Physics – Ray Optics and Optical Instruments
Class 12 Physics (Chapter 9) • 100 Multiple Choice Questions
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Question 1
The relation between focal length (f) and radius of curvature (R) of a spherical mirror is:
Correct Answer: f = R / 2
Question 2
Mirror formula relating object distance (u), image distance (v), and focal length (f) is:
Correct Answer: 1/f = 1/v + 1/u
Question 3
Refractive index of a medium with respect to vacuum is defined as (where c is speed in vacuum and v in medium):
Correct Answer: n = c / v
Question 4
Snell's Law of refraction states that:
Correct Answer: sin i / sin r = constant (n₂ / n₁)
Question 5
Total Internal Reflection (TIR) occurs when light travels from:
Correct Answer: Denser to rarer medium at an angle greater than critical angle
Question 6
The relation between critical angle (i_c) and refractive index (n) is:
Correct Answer: sin i_c = 1 / n
Question 7
Lens Maker's Formula is given by:
Correct Answer: 1/f = (n - 1) * (1/R₁ - 1/R₂)
Question 8
SI unit of optical power of a lens is:
Correct Answer: Diopter (D)
Question 9
Power of a lens (P) in terms of its focal length (f in meters) is:
Correct Answer: P = 1 / f
Question 10
When two thin lenses of focal lengths f₁ and f₂ are kept in contact, the equivalent focal length (F) is given by:
Correct Answer: Both (A) and (C)
Question 11
The angle of deviation (δ) through a prism of angle A and refractive index n for small angle is:
Correct Answer: δ = (n - 1) A
Question 12
At minimum deviation position in a prism:
Correct Answer: Angle of incidence equals angle of emergence (i = e)
Question 13
The phenomenon of splitting of white light into its constituent colors is called:
Correct Answer: Dispersion
Question 14
Which color deviates the most when white light passes through a prism?
Correct Answer: Violet
Question 15
Which color deviates the least when white light passes through a prism?
Correct Answer: Red
Question 16
The blue color of the sky is due to:
Correct Answer: Rayleigh scattering of light
Question 17
According to Rayleigh scattering, the intensity of scattered light (I) is inversely proportional to:
Correct Answer: Fourth power of wavelength (λ⁴)
Question 18
Magnifying power (M) of a simple microscope (magnifying glass) for normal adjustment is:
Correct Answer: M = 1 + D / f
Question 19
Magnifying power of an astronomical telescope in normal adjustment is:
Correct Answer: M = -f_o / f_e
Question 20
Length of an astronomical telescope in normal adjustment is:
Correct Answer: L = f_o + f_e
Question 21
Refractive index of water with respect to air is 4/3 and glass is 3/2. What is refractive index of glass with respect to water?
Correct Answer: 9 / 8
Question 22
What is the focal length of a plane mirror?
Correct Answer: Infinity
Question 23
What is the power of a plane mirror?
Correct Answer: Zero
Question 24
An air bubble inside water behaves like a:
Correct Answer: Diverging (concave) lens
Question 25
When a convex lens of refractive index n₁ is immersed in a liquid of refractive index n₂ (where n₂ > n₁), its focal length:
Correct Answer: Increases and nature of lens changes (behaves as concave lens)
Question 26
The critical angle for light passing from glass to air is approximately:
Correct Answer: 42°
Question 27
Optical fibers work on the principle of:
Correct Answer: Total Internal Reflection (TIR)
Question 28
Mirage (illusion of water on hot roads) is caused by:
Correct Answer: Total internal reflection of light due to varying refractive index of air layers
Question 29
Brilliant sparkle of a diamond is primarily due to:
Correct Answer: Multiple total internal reflections inside cleverly cut facets
Question 30
The least distance of distinct vision (D) for a normal human eye is:
Correct Answer: 25 cm
Question 31
Myopia (nearsightedness) can be corrected by using a:
Correct Answer: Concave lens
Question 32
Hypermetropia (farsightedness) can be corrected by using a:
Correct Answer: Convex lens
Question 33
Astigmatism is corrected by using a:
Correct Answer: Cylindrical lens
Question 34
Presbyopia is usually corrected by using:
Correct Answer: Bifocal lenses
Question 35
Magnifying power of a compound microscope is given by:
Correct Answer: M = -(v_o / u_o) · (1 + D / f_e)
Question 36
Resolving power of a microscope is proportional to:
Correct Answer: 1 / λ (or numerical aperture / wavelength)
Question 37
Resolving power of a telescope depends on:
Correct Answer: Diameter of objective lens (D / 1.22λ)
Question 38
Why are reflecting telescopes preferred over refracting telescopes for astronomical observations?
