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Optics — GK Questions with Answers

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36 questions
  1. In which medium does light travel fastest?
    1. water
    2. vacuum
    3. glass
    4. diamond
    Show answer

    Answer: (B) vacuum

    The speed of light is highest in vacuum (3 × 10⁸ m/s); the higher the refractive index, the lower the speed.

  2. Two plane mirrors are at 90°. How many images of an object between them will form?
    1. 2
    2. infinite
    3. 4
    4. 3
    Show answer

    Answer: (D) 3

    n = 360/90 − 1 = 3.

  3. Why is "AMBULANCE" written reversed on an ambulance?
    1. so the driver ahead reads it correctly in the rear-view mirror
    2. for beauty
    3. to shine at night
    4. for legal reasons
    Show answer

    Answer: (A) so the driver ahead reads it correctly in the rear-view mirror

    A mirror causes lateral inversion, so the reversed word looks correct in the mirror of the vehicle ahead.

  4. Which mirror is fitted in vehicles to see behind?
    1. concave
    2. plane
    3. convex
    4. cylindrical
    Show answer

    Answer: (C) convex

    A convex mirror always gives an erect image and a very wide field of view.

  5. Which mirror is used as a shaving mirror?
    1. convex
    2. plane
    3. concave
    4. parabolic convex
    Show answer

    Answer: (C) concave

    With the face between P and F of a concave mirror, an erect, enlarged image forms.

  6. What image does an object at the centre of curvature of a concave mirror form?
    1. real, inverted, same size
    2. virtual, enlarged
    3. real, diminished
    4. at infinity
    Show answer

    Answer: (A) real, inverted, same size

    With the object at C the image is also at C, the same size as the object, real and inverted.

  7. Where will the image form for an object 20 cm from a concave mirror of focal length 15 cm?
    1. 12 cm behind
    2. 60 cm behind
    3. 30 cm in front
    4. 60 cm in front
    Show answer

    Answer: (D) 60 cm in front

    1/v = −1/15 + 1/20 = −1/60 → v = −60 cm, that is, 60 cm in front of the mirror, real and three times enlarged.

  8. The power of a lens is −2 D. What lens is it and what is its focal length?
    1. convex, 50 cm
    2. concave, 50 cm
    3. convex, 2 m
    4. concave, 2 cm
    Show answer

    Answer: (B) concave, 50 cm

    Negative power belongs to a concave lens; f = 1/P = −0.5 m = −50 cm.

  9. Where is the object placed from the lens in a camera?
    1. beyond 2F
    2. at F
    3. inside F
    4. between F and 2F
    Show answer

    Answer: (A) beyond 2F

    With the object beyond 2F a small, inverted, real image forms on the sensor.

  10. An air bubble trapped in water acts like a —
    1. convex lens
    2. plane mirror
    3. concave lens
    4. prism
    Show answer

    Answer: (C) concave lens

    The bubble’s medium (air) is rarer than the surrounding water, so though convex in shape it acts as a diverging (concave) lens.

  1. Two thin lenses of +4 D and −2 D are kept in contact. Focal length of the combination?
    1. 25 cm
    2. 50 cm
    3. −50 cm
    4. 1 m
    Show answer

    Answer: (B) 50 cm

    P = 4 − 2 = +2 D, f = 1/2 m = 50 cm.

  2. Which quantity does not change in refraction of light?
    1. speed
    2. wavelength
    3. direction
    4. frequency
    Show answer

    Answer: (D) frequency

    The source decides the frequency; on changing the medium the speed and wavelength change.

  3. Why does a pond full of water look shallower than it is?
    1. reflection
    2. diffraction
    3. scattering
    4. refraction
    Show answer

    Answer: (D) refraction

    Rays from the bottom bend away from the normal while going from water to air, so the bottom looks raised.

  4. Stars twinkle but planets do not. Reason?
    1. stars are hot
    2. atmospheric refraction and stars being point sources
    3. planets reflect
    4. scattering
    Show answer

    Answer: (B) atmospheric refraction and stars being point sources

    Changing air layers keep bending the light of point-source stars; nearby planets are extended sources and the effect averages out.

  5. Which condition is necessary for total internal reflection?
    1. light goes from rarer to denser
    2. i = 0
    3. light goes from denser to rarer and i > C
    4. i < C
    Show answer

    Answer: (C) light goes from denser to rarer and i > C

    TIR happens only for light going from denser to rarer, at an angle of incidence larger than the critical angle.

  6. Why does diamond sparkle more than glass?
    1. diamond has a small critical angle, TIR happens again and again
    2. diamond is hard
    3. diamond absorbs light
    4. diamond emits light
    Show answer

    Answer: (A) diamond has a small critical angle, TIR happens again and again

    Diamond has n = 2.42 and a critical angle ≈ 24.4°, so light entering it undergoes TIR again and again.

  7. What causes a mirage in a desert?
    1. scattering
    2. refraction and total internal reflection
    3. diffraction
    4. polarisation
    Show answer

    Answer: (B) refraction and total internal reflection

    With hot rare air near the ground and cool dense air above, light bends more and more, undergoes TIR and goes up.

