12th Class Physics Ray Optics Question Bank

done Ray Optics Conceptual Problems

• question_answer1) An object is placed between two plane min-ors inclined at angle \[\theta \] with each other. What is the total number of images formed?

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• question_answer2) Will the reflected rays converge at a point when a parallel beam of light is incident on a concave mirror of large aperture?

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• question_answer3) Two concave mirrors have the same focal length but the aperture of one is larger than that of the other. Which mirror forms the sharper image and why?

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• question_answer4) A concave mirror is held in water. What should be the change in the focal length of the mirror?

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• question_answer5) How will you distinguish between a plane, a concave and a convex mirror without touching its surface?

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• question_answer6) A man stands in front of a mirror of special shape. He finds that his image has a very small head, a fat body and legs of normal size. What can we say about the shapes of the three parts of the mirror?

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• question_answer7) What is the advantage of using a parabolic concave mirror (as compared to a convex lens) as objective of a telescope?

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• question_answer8) Why is a concave mirror preferred to a plane mirror for shaving?

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• question_answer9) A parallel beam of light strikes a (i) plane mirror (ii) concave mirror and (iii) convex mirror. What is the deviation produced in each case in terms of the angle of incidence?

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• question_answer10) Does refraction of light mean just a change in the direction of light propagation?

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• question_answer11) Can the absolute value of refractive index of a medium be less than unity?

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• question_answer12) Can the relative refractive index of a medium w. r. t. another medium be less than unity?

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• question_answer13) What is the cause of refraction of light?

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• question_answer14) Why does a ray of light bend towards normal as it passes from air to glass?

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• question_answer15) For the same angle of incidence, the angles of refraction in three different media A, B and C are \[\mathbf{15}{}^\circ ,\text{ }\mathbf{25}{}^\circ \] and \[\mathbf{35}{}^\circ \] respectively. In which medium will the velocity of light be minimum?

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• question_answer16) Do the frequency and wavelength change when light passes from a rarer to a denser medium?

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• question_answer17) An ink mark on a sheet of paper is viewed through a glass slab of thickness t and refractive index p. What is the distance by which the mark appears to be raised?

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• question_answer18) On what factors, does the normal shift through a refracting medium depend?

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• question_answer19) Does critical angle depend on colour of light? Explain.

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• question_answer20) The critical angle for glass-air interface is \[{{i}_{c}}\]. Will the critical angle for glass-water interface be greater than or less than \[{{i}_{c}}\]?

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• question_answer21) An air bubble in a jar of water shines brightly. Why?

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• question_answer22) Why does a diamond sparkle?

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• question_answer23) What happens to the shining of diamond if it is dipped in a transparent oil?

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• question_answer24) During summer noon, why do the trees and houses on the other side of an open ground appear to be shaking?

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• question_answer25) If a plane glass slab is placed on letters of different colours, then red coloured letters appear more raised up. Why? The apparent shift caused by a slab of thickness \['t'\] is given by \[d=t\left( 1-\frac{1}{\mu } \right)\] As refractive index of glass is maximum for red light, so red coloured letters are more raised up.

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• question_answer26) A water tank is 4 m deep. A candle flame is held 6 m above the water level. For water, \[\mu =4/3\]. Where will the image of the candle flame be formed?

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• question_answer27) Does refraction in a water tank make apparent depth same throughout?

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• question_answer28) A fish in a water tank sees the outside world as if it is at the vertex of a cone, such that the circular base of the cone is at the surface of water. If the depth of the fish is d and the critical angle for water-air interface is\[{{i}_{c}}\], what is the radius of the circle?

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• question_answer29) When does a convex lens behave as a concave lens?

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• question_answer30) What type of a lens is an air bubble inside water? Give reason also.

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• question_answer31) Under what condition does a biconvex lens of glass having a certain refractive index act as a plane glass sheet when immersed in a liquid?

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• question_answer32) A lens immersed in a transparent liquid is not visible. Under what condition can this happen?

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• question_answer33) What is lens maker's formula? Why is it so called?

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• question_answer34) A lens whose radii of curvature are different is forming the image of an object placed on its axis. If the lens is placed with its faces reversed, will the position of the image change?

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• question_answer35) What is the focal length and power of a rectangular glass slab?

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• question_answer36) Sun glasses (goggles) have curved surfaces but they do not have any power. Why?

