Ch-9: Ray Optics and Optical Instruments20 Questions
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1. The mirror formula relating object distance (u), image distance (v), and focal length (f) is:
A. 1/v + 1/u = 1/f
B. 1/v − 1/u = 1/f
C. v + u = f
D. vu = f
Answer: A. 1/v + 1/u = 1/f
Explanation: For spherical mirrors, using the standard sign convention, the relation between object distance, image distance, and focal length is 1/v + 1/u = 1/f.
2. Linear magnification produced by a spherical mirror is given by:
A. m = v/u
B. m = −v/u
C. m = u/v
D. m = −u/v
Answer: B. m = −v/u
Explanation: Magnification for a mirror is defined as the ratio of image height to object height, and using sign convention it works out to m = −v/u.
3. The Lens Maker's Formula relates focal length to:
A. Only the refractive index of the lens
B. The refractive index of the lens material and radii of curvature of its surfaces
C. Only the radii of curvature
D. The wavelength of incident light
Answer: B. The refractive index of the lens material and radii of curvature of its surfaces
Explanation: The Lens Maker's formula, 1/f = (n−1)(1/R1 − 1/R2), connects the focal length of a lens to the refractive index of its material and the radii of curvature of both surfaces.
4. The power of a lens of focal length f (in metres) is given by:
A. P = f
B. P = 1/f, measured in dioptre
C. P = f², measured in dioptre
D. P = 2f
Answer: B. P = 1/f, measured in dioptre
Explanation: Power of a lens is defined as the reciprocal of its focal length expressed in metres, P = 1/f, and is measured in dioptres (D).
5. When two thin lenses of powers P1 and P2 are placed in contact, the net power of the combination is:
A. P = P1 × P2
B. P = P1 + P2
C. P = P1 − P2
D. P = P1/P2
Answer: B. P = P1 + P2
Explanation: For thin lenses in contact, the equivalent power of the combination is simply the algebraic sum of individual powers, P = P1 + P2.
6. The refractive index of a medium is defined as the ratio of:
A. Speed of light in vacuum to speed of light in the medium
B. Speed of light in the medium to speed of light in vacuum
C. Wavelength in vacuum to wavelength in medium
D. Frequency in medium to frequency in vacuum
Answer: A. Speed of light in vacuum to speed of light in the medium
Explanation: Refractive index n = c/v, the ratio of the speed of light in vacuum (c) to its speed in the given medium (v), indicating how much the medium slows light down.
7. Snell's law of refraction is expressed as:
A. n1 sinθ1 = n2 sinθ2
B. n1 cosθ1 = n2 cosθ2
C. n1/sinθ1 = n2/sinθ2
D. n1 θ1 = n2 θ2
Answer: A. n1 sinθ1 = n2 sinθ2
Explanation: Snell's law states that the ratio of sines of the angle of incidence and refraction is constant for a given pair of media, expressed as n1 sinθ1 = n2 sinθ2.
8. Total internal reflection occurs when light travels:
A. From a rarer to a denser medium at any angle
B. From a denser to a rarer medium at an angle greater than the critical angle
C. From a denser to a rarer medium at any angle
D. Only in vacuum
Answer: B. From a denser to a rarer medium at an angle greater than the critical angle
Explanation: Total internal reflection occurs when light passes from a denser to a rarer medium and the angle of incidence exceeds the critical angle, causing all light to reflect back.
9. Optical fibres transmit light signals using the principle of:
A. Diffraction
B. Total internal reflection
C. Polarization
D. Interference
Answer: B. Total internal reflection
Explanation: Optical fibres guide light along their length through repeated total internal reflection at the core-cladding boundary, allowing signals to travel with minimal loss.
10. The splitting of white light into its constituent colours by a prism is called:
A. Reflection
B. Dispersion
C. Diffraction
D. Polarization
Answer: B. Dispersion
Explanation: Dispersion is the phenomenon where a prism separates white light into its component colours because each wavelength refracts by a slightly different amount.
