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CBSE Important Questions

CBSE Guess > Papers > Important Questions > Class XII > 2011 > Physics > Physics By Mr. Ajay Sharma

CBSE CLASS XII

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Ray Optics And Optical Instrument

(Q.1) Name the physical principle on which the working of optical fibres is based. ( 1 mark )

(Q.2) What is a dioptre? ( 1 mark )

(Q.3) How does the focal length of a convex lens change if monochromatic red light is used instead of monochromatic blue light? ( 1 mark )

(Q.4) Define refractive index. ( 1 mark )

(Q.5) When light undergoes refraction, what happens to its frequency? (1mark )

(Q.6) What are the factors on which the values of refractive index depend? (1 mark )

(Q.7) What is lens? ( 1 mark )

(Q.8) Define optical centre of a lens. ( 1 mark )

(Q.9) A convex lens and a concave lens, each having focal length 50 cm, are placed in contact. What is the focal length and power of the combination? ( 1 mark )

(Q.10) Define resolving power of a telescope. (H) ( 1 mark )

(Q.11) If a ray entered from air to water the refractive index will be, if refractive index for water is ½ and for air ¾ (H) ( 1 mark )

(a) 3/2
(b) 2/3
(c) 2
(d) 3/4

(Q.12) Which of the following is not a property of light? ( 1 mark )

(a) It can travel through vacuum
(b) It requires material medium for propagation
(c) It involves transportation of energy
(d) It has a finite speed

(Q.13) Which of the following is not the property of light according to the corpuscular theory? (H) ( 1 mark )

(a) Light travels in a straight line.
(b) The velocity of light is more in air than in water.
(c) The velocity of light changes on refraction.
(d) The velocity of light does not change on reflection.

( 1 mark )

(Q.14) In the above figure of Cartesian sign convention which of the following you find wrong ( 1 mark )

(a) A and B
(b) C and D
(c) all distances are measured from P
(d) the focal length of the mirror shown in the figure is negative.

(Q.15) In a compound microscope, the intermediate image is ( 1 mark )

(a) virtual, erect and magnified.
(b) real, erect and magnified.
(c) virtual,inverted and magnified.
(d) real, inverted and magnified.

(Q.16) A man runs towards the plane mirror at a speed of 4 m/s. The relative speed of his image with respect to him will be: ( 1 mark )

(a) 4m/s
(b) 8m/s
(c) 10m/s
(d) 2m/s

(Q.17) In winter season, one cannot see through in the fog. This is because: ( 1 mark )

(a) fog reflects the light
(b) fog refracts the light
(c) fog scatters the light
(d) fog polarises the light

(Q.18) A ray of light enters from a denser medium into a rarer medium. The speed of light in rarer medium is four times that in denser medium. What is the critical angle for total internal refraction to take place ( 1 mark )

(a) sin–1 (1/4)
(b) sin–1(1/2)
(c) tan–1(1/4)
(d) sin–1 (4 )

(Q.19) The critical angle for water w. r. t air is . What is the angular range in which the fish just below the surface of water can see the objects outside? ( 1 mark )

(a) ϴ / 2
(b) ϴ
(c) 2 ϴ
(d) ϴ/4

(Q.20) Visible spectrum is the band of ( 1 mark )

(a) seven colors
(b) only five colors
(c) only six colors
(d) only two colors

(Q.21) Inverse square law for illumination is valid for ( 1 mark )

(a) Isotropic point source
(b) search light
(c) cylindrical source
(d) all type of sources

(Q.22) A convex mirror forms an image which is 1/2n times the size of the object. If the focal length of the mirror is 2f , then the distance of the object is: (H) ( 1 mark )

(a) (2n – 1)f
(b) (2n – 1)2f
(c) (2n – 1)/2f
(d) 2f/(2n – 1)

(Q.23) We have a right-angled isosceles prism. Its refractive index is 1.5. If we make incidence a ray of light normally on one of the two perpendicular surfaces, which of the following phenomenon will take place? (1mark )

(a) Dispersion
(b) Total internal reflection
(c) Refraction
(d) Reflection

(Q.24) A ray of light passes through an equilateral prism such that the angle of incidence is equal to the angle of emergence. If the angle of incidence is 3/4th of the angle of prism, what is the angle of deviation? (1mark )

(a) 60o
(b) 30o
(c) 90o
(d) 45o

(Q.25) A convex lens of power P is immersed in water. How willits power change? ( 1 mark )

(a) Increases
(b) Remains unchanged
(c) Decreases
(d) Increases for red colour and decreases for blue colour.