Correct Answer: They are free from chromatic and spherical aberrations and have higher light gathering power
Question 39
Apparent depth of a tank filled with water of refractive index n to a depth d is given by:
Correct Answer: d_apparent = d / n
Question 40
What is the apparent shift in the position of an object placed at depth d in a medium of refractive index n?
Correct Answer: Shift = d (1 - 1/n)
Question 41
When light reflects from a denser medium boundary, what is the phase change introduced?
Correct Answer: Ï€ radians (or 180°)
Question 42
Why does the sun appear reddish at sunrise and sunset?
Correct Answer: Scattering of shorter wavelengths (blue/violet) leaves mostly red light reaching the observer
Question 43
What is the magnification produced by a convex mirror always?
Correct Answer: Positive and less than 1 (virtual, erect, and diminished)
Question 44
Why are convex mirrors used as rear-view mirrors in vehicles?
Correct Answer: They give an erect, diminished image and have a wide field of view
Question 45
Why are concave mirrors used in dentist's head mirrors or shaving mirrors?
Correct Answer: They can produce an enlarged, erect image when object is close to the focus
Question 46
What is the focal length of a convex lens in air if its refractive index is 1.5 and radii of curvature of both surfaces are 20 cm?
Correct Answer: 20 cm
Question 47
When a ray of light enters from a rarer medium to a denser medium, which of the following properties changes?
Correct Answer: Speed and wavelength (frequency remains constant)
Question 48
What is the magnifying power of a lens of focal length 5 cm used as a simple microscope for normal adjustment (D = 25 cm)?
Correct Answer: 6
Question 49
The ability of an optical instrument to distinguish between two closely spaced objects is called:
Correct Answer: Resolving power
Question 50
What is the cause of chromatic aberration in lenses?
Correct Answer: Variation of refractive index with wavelength (dispersion)
Question 51
What is spherical aberration in lenses?
Correct Answer: Marginal rays and paraxial rays focus at different points
Question 52
When an object is placed at the focus of a convex lens, where is the image formed?
Correct Answer: At infinity
Question 53
For which position of an object does a convex lens form a virtual and magnified image?
Correct Answer: Between optical center and focus (u < f)
Question 54
What is the linear magnification (m) of a lens in terms of image distance v and object distance u?
Correct Answer: m = v / u
Question 55
What is the linear magnification (m) of a spherical mirror in terms of v and u?
Correct Answer: m = -v / u
Question 56
The refractive index of diamond is approximately:
Correct Answer: 2.42
Question 57
What is the refractive index of pure water?
Correct Answer: 1.33 (or 4/3)
Question 58
When a ray of light passes undeviated through the interface of two media, the angle of incidence is:
Correct Answer: 0° (Normal incidence)
Question 59
What is the focal length of a concave lens of power -2.0 Diopter?
Correct Answer: -50 cm
Question 60
What type of image is formed by a concave mirror when an object is placed beyond its center of curvature?
Correct Answer: Real, inverted, and diminished
Question 61
The formula relating refraction at a spherical surface is: (n₂/v - n₁/u = (n₂ - n₁)/R). This holds true for:
Correct Answer: Both convex and concave spherical refracting surfaces
Question 62
If the refractive index of a medium is 2, what is the speed of light in that medium?
Correct Answer: 1.5 × 10⁸ m/s
Question 63
Why does a swimming pool appear shallower than its actual depth?
Correct Answer: Refraction of light rays passing from water to air
Question 64
What is the angle of deviation when a ray of light suffers normal incidence on a glass slab?
Correct Answer: 0°
Question 65
A concave mirror gives a virtual, erect, and magnified image of an object when it is placed:
Correct Answer: Between pole and focus (u < f)
Question 66
Which of the following optical instruments uses a prism or a system of lenses to erect the inverted image?
Correct Answer: Terrestrial telescope (or binocular)
Question 67
What is the magnifying power of an astronomical telescope when final image is formed at the least distance of distinct vision (D)?
Correct Answer: M = -(f_o / f_e) * (1 + f_e / D)
Question 68
What is the main advantage of Cassegrain reflecting telescope over Newtonian reflecting telescope?