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• question_answer37) A convex lens forms the image of the sun at a distance of 20 cm. Where will the image be formed when another lens of the same aperture but double the power is used?

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• question_answer38) Why is the power of a lens measured as the reciprocal of its focal length?

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• question_answer39) What happens to a focal length of a convex lens, when it is immersed in water?

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• question_answer40) A lens of glass is immersed in water. What will be its effect on the power of the lens?

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• question_answer41) The focal length of an equiconvex lens e is equal to the radius of curvature of either face. What is the refractive index of the lens material?

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• question_answer42) The radii of curvature of both the surfaces of a lens are equal. If one of the surfaces is made plane by grounding, how will the focal length and power change?

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• question_answer43) Two thin lenses of powers\[~+\mathbf{2D}\] and \[+\text{ }\mathbf{5}\text{ }\mathbf{D}\] are placed in contact. Will the focal length of the combination increase, or decrease?

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• question_answer44) How does the focal length of a convex lens change if monochromatic red light is used instead of violet light?

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• question_answer45) In Fig. path of a parallel beam of light passing through a convex lens of refractive index j\[{{\mu }_{g}}\]kept in a medium of refractive index\[{{\mu }_{m}}\] is shown. \[Is\] (i) \[{{\mu }_{g}}={{\mu }_{w}}\] or (ii) \[{{\mu }_{g}}>{{\mu }_{w}}\]or (iii) \[{{\mu }_{g}}<{{\mu }_{w}}\]?

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• question_answer46) A ray of light after refraction through a concave lens emerges parallel to the principal axis. Under what condition can it happen?

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• question_answer47) The layered lens shown in Fig. is made of two different transparent materials. A point object is placed on its axis. How many images does it form?

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• question_answer48) The line AB in the ray diagram of Fig. represents a lens. State whether the lens represented by AB is convex or concave?

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• question_answer49) The line AB in the ray diagram of Fig. represents a lens. State whether the lens represented by AB is convex or concave?

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• question_answer50) The image of an object formed by a lens on the screen is not in sharp focus. Suggest a method to get clear focusing of the image on the screen without disturbing the position of the object, the lens or the screen.

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• question_answer51) Explain why one prefers even in summer a black umbrella to a white one?

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• question_answer52) How is the refractive index of a medium related to the wavelength of incident light?

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• question_answer53) The wavelength of red light is almost twice that of violet light. Is the refractive index of a material also greater for red colour than that for violet colour?

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• question_answer54) Does the angle of deviation of a small-angled prism depend on angle of incidence?

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• question_answer55) Out of blue and red light which is deviated more by a prism? Give reason. Or Rays of red and blue light are incident on a given prism. Explain, which will have the larger value of \[{{\delta }_{m}},\] the angle of minimum deviation?

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• question_answer56) Does dispersive power of a prism material depend on the shape, size and angle of the prism?

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• question_answer57) A ray of light is normally incident on one face of an equilateral prism. Trace the course of the ray through the prism and emerging from it.

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• question_answer58) How does the intensity \[I\] of scattered light vary with the wavelength \[\lambda \] of the incident light?

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• question_answer59) Why does the sky appear blue? What will it look like on the moon?

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• question_answer60) What will be the colour of the sky in the ' absence of atmosphere?

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• question_answer61) Why do clouds appear white?

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• question_answer62) Eye is more sensitive to yellow colour. Why do then we use traffic light stop signals of red colour?

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• question_answer63) Why is yellow sodium light used for illumination in foggy conditions?

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• question_answer64) Why is the smoke of cigarette blue?

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• question_answer65) What is the cause of blue colour of ocean?

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• question_answer66) Explain giving reason why the sun looks reddish at sunrise and sunset.

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• question_answer67) Why do sometimes we observe haloes (rings) round the sun or the moon?

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• question_answer68) Why do we observe a spectrum in the minimum deviation position?

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• question_answer69) What is the difference between a spectroscope, spectrograph and spectrometer?

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• question_answer70) How is rainbow formed in the sky?

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• question_answer71) Why is the sequence of colours in the secondary rainbow reverse of that in the primary rainbow?

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• question_answer72) Why is the secondary rainbow fainter than the primary rainbow?

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• question_answer73) What are the two possible values of the magnifying powers of a simple microscope made of convex lens of focal length \[\text{f = 5}\text{.0 cm}\]

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• question_answer74) Why do we prefer a magnifying glass of smaller focal length?

