Light Optics Competitive Exams
100 questions · General Science
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Question 1 of 100
Polarisation of light proves that light waves are
ExplanationOnly transverse waves can be polarised.
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Question 2 of 100
Which mirror is used by dentists to see enlarged images of teeth?
ExplanationWhen the object is within the focal length, a concave mirror forms an enlarged, erect, virtual image.
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Question 3 of 100
Which of the following is NOT a property of laser light?
ExplanationLasers are coherent, monochromatic and highly directional.
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Question 4 of 100
A mirage is formed in deserts due to
ExplanationHot air near the ground is rarer, so light from the sky bends upward and appears to come from a water-like surface.
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Question 5 of 100
The least distance of distinct vision (near point) for a normal eye is about
ExplanationFar point of a normal eye is at infinity.
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Question 6 of 100
The rear-view mirror of vehicles is a
ExplanationIt gives an erect, diminished image and a wide field of view.
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Question 7 of 100
The mirror formula is
ExplanationIt relates focal length with image distance (v) and object distance (u), using the sign convention.
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Question 8 of 100
Among visible colours, the photons with the highest energy belong to
ExplanationViolet has the highest frequency in the visible spectrum, so E = hf is largest.
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Question 9 of 100
The bending of light when it passes from one transparent medium to another is called
ExplanationIt happens because the speed of light changes in different media.
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Question 10 of 100
A lunar eclipse occurs when
ExplanationThe Earth's shadow falls on the Moon.
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Question 11 of 100
An object is placed at 2f from a convex lens. The image is
ExplanationHere u = v = 2f and m = −1.
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Question 12 of 100
The image formed by the lens of a camera is
ExplanationThe object lies beyond 2f of the camera's convex lens.
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Question 13 of 100
The lens maker's formula is
ExplanationIt relates focal length with refractive index and radii of curvature.
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Question 14 of 100
The three primary colours of light are
ExplanationMixing them in the right proportion gives white light.
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Question 15 of 100
In Young's double slit experiment, the fringe width is given by
ExplanationD is slit-to-screen distance and d is the slit separation.
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Question 16 of 100
Stars twinkle because of
ExplanationThe starlight path keeps changing slightly as it passes through turbulent air layers.
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Question 17 of 100
A pencil placed in a glass of water appears bent at the water surface due to
ExplanationLight bends as it goes from water to air.
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Question 18 of 100
An astronomical telescope in normal adjustment has an objective of focal length 100 cm and an eyepiece of focal length 5 cm. Its magnifying power and tube length are
ExplanationM = fₒ/fₑ = 20; length = fₒ + fₑ = 105 cm.
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Question 19 of 100
A convex lens is also called a
ExplanationIt is thicker at the centre and converges parallel rays to a point.
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Question 20 of 100
The SI unit of power of a lens is
ExplanationPower is the reciprocal of focal length in metres.
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Question 21 of 100
In the electromagnetic spectrum, the radiation with the highest frequency is
ExplanationThe order of increasing frequency is radio, microwave, infrared, visible, ultraviolet, X-rays and gamma rays.
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Question 22 of 100
The wavelength of 600 nm light in vacuum, when it enters glass of refractive index 1.5, becomes
ExplanationFrequency stays constant, so λ' = λ/n = 600/1.5 = 400 nm.
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Question 23 of 100
A concave lens of focal length 20 cm has an object placed 20 cm from it. The image distance is
Explanation1/v = 1/f + 1/u = −1/20 − 1/20 = −1/10, so v = −10 cm (virtual).
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Question 24 of 100
White light is made of
ExplanationA prism splits white light into violet, indigo, blue, green, yellow, orange and red.
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Question 25 of 100
The image formed by a plane mirror is
ExplanationIt is also erect and laterally inverted.
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Question 26 of 100
Interference of light shows that light has
ExplanationAlternate bright and dark fringes arise from superposition of waves.
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Question 27 of 100
The relation between focal length (f) and radius of curvature (R) of a spherical mirror is
ExplanationThe focus lies midway between the pole and the centre of curvature.
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Question 28 of 100
Danger signals are red because red light is
ExplanationRed has the longest visible wavelength.
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Question 29 of 100
According to the law of reflection, the angle of incidence is
ExplanationAlso, the incident ray, reflected ray and normal lie in the same plane.
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Question 30 of 100
The wavelength range of visible light is approximately
ExplanationViolet is near 400 nm and red near 700 nm.
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Question 31 of 100
Short-sightedness is medically called
ExplanationA short-sighted person can see near objects clearly but not distant ones.
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Question 32 of 100
A compound microscope consists of
ExplanationThe objective has a short focal length and forms a magnified real image, which the eyepiece magnifies further.
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Question 33 of 100
According to Huygens' principle, every point on a wavefront acts as
ExplanationThe new wavefront is the envelope of these secondary wavelets.
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Question 34 of 100
Which of the following gives better resolution in an optical microscope?