Correct Answer: It has a folded shorter tube length for a long focal length
Question 69
The refractive index of a prism depends on:
Correct Answer: Wavelength of light (and material of prism)
Question 70
Why do stars twinkle at night?
Correct Answer: Atmospheric refraction of starlight through varying layers of air
Question 71
What is the angle of deviation (δ) for a ray reflecting off a plane mirror at angle of incidence i?
Correct Answer: δ = 180° - 2i
Question 72
If a person stands in front of a plane mirror at distance d, what is the distance between the person and their image?
Correct Answer: 2d
Question 73
What minimum size of a plane mirror is required for a person of height H to see their full image?
Correct Answer: H / 2
Question 74
What is the focal length of a combination of two lenses in contact with powers P₁ and P₂?
Correct Answer: 1 / (P₁ + P₂)
Question 75
If a lens is cut vertically into two identical halves along its principal axis, what happens to the focal length of each half?
Correct Answer: Remains unchanged
Question 76
If a lens is cut horizontally perpendicular to its principal axis into two halves, what happens to the focal length of each half?
Correct Answer: Doubles
Question 77
What is the angle of deviation (δ) for a ray passing through a prism at minimum deviation D_m with prism angle A?
Correct Answer: n = sin((A + D_m)/2) / sin(A/2)
Question 78
Why does the sun appear oval or flattened at sunset and sunrise?
Correct Answer: Due to atmospheric refraction of light rays from different parts of the solar disc
Question 79
What is the magnifying power of a telescope when final image is formed at infinity (normal adjustment)?
Correct Answer: M = -f_o / f_e
Question 80
Which mirror has a virtual focus?
Correct Answer: Convex mirror
Question 81
Which lens has a virtual focus?
Correct Answer: Concave lens
Question 82
What is the focal length of a glass prism?
Correct Answer: Infinity
Question 83
What is the position of an object to get a real image of the same size as the object using a convex lens?
Correct Answer: At 2f (twice the focal length)
Question 84
What is the magnification of an image formed when object is at 2f of a convex lens?
Correct Answer: -1
Question 85
Which phenomenon is responsible for the formation of rainbows in the sky?
Correct Answer: Dispersion, refraction, and total internal reflection of sunlight by water droplets
Question 86
What is the angle subtended by a rainbow at the observer's eye for primary rainbow?
Correct Answer: 40° to 42°
Question 87
Why does a clean glass test tube immersed in water appear silvery-white when viewed from certain angles?
Correct Answer: Due to total internal reflection of light at the glass-air interface inside the tube
Question 88
What is the focal length of a combination of two lenses in contact having powers +2D and -3D?
Correct Answer: -1 meter (-100 cm)
Question 89
What is the focal length of a lens system when power of combination is zero?
Correct Answer: Infinity
Question 90
What kind of lens is thinner at the center and thicker at the edges?
Correct Answer: Concave lens
Question 91
What kind of lens is thicker at the center and thinner at the edges?
Correct Answer: Convex lens
Question 92
The magnification of a compound microscope can be increased by:
Correct Answer: Using objective and eyepiece of smaller focal lengths
Question 93
To get high magnifying power in an astronomical telescope, we need:
Correct Answer: Large focal length objective and small focal length eyepiece
Question 94
What is the angle of deviation (δ) for normal incidence on a prism?
Correct Answer: Depends on angle of emergence (δ = e - A)
Question 95
The relation between angle of prism (A), angle of deviation (δ), angle of incidence (i), and angle of emergence (e) is:
Correct Answer: A + δ = i + e
Question 96
What is the sum of the two refracting angles inside a prism (r₁ + r₂)?
Correct Answer: Equal to prism angle A (r₁ + r₂ = A)
Question 97
Which mirror forms an image of the same size as the object for all positions of the object?
Correct Answer: Plane mirror
Question 98
What happens to the focal length of a concave mirror when it is immersed in water?
Correct Answer: Remains unchanged (focal length of mirrors is independent of medium)
Question 99
What is the resolving limit of an eye or instrument defined as?
Correct Answer: Minimum angle subtended by two point objects that can be just distinguished
Question 100
Why does a diamond sparkle more brilliantly than a piece of ordinary glass cut to the same shape?
Correct Answer: Diamond has a much lower critical angle (high refractive index), allowing more total internal reflections
