye.

13. Q13.The focal length of a plane mirror is:

    • A.zero
    • B.infinity✓ Correct Answer
    • C.equal to its size
    • D.one metre

    Explanation: Parallel rays remain parallel after reflection.

14. Q14.The SI unit of the power of a lens is:

    • A.dioptre✓ Correct Answer
    • B.watt
    • C.lumen
    • D.candela

    Explanation: Power is the reciprocal of focal length in metres.

15. Q15.A mirage in a desert is caused by:

    • A.total internal reflection due to varying air density✓ Correct Answer
    • B.diffraction of sound
    • C.refraction of sound
    • D.dispersion of heat

    Explanation: Hot air near the ground bends light upward.

16. Q16.Which colour has the longest wavelength in visible light?

    • A.violet
    • B.blue
    • C.red✓ Correct Answer
    • D.green

    Explanation: Violet has the shortest.

17. Q17.When white light passes through a prism, which colour is deviated the most?

    • A.red
    • B.yellow
    • C.green
    • D.violet✓ Correct Answer

    Explanation: Violet has the highest refractive index in glass.

18. Q18.The lens formula is:

    • A.1/v + 1/u = 1/f
    • B.v - u = f
    • C.v u = f
    • D.1/v - 1/u = 1/f✓ Correct Answer

    Explanation: It uses the Cartesian sign convention.

19. Q19.A diamond sparkles mainly because of:

    • A.its high melting point
    • B.its hardness
    • C.its black colour
    • D.total internal reflection inside it✓ Correct Answer

    Explanation: Its high refractive index gives a small critical angle.

20. Q20.A lens of focal length 50 cm has a power of:

    • A.+0.5 dioptre
    • B.+5 dioptre
    • C.+2 dioptre✓ Correct Answer
    • D.+50 dioptre

    Explanation: P = 1/f = 1/0.5 m.