ExplanationResolving power ∝ 1/λ.
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Question 35 of 100
A concave mirror has a focal length of 15 cm. An object is placed 30 cm from it. The image is formed at
ExplanationThe object is at C (u = −30): 1/v = 1/f − 1/u gives v = −30 cm and m = −v/u = −1.
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Question 36 of 100
A concave lens is also called a
ExplanationIt is thinner at the centre and spreads parallel rays outward.
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Question 37 of 100
In dispersion by a glass prism, the colour deviated the most is
ExplanationViolet has the highest refractive index in glass.
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Question 38 of 100
In a plane mirror, the right hand of an object appears as the left hand in the image. This is called
ExplanationLeft and right are interchanged.
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Question 39 of 100
Two thin lenses of powers +3 D and −1 D are placed in contact. The combined power and focal length are
ExplanationP = P₁ + P₂ = 2 D; f = 1/P = 0.5 m = 50 cm.
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Question 40 of 100
The image formed on the retina is
ExplanationThe brain interprets it as erect.
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Question 41 of 100
Diffraction of light is the
ExplanationIt is significant when the obstacle size is comparable to the wavelength.
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Question 42 of 100
The photoelectric effect shows that light also has
ExplanationLight is emitted and absorbed in quanta called photons.
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Question 43 of 100
The part of the eye that controls the amount of light entering it is the
ExplanationThe iris widens or narrows the pupil.
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Question 44 of 100
A ray of light travels from air into a medium of refractive index √3 at an angle of incidence of 60°. The angle of refraction is
Explanationsin r = sin 60°/√3 = (√3/2)/√3 = 1/2, so r = 30°.
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Question 45 of 100
The Brewster angle for a transparent medium of refractive index √3 is
Explanationtan θ_B = n = √3, so θ_B = 60°; reflected light is then completely polarised.
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Question 46 of 100
A light-year is a unit of
ExplanationIt is the distance light travels in one year, about 9.46 × 10¹⁵ m.
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Question 47 of 100
A pinhole camera forms an image that is
ExplanationLight rays cross at the pinhole, so the image is upside down.
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Question 48 of 100
The formation of shadows shows that light travels in
ExplanationLight travels in straight lines (rectilinear propagation); an opaque object blocks it and forms a shadow.
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Question 49 of 100
The cells of the retina responsible for colour vision are the
ExplanationRods work in dim light; cones work in bright light and detect colour.
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Question 50 of 100
A rainbow is formed due to
ExplanationRaindrops refract, reflect and disperse sunlight into its colours.
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Question 51 of 100
Sunlight takes about ______ to reach the Earth.
ExplanationAverage Sun–Earth distance is 150 million km: 1.5 × 10¹¹ / 3 × 10⁸ ≈ 500 s.
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Question 52 of 100
The image in the human eye is formed on the
ExplanationThe retina contains light-sensitive cells.
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Question 53 of 100
A magnifying glass is a
ExplanationIt forms an enlarged, erect, virtual image.
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Question 54 of 100
Which colour of visible light has the longest wavelength?
ExplanationRed has the longest wavelength and violet the shortest.
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Question 55 of 100
The refractive index of a medium is defined as
Explanationn = c/v, so n ≥ 1 for all media.
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Question 56 of 100
The power of a concave lens is
ExplanationConcave lenses have negative focal lengths.
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Question 57 of 100
A red rose appears red in white light because it
ExplanationThe colour of an opaque object is the colour it reflects.
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Question 58 of 100
If the refractive index of glass is 1.5, the speed of light in glass is
Explanationv = c/n = 3 × 10⁸ / 1.5 = 2 × 10⁸ m/s.
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Question 59 of 100
Sustained interference requires sources that are
ExplanationCoherent sources have a constant phase difference.
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Question 60 of 100
Persistence of vision is about
ExplanationThis is how successive pictures in a film appear continuous.
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Question 61 of 100
Chromatic aberration in a simple lens is due to
ExplanationA lens refracts violet more than red, so the colours focus at different points.
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Question 62 of 100
A concave mirror of focal length 10 cm forms an image of an object placed 15 cm away. The image distance is
Explanation1/v = 1/f − 1/u = −1/10 + 1/15 = −1/30, so |v| = 30 cm (real, magnified ×2).
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Question 63 of 100
When an object is placed at the centre of curvature of a concave mirror, the image is
Explanationu = v = 2f, m = −1.
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Question 64 of 100
The prisms in binoculars and periscopes work using
ExplanationRight-angled prisms reflect light through 90° or 180° without loss.
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Question 65 of 100
The Sun appears red at sunrise and sunset because
ExplanationMostly red light reaches the eye.
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Question 66 of 100
The sky appears blue because
ExplanationRayleigh scattering is proportional to 1/λ⁴.
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Question 67 of 100
A solar eclipse occurs when
ExplanationThe Moon's shadow falls on part of the Earth.