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• question_answer75) In which direction relative to the normal, does a ray of light bend, when it enters obliquely a medium in which its speed is increased?

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• question_answer76) The size of the moon is much smaller than stars, still it appears bigger. Why?

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• question_answer77) The sun appears to be a small disc, even though its diameter is \[\mathbf{1}{{\mathbf{0}}^{\mathbf{9}}}\] m, why?

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• question_answer78) The diameter of the sun is several hundred times larger than the moon, still at the time of the solar eclipse, the entire sun is covered by the moon. How?

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• question_answer79) An object is first seen in red light and then in violet light through a simple microscope. In which case is the magnifying power of simple microscope greater?

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• question_answer80) Why is the focal length of an objective in compound microscope little shorter than the focal length of the eyepiece?

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• question_answer81) Why should the objective of a telescope have a larger focal length?

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• question_answer82) Why is the power of the objective of a telescope kept as small as possible?

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• question_answer83) Why is the aperture of objective lens or mirror of a telescope taken as large as possible?

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• question_answer84) How much intensity of the image is increased if the diameter of the objective of a telescope is doubled?

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• question_answer85) Two lenses of focal lengths 5 cm and 50 cm are to be used for making a telescope. Which lens will you use for the objective?

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• question_answer86) Can a microscope function as a telescope by inverting it? Can a telescope function as a microscope?

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• question_answer87) Can a terrestrial telescope be used for observing astronomical objects?

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• question_answer88) Can we increase the range of a telescope by increasing the diameter of its objective?

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• question_answer89) How does the magnifying power of a telescope change on increasing the diameter of its objective?

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• question_answer90) Two telescopes have same magnifying power, but the diameters of their apertures are different. What will be the difference in the final images formed by them?

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• question_answer91) A fly is sitting on the objective of a telescope. What will be its effect on the final image of the distant object?

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• question_answer92) What is the difference in the construction of the objective of a telescope and that of a microscope?

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• question_answer93) How will you distinguish between a compound microscope and a telescope just by seeing them?

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• question_answer94) Why should the objective of a micro- scope be of small aperture?

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• question_answer95) A telescope has been adjusted for the relaxed eye. You are asked to adjust it for the least distance of distinct vision, then how will you change the distance between the two lenses?

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• question_answer96) Bees can see objects in the ultra-violet light while human beings cannot do so. Why?

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• question_answer97) A chicken wakes up early in the morning and goes to sleep by sunset. Why?

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• question_answer98) Why do some people use bi-focal lenses?

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• question_answer99) A boy uses spectacles of focal length\[-\mathbf{50}\text{ }\mathbf{cm}\]. Name the defect of vision he is suffering from. Compute the power of the lens.

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• question_answer100) An astronomical telescope uses lenses of power 10 D and 1 D. What is its magnifying power in normal adjustment?

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• question_answer101) What are real and virtual images? Distinguish between them.

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• question_answer102) (a) An object is placed between two plane mirrors inclined at \[\mathbf{60}{}^\circ \] to each other. How many images do you expect to see? (b) An object is placed between two plane parallel mirrors. Why do the distant images get fainter and fainter? (c) Why are mirrors used in search-lights parabolic and not concave spherical? (d) If you were driving a car, what type of mirror would you prefer to use for observing traffic at your back?

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• question_answer103) Answer the following questions : (a) A man holding a lighted candle in front of a thick glass mirror and viewing it obliquely sees a number of images of the candle. What is the origin of these multiple images? (b) You read a newspaper because of the light that it reflects. Then why do you not see even a faint image of yourself in the newspaper? (c) The wall of a room is covered with a perfect plane mirror, and two movie films are made, one recording the movement of a man and the other of his mirror image From viewing the films later, can an outsider tell which is the mirror image film?

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• question_answer104) A boy \[\mathbf{1}.\mathbf{50}\text{ }\mathbf{m}\] tall with his eye level at \[\mathbf{1}.\mathbf{38}\text{ }\mathbf{m}\] stands before a mirror fixed on a wall. Indicate by means of a ray diagram how the mirror should be positioned so that he can view himself fully. What should be the minimum length of the mirror? Does the answer depend on the eye level?

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• question_answer105) Suppose that the left half of a concave minor's reflecting surface is covered with non - reflective soot. What effect will this have on the image of an object placed in front of the mirror?