  8. Which colour bends least in the dispersion of white light?
    1. violet
    2. green
    3. blue
    4. red
    Show answer

    Answer: (D) red

    The refractive index of glass is lowest for red, so red bends the least.

  9. How many internal reflections take place inside a water drop in a primary rainbow?
    1. one
    2. zero
    3. two
    4. three
    Show answer

    Answer: (A) one

    One in the primary and two in the secondary rainbow; in the secondary the order of colours is reversed.

  10. Where must the Sun be to see a rainbow?
    1. in front
    2. overhead
    3. behind your back
    4. anywhere
    Show answer

    Answer: (C) behind your back

    A rainbow is seen at ≈ 42° from the direction opposite the Sun, so the Sun must be behind you.

  1. What colour forms on mixing red and green light?
    1. brown
    2. cyan
    3. yellow
    4. white
    Show answer

    Answer: (C) yellow

    Additive mixing of light: red + green = yellow; all three primaries give white.

  2. How will a green leaf look in red light?
    1. black
    2. red
    3. green
    4. yellow
    Show answer

    Answer: (A) black

    A green leaf absorbs red light, and there is no green light there for it to reflect.

  3. In which part of the eye does most refraction of light take place?
    1. eye lens
    2. pupil
    3. retina
    4. cornea
    Show answer

    Answer: (D) cornea

    About two-thirds of the total refracting power of the eye comes from the cornea; the lens does fine focusing.

  4. Which cells of the retina recognise colours?
    1. rods
    2. cones
    3. nerve cells
    4. yellow cells
    Show answer

    Answer: (B) cones

    Cone cells recognise colours in bright light; rods work in dim light.

  5. What is the least distance of distinct vision for a normal eye?
    1. 25 cm
    2. 10 cm
    3. 50 cm
    4. 1 m
    Show answer

    Answer: (A) 25 cm

    The near point of a normal eye is 25 cm and the far point is infinity.

  6. In near-sightedness (myopia) a person —
    1. sees near things blurred
    2. cannot see colours
    3. sees distant things blurred
    4. cannot see at night
    Show answer

    Answer: (C) sees distant things blurred

    In myopia the image forms in front of the retina, distant objects look blurred; it is corrected by a concave lens.

  7. Which part of the eye is affected in cataract?
    1. cornea
    2. eye lens
    3. retina
    4. optic nerve
    Show answer

    Answer: (B) eye lens

    In cataract the eye lens becomes cloudy; it is removed and an artificial lens is fitted.

  8. Astigmatism is corrected by which lens?
    1. convex
    2. concave
    3. bifocal
    4. cylindrical
    Show answer

    Answer: (D) cylindrical

    The defect is due to different curvature of the cornea in different directions; a cylindrical lens corrects it.

  9. The objective of an astronomical telescope has a focal length of 100 cm and the eyepiece 5 cm. Magnification?
    1. 5
    2. 500
    3. 105
    4. 20
    Show answer

    Answer: (D) 20

    M = f₀/fₑ = 100/5 = 20; tube length f₀ + fₑ = 105 cm.

  10. The sky looks blue because of —
    1. reflection of blue
    2. more scattering of blue by air molecules
    3. absorption of blue
    4. reflection of the sea
    Show answer

    Answer: (B) more scattering of blue by air molecules

    In Rayleigh scattering I ∝ 1/λ⁴, so short-wavelength blue light scatters the most.

  1. Why are danger signals red?
    1. red is the prettiest
    2. red travels fastest
    3. red is scattered the least
    4. red has the shortest wavelength
    Show answer

    Answer: (C) red is scattered the least

    Red has the longest wavelength, scatters the least and is seen far even in haze.

  2. The date of discovery of the Raman effect (28 February) is observed in India as —
    1. National Science Day
    2. Teachers’ Day
    3. National Technology Day
    4. Engineers’ Day
    Show answer

    Answer: (A) National Science Day

    C. V. Raman discovered the Raman effect on 28 February 1928; this day is National Science Day (National Technology Day is 11 May).

  3. Seeing the path of a sunbeam in a dusty room is due to —
    1. reflection
    2. Tyndall effect
    3. interference
    4. polarisation
    Show answer

    Answer: (B) Tyndall effect

    Colloidal particles scatter light and the path of the beam becomes visible — the Tyndall effect.

  4. In Young’s experiment, if the distance between the two slits is doubled, the fringe width —
    1. doubles
    2. stays the same
    3. becomes four times
    4. halves
    Show answer

    Answer: (D) halves

    β = λD/d — d doubled means β halved.

  5. Why do soap bubbles look coloured?
    1. interference in a thin film
    2. dispersion
    3. scattering
    4. polarisation
    Show answer

    Answer: (A) interference in a thin film

    Interference of rays reflected from the two surfaces of the thin film of the bubble shows different colours at different places.

  6. Which event proves that light is a transverse wave?
    1. interference
    2. diffraction
    3. polarisation
    4. reflection
    Show answer

    Answer: (C) polarisation

    Polarisation happens only in transverse waves; interference and diffraction also happen in longitudinal waves (sound).

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