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Question 68 of 100
A microscope is used to see
ExplanationIt gives a magnified image of tiny objects.
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Question 69 of 100
A convex mirror always forms an image that is
ExplanationThe image lies behind the mirror between the pole and focus.
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Question 70 of 100
An object between the pole and the focus of a concave mirror gives an image that is
ExplanationThis is why concave mirrors are used as shaving mirrors.
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Question 71 of 100
Total internal reflection can occur only when light travels from
ExplanationBoth conditions must be met.
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Question 72 of 100
The critical angle for a medium of refractive index 2 (relative to air) is
Explanationsin C = 1/n = 1/2, so C = 30°.
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Question 73 of 100
A telescope is used to see
ExplanationIt collects light from far-away objects and magnifies them.
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Question 74 of 100
Reflection from a smooth, polished surface such as a mirror is called
ExplanationParallel rays remain parallel after reflection from a smooth surface.
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Question 75 of 100
In Young's experiment, λ = 500 nm, D = 1 m and d = 1 mm. The fringe width is
Explanationβ = λD/d = (500 × 10⁻⁹ × 1)/(10⁻³) = 5 × 10⁻⁴ m = 0.5 mm.
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Question 76 of 100
The angle between the incident ray and the normal at the point of incidence is called the
ExplanationThe normal is the line perpendicular to the surface at the point where the ray strikes.
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Question 77 of 100
Hypermetropia (long-sightedness) is corrected using a
ExplanationThe convex lens converges rays before they enter the eye.
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Question 78 of 100
Light from the Moon is
ExplanationThe Moon is a non-luminous object that reflects sunlight.
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Question 79 of 100
Mixing red and green light produces
ExplanationRed + green = yellow, green + blue = cyan and red + blue = magenta.
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Question 80 of 100
The colours seen on soap bubbles and oil films are caused by
ExplanationLight reflected from the two surfaces of the thin film interferes.
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Question 81 of 100
The magnifying power of a simple microscope (image at the near point, D = 25 cm) of focal length 5 cm is
ExplanationM = 1 + D/f = 1 + 25/5 = 6.
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Question 82 of 100
A coin lies at the bottom of a pool 12 m deep (n = 4/3). To an observer above, it appears at a depth of
ExplanationApparent depth = real depth / n = 12 / (4/3) = 9 m.
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Question 83 of 100
Optical fibres work on the principle of
ExplanationLight is trapped inside the core by repeated total internal reflection.
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Question 84 of 100
The power of a lens of focal length 50 cm is
ExplanationP = 1/f(m) = 1/0.5 = 2 D; positive for a convex lens.
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Question 85 of 100
The speed of light in vacuum is approximately
Explanationc ≈ 300,000 km/s.
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Question 86 of 100
Light is a form of
ExplanationLight carries energy and travels as electromagnetic waves.
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Question 87 of 100
A transparent object
ExplanationGlass and clear water are transparent.
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Question 88 of 100
The lens formula is
ExplanationUsed with the Cartesian sign convention for lenses.
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Question 89 of 100
A red rose seen in pure green light appears
ExplanationIt absorbs green light and has no red light to reflect.
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Question 90 of 100
Which of the following is a luminous object?
ExplanationLuminous objects produce their own light; the Moon only reflects sunlight.
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Question 91 of 100
Which of the following is an opaque material?
ExplanationOpaque materials do not allow light to pass through.
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Question 92 of 100
A convex mirror of focal length 20 cm forms an image of an object placed 20 cm in front of it. The image is formed
Explanation1/v = 1/f − 1/u = 1/20 + 1/20 = 1/10, so v = +10 cm.
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Question 93 of 100
A convex lens of focal length 10 cm forms an image of an object placed 30 cm from it. The image distance is
Explanation1/v = 1/f + 1/u = 1/10 − 1/30 = 1/15, so v = 15 cm.
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Question 94 of 100
White light splits into colours in a prism because
ExplanationDifferent colours bend by different angles.
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Question 95 of 100
Presbyopia, common in old age, is corrected by
ExplanationIt is due to weakening of ciliary muscles and loss of lens flexibility.
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Question 96 of 100
The energy of a photon is given by
Explanationh is Planck's constant (6.63 × 10⁻³⁴ J·s).
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Question 97 of 100
Myopia (short-sightedness) is corrected using a
ExplanationThe concave lens diverges rays so the image falls on the retina.
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Question 98 of 100
The refractive index of a prism material of angle 60° whose angle of minimum deviation is 30° is
Explanationn = sin[(A + D)/2] / sin(A/2) = sin 45° / sin 30° = √2.
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Question 99 of 100
The transparent front part of the human eye is called the
ExplanationMost of the refraction of light in the eye occurs at the cornea.
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Question 100 of 100
A ray of light going from air into glass bends
ExplanationIt enters a denser (higher n) medium and slows down.
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