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• question_answer106) A mobile phone lies along the principal axis of a concave mirror, as shown in Fig. Show by suitable diagram the formation of its image. Explain why the magnification is not uniform, and distortion will occur depending on the location of the mobile with respect to the mirror.

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• question_answer107) A person standing before a concave mirror cannot see his inverted image unless he stands beyond the centre of curvature. Why?

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• question_answer108) The distances of an object and its real image, measured from the focus of a concave mirror, are a and b respectively. Show that \[{{f}^{2}}=ab\].

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• question_answer109) A beam of light converges at a point on the screen. A plane parallel glass plate is introduced in the path of this converging beam. How will the point of convergence be affected? Draw the relevant ray diagram.

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• question_answer110) A straight rod appears bent in water. Why?

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• question_answer111) A microscope is focussed on a dot at the bottom of a beaker. Some oil is poured into the beaker to a height of \[y\] cm and it is found necessary to raise the microscope through a vertical distance of \[x\] cm to bring the dot again into focus. Express refractive index of oil in terms of \[x\] and \[y\].

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• question_answer112) What is total internal reflection? Under what conditions does it take place? Or (i) State the principle on which the working of an optical fibre is based. (ii) What are the necessary conditions for this phenomenon to occur?

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• question_answer113) A light ray travels from medium 1 of refractive index \[{{\mu }_{1}}\] to medium 2 of refractive index \[{{\mu }_{2}}\], where \[{{\mu }_{2}}<{{\mu }_{1}}\] Write an expression for critical angle of incidence.

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• question_answer114) A ray of light while travelling from a denser to a rarer medium undergoes total reflection. Derive the expression for the critical angle in terms of the speed of light in the respective media.

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• question_answer115) An empty test tube dipped into water in a beaker appears silvery, when viewed from a suitable direction. Why?

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• question_answer116) A right-angle prism is placed before an object in the two positions shown in Fig. The prism is made of crown glass with critical angle equal to\[\mathbf{41}{}^\circ \]. Trace the paths of two rays from P and Q normal to the hypotenuse in Fig. (a), and parallel to the hypotenuse in Fig. (b). (a) (b)

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• question_answer117) Explain the twinkling of stars. Why do the planets not show twinkling effect?

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• question_answer118) Only the stars near the horizon twinkle while those overhead do not twinkle. Why?

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• question_answer119) (a) A concave mirror and a convex lens are held in water. What change, if any, do you expect to find in the focal length of either? (b) On a hot summer day in a desert, one sees the reflected image of distant parts of the sky. (This is sometimes mistaken by the observer to be the reflection of the sky in some distant lake of water. This illusion is called a mirage). Explain. (c) What is the twinkling effect of starlight due to? (d) Watching the sunset on a beach, one can see the sun for several minutes after it has 'actually set?. Explain.

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• question_answer120) Define focus and principal focus of a lens.

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• question_answer121) Distinguish between a convex lens and a concave lens.

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• question_answer122) A converging and a diverging lens of equal focal lengths are placed coaxially in contact. Find the power and focal length of the combination.

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• question_answer123) What changes in the focal length of a (i) concave mirror and (ii) convex lens occur, when the incident violet light on them is replaced with red light?

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• question_answer124) If the wavelength of incident light on a (i) concave mirror, and (ii) convex lens is increased, how will the focal length of each of these change?

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• question_answer125) A thin converging lens has a focal length\['f'\] in air. If it is completely immersed in a liquid, briefly explain how the focal length of the lens will vary?

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• question_answer126) The image of a candle is formed by a convex lens on a screen. The lower half of the lens is painted black to make it completely opaque. Draw the ray diagram to show the image formation. How will this image be different from the one obtained when the lens is not painted black?

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• question_answer127) A convex lens made of material of refractive index \['{{n}_{2}}'\] is held in a reference medium of refractive index \['{{n}_{1}}'\]. Trace the path of a parallel beam of light passing through the lens when (i) \[{{n}_{1}}={{n}_{2}},\] (ii) \[{{n}_{1}}<{{n}_{2}}\] and (iii) \[{{n}_{1}}>{{n}_{2}}\]

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• question_answer128) A concave lens made of material of refractive index \['{{n}_{2}}'\] is held in a reference medium of refractive index \['{{n}_{1}}'\]. Trace the path of parallel beam of light passing through the lens when: (i) \[{{n}_{1}}={{n}_{2}},\] (ii) \[{{n}_{1}}<{{n}_{2}}\]and (iii\[{{n}_{1}}>{{n}_{2}}\]

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• question_answer129) A magician during a show makes a glass lens \[\mu =1.5\]disappear in a trough of liquid. What is the refractive index of the liquid? Is the liquid water?