11. The magnifying power of a simple/compound microscope is defined as the ratio of:
A. Focal lengths of the two lenses
B. The angle subtended by the final image at the eye to the angle subtended by the object at the eye when placed at the near point
C. Object distance to image distance
D. Aperture to focal length
Answer: B. The angle subtended by the final image at the eye to the angle subtended by the object at the eye when placed at the near point
Explanation: Magnifying power of an optical instrument is the ratio of the angle subtended by the image (viewed through the instrument) to the angle subtended by the object when viewed directly at the near point.
12. The magnifying power of an astronomical telescope in normal adjustment is given by:
A. M = fo/fe
B. M = fe/fo
C. M = fo × fe
D. M = fo + fe
Answer: A. M = fo/fe
Explanation: For an astronomical telescope in normal adjustment, the magnifying power equals the ratio of the focal length of the objective lens to that of the eyepiece, M = fo/fe.
13. The near point of a normal human eye is generally taken as:
A. 10 cm
B. 25 cm
C. 50 cm
D. Infinity
Answer: B. 25 cm
Explanation: For a normal human eye, the least distance of distinct vision (near point) at which objects can be seen clearly without strain is conventionally taken as 25 cm.
14. Myopia (short-sightedness) is corrected using:
A. A convex lens
B. A concave lens
C. A cylindrical lens
D. A plane mirror
Answer: B. A concave lens
Explanation: In myopia, the image forms in front of the retina; a concave (diverging) lens is used to diverge the incoming rays slightly so the image forms exactly on the retina.
15. Hypermetropia (long-sightedness) is corrected using:
A. A concave lens
B. A convex lens
C. A bifocal lens only
D. No correction is possible
Answer: B. A convex lens
Explanation: In hypermetropia, the image forms behind the retina; a convex (converging) lens is used to converge light more so that the image falls correctly on the retina.
16. A rainbow is formed in the sky due to a combination of:
A. Reflection only
B. Refraction, total internal reflection, and dispersion of sunlight in water droplets
C. Diffraction only
D. Polarization only
Answer: B. Refraction, total internal reflection, and dispersion of sunlight in water droplets
Explanation: A rainbow forms when sunlight undergoes refraction while entering water droplets, total internal reflection inside them, and further refraction and dispersion while exiting.
17. The blue colour of the clear sky is explained by:
A. Total internal reflection
B. Rayleigh scattering of sunlight by atmospheric molecules, which is stronger for shorter wavelengths
C. Dispersion by clouds
D. Polarization by the atmosphere
Answer: B. Rayleigh scattering of sunlight by atmospheric molecules, which is stronger for shorter wavelengths
Explanation: Rayleigh scattering intensity varies as 1/λ⁴, so shorter blue wavelengths scatter far more strongly than longer red wavelengths, giving the sky its blue appearance.
18. The reddish appearance of the Sun at sunrise and sunset is due to:
A. Absorption of red light by the atmosphere
B. Preferential scattering of shorter wavelengths, leaving mostly longer (red) wavelengths to reach the observer
C. Refraction by clouds
D. Total internal reflection in the atmosphere
Answer: B. Preferential scattering of shorter wavelengths, leaving mostly longer (red) wavelengths to reach the observer
Explanation: At sunrise/sunset, sunlight travels a much longer path through the atmosphere; most blue light is scattered away en route, so the light reaching the eye appears predominantly red.
19. The focal length of a plane mirror is:
A. Zero
B. Infinite
C. Equal to the radius of curvature
D. Negative and finite
Answer: B. Infinite
Explanation: A plane mirror can be regarded as a spherical mirror with an infinite radius of curvature, and hence its focal length is also infinite.
20. A compound microscope typically uses:
A. One concave lens
B. Two convex lenses — an objective and an eyepiece
C. One convex mirror
D. Two concave mirrors
Answer: B. Two convex lenses — an objective and an eyepiece
Explanation: A compound microscope consists of two convex lenses, a short focal length objective lens near the object and a longer focal length eyepiece, to achieve high magnification.
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