BPSC CCE Prelims
Light and Optics Previous Year Questions (PYQs)
Practice solved questions for Light and Optics with detailed step-by-step solutions, key insights, and trend analysis for BPSC CCE PRELIMS.
Solved Previous Year Questions
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The focal distance of a plane mirror is
Detailed Explanation:
Focal length of a mirror is f = R/2, where R is the radius of curvature.
A plane mirror has zero curvature (flat surface), so R = ∞. Therefore, f = ∞/2 = ∞. Parallel rays reflecting from a plane mirror remain parallel and never converge or diverge to form a focus point.
In the following question, out of four words given below, three are alike in some manner and fourth word is different. Find the different one.
Detailed Explanation:
Telescope, Microscope, and Periscope are optical instruments that use lenses and mirrors to view objects (distant, tiny, or around obstacles).
Stethoscope is an acoustic medical device used to listen to internal body sounds (heartbeat, lung sounds), not for viewing.
As the diameter of objective lens of a telescope increases, the resolution of telescope
Detailed Explanation:
Resolution of a telescope is its ability to distinguish between two closely spaced objects and is inversely proportional to the angular resolution θ.
According to Rayleigh's criterion, θ = 1.22λ/D, where λ is the wavelength of light and D is the diameter of the objective lens. As D increases, θ decreases, meaning the telescope can resolve finer details — hence resolution increases.
A larger aperture also collects more light, further improving image clarity and the ability to separate closely spaced objects.
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Which of the following is responsible for production of shadow?
Detailed Explanation:
Shadows are formed because light travels in straight lines, a property called rectilinear propagation.
When an opaque object blocks light's path, it cannot bend around the obstacle, creating a dark region (shadow) behind it. Polarisation relates to wave orientation, diffraction to bending at edges, and interference to wave overlap patterns—none directly cause shadows.
Large number of thin strips of black paint are made on the surface of a convex lens of focal length 20 cm to catch the image of a white horse. The image will be
Detailed Explanation:
Black paint strips block portions of the convex lens, reducing the amount of light passing through each section of the lens.
Every point on the horse emits light that passes through multiple parts of the lens, so blocking some strips reduces overall brightness but does not create stripes in the image—the image remains a complete horse with lower intensity.
What type of lens is used in magnifying glass?
Detailed Explanation:
A convex lens is thicker at the center and converges light rays. When an object is placed between the lens and its focal point, it produces a magnified, upright, virtual image—the principle behind magnifying glasses.
Concave lenses and plano-concave lenses diverge light and reduce image size, while convex mirrors also produce diminished images, making them unsuitable for magnification.
Therefore, a convex lens is used in magnifying glasses, making Option 2 the correct answer.
The speed of light will be minimum while passing through
Detailed Explanation:
The speed of light in a medium is inversely proportional to its refractive index.
- Refractive Index Values: Vacuum = 1.0, Air ≈ 1.0003, Water ≈ 1.33, Glass ≈ 1.5–1.65.
- Glass has the highest refractive index among the given options, resulting in the slowest speed of light.
- Therefore, light travels slowest through glass, making Option 3 the correct answer.
The 'theory of relativity' is presented by which scientist?
Detailed Explanation:
The Theory of Relativity was formulated by Albert Einstein in two parts: Special Relativity (1905) and General Relativity (1915).
- Special Relativity: Established that the speed of light is constant and the laws of physics are the same for all non-accelerating observers.
- General Relativity: Described gravity as the curvature of spacetime caused by mass and energy, replacing Newtonian gravity.
- Therefore, Albert Einstein is the scientist who presented the theory of relativity, making Option 4 the correct answer.
A person standing in front of a mirror finds his image larger than himself This implies that the mirror is
Detailed Explanation:
A concave mirror produces a magnified (larger) virtual image when the object is placed between the pole and focus—commonly seen in makeup/shaving mirrors.
A plane mirror always produces same-size images, a convex mirror always produces diminished images, and a cylindrical mirror with bulging side outwards distorts rather than uniformly magnifies.
Therefore, only a concave mirror produces a larger image when the person stands close enough, making Option 1 the correct answer.
Twinkling of stars in clear sky during nighttime can be explained with
Detailed Explanation:
Stellar twinkling (or scintillation) occurs because starlight passes through Earth's atmosphere with continuously varying refractive index due to turbulent air layers of different temperatures and densities. This causes the light path to bend unpredictably, making stars appear to twinkle.
Reflection involves bouncing back of light, polarization relates to light wave orientation, and interference involves superposition of waves—none explain atmospheric twinkling.
Therefore, refraction of light is the correct physical phenomenon, making Option 1 the correct answer.
When a soap film on the water is seen in the daytime, it shows beautiful colours. This phenomenon is due to
Detailed Explanation:
Soap film has two surfaces (top and bottom) that reflect light. When white light hits these surfaces, waves reflected from the top and bottom surfaces overlap and interfere, causing certain wavelengths to cancel out and others to amplify, producing the vibrant colours observed.
Diffraction occurs when light bends around edges, refraction is bending due to medium change, and polarization restricts light vibrations to one plane—none explain the colour pattern in soap films.
Therefore, interference of light is responsible for soap film colours, making Option 4 the correct answer.
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