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• question_answer130) Derive the lens formula giving the relation between \[u,\upsilon \]and \[f\] for a thin convex lens Define the term 'linear magnification' and draw a graph showing the variation of linear magnification with image distance for a thin convex lens. How can this graph be used for finding the focal length of the lens?

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• question_answer131) (a) People usually prefer light-coloured dresses during summer and dark dresses during winter. Why? (b) How would a blue object appear under sodium lamp light? (c) What does a welder protect against when he wears a mask? (d) Explain why the sky is blue, and the sun appears red at sunset.

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• question_answer132) Give reasons for the following observations on the surface of moon: (i) sunrise and sunset are abrupt (ii) sky appears dark (iii) A rainbow is never formed.

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• question_answer133) Refractive indices of glass for violet yellow and red colours are \[{{\mu }_{V}},{{\mu }_{\Upsilon }}\]and \[{{\mu }_{R}}\]respective! Write these in decreasing order of value. Justify your answer.

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• question_answer134) A plot, between the angle of deviation \[(\delta )\] and angle of incidence (i), for a triangular prism is shown in Fig. Explain why any given value of \['\delta '\] corresponds to two values of angle of incidence. State the significance of point 'P' on the graph. Use this information to derive an expression for refractive index of the material of the prism.

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• question_answer135) A glass prism causes dispersion while a glass plate does not. Why?

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• question_answer136) A beam of white light on passing through a hollow prism gives no spectrum. Why?

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• question_answer137) Why does a parallel faced glass slab neither produce dispersion nor deviation?

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• question_answer138) Dispersion is caused by refraction not by reflection. Why?

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• question_answer139) What are the advantages of binocular vision?

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• question_answer140) What are the purposes of using optical instruments?

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• question_answer141) Distinguish between linear magnification and angular magnification. When are these two magnifications equal?

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• question_answer142) How will the magnifying power of a refracting type astronomical telescope be affected on increasing for its eyepiece (i) the focal length and (ii) the aperture? Justify your answer.

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• question_answer143) Two convex lenses A and B of an astronomical telescope having focal lengths 5 cm and 20 cm respectively, are arranged as shown in Fig. (i) Which one of the two lenses you will select to use as the objective lens and why? (ii) What should be the change in the distance between the lenses to have the telescope in its normal adjustment position? (iii) Calculate the magnifying power of the telescope in the normal adjustment position.

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• question_answer144) Using the data given below, state which two of the given lenses will you prefer to construct a best possible (i) telescope (ii) microscope. Also indicate which of the selected lenses is to be used as an objective and as an eyepiece in each case.      

  Lenses	Power (P)	Aperture (A)
  \[{{L}_{1}}\]	\[6D\]	\[1cm\]
  \[{{L}_{2}}\]	\[3D\]	\[8cm\]
  \[{{L}_{3}}\]	\[10D\]	\[1cm\]

            
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• question_answer145) Four double convex lenses, with the following specifications are available:  

  Lens	Focal length	Aperture
  A	100 cm	10 cm
  B	100 cm	5 cm
  C	10 cm	2 cm
  D	5 cm	2 cm

    Which two of the given four lenses, should be selected as the objective and eyepiece to construct an astronomical telescope and why? What will be the magnifying power and normal length of the telescope tube so constructed? Write the advantages of reflecting type telescope over such a telescope. Or Which two of the above four lenses should be selected as objective and eyepiece of a compound microscope and why? How can the magnifying power of such a microscope be increased? Draw a labelled ray diagram for the image formation in such a microscope.                 
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• question_answer146) In a telescope, the objective has a large aperture while the eyepiece has a small aperture. Why?

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• question_answer147) Which two main considerations are kept in mind while designing the 'objective? of an astronomical telescope? Obtain an expression for the angular magnifying power and the length of the tube of an astronomical telescope in its 'normal adjustment' position.

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• question_answer148) How is the working of a telescope different from a microscope?

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• question_answer149) Distinguish between a microscope and a telescope.

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