(Q.26) A biconvex lens behaves as diverging lens in water and a converging lens in air. The refractive index of water is 1.33 and that of air is 1. What can we say about the refractive index n of the material of lens? (1mark )

(a) n < 0
(b) 0 < n < 1
(c) 1 < n <1.33
(d) n > 1.33

(Q.27) A convex lens produces a real image m times the size of the object. What is the distance of the object from the lens? ( 1 mark )

(Q.28) A parallel beam is incident on a convex lens of focal length f. It is then put in contact with a concave lens of focal length f/2. What will happen to the image? ( 1 mark )

(a) Real, v = f/2
(b) Real, v = f
(c) Virtual, v=f/2
(d) Virtual, v =f

(Q.29) Ray optics is valid, when characteristic dimensions are ( 1 mark )

(a) Much smaller than the wavelength of light
(b) Much larger than the wavelength of light
(c) of the same order as the wavelength of light
(d) of the order of one millimeter

(Q.30) A plane mirror produces a magnification of ( 1 mark )

(a) -1
(b) +1
(c) zero
(d) betwen 0 and + ∞

(Q.31) A person cannot see the objects distinctly, when it is placed at a distance less than 200cm. What is the power of the spectacles that he should use to see clearly the objects placed at 25cm? ( 1 mark )

(a) 28 cm
(b) 30 cm
(c) 28.5 cm
(d) 29.5 cm

(Q.32) Total internal reflection takes place when light is incident : ( 1 mark )

(a) on a concave mirror
(b) from air on a plane glass surface at a certain given angle
(c) from air on a plane surface at any angle
(d) from inside glass placed in water at a certain given angle

(Q.33) Which of the following optical components forms a perfect image free from all aberrations? ( 1 mark )

(a) A plane mirror
(b) A spherical mirror
(c) A spherical lens
(d) A cylindrical lens

(Q.34) A convex lens of glass is immersed in water, compared to its power in air, its power in water will ( 1 mark )

(a) diminish for red light and increase for blue light
(b) remain the same
(c) increase
(d) diminishes

(Q.35) When light travels from one medium to another, which are separated by a sharp boundary, the characteristic which does not change is: ( 1 mark )

(a) velocity
(b) wavelength
(c) frequency
(d) amplitude

(Q.36) An object is placed at the focus of a convex mirror. If the focal length of the mirror be 2f , then the distance of the image from the pole of the mirror is ( 1 mark )

(a) Less than 2f
(b) Equal to 2f
(c) More than f
(d) Equal to f

(Q.37) An object is placed at 20 cm in front of a concave mirror of radius of curvature 15 cm. What is the position of image? ( 1 mark )

(a) -20 cm
(b) 40 cm
(c) -60 cm
(d) 60 cm

(Q.38) An object is placed at 10 cm in front of a concave mirror of radius of curvature 15 cm. What is the magnification of image? ( 1 mark )

(a) -4
(b) -3
(c) 3
(d) 4

(Q.39) The minimummagnifying power of a telescope is M. If focal length of its eye lens is halved, the magnifying power will become (H) ( 1 mark )

(a) M / 2
(b) 2 M
(c) 3 M
(d) 4 M

(Q.40) Light from a point source in air falls on a spherical glass surface (n= 1.5 and radius of curvature = 10 cm). The distance of light source from the glass surface is 200 cm At what position the image is formed? (1mark)

(a) 100/5 cm
(b) 200 cm
(c) 100/3 cm
(d) 200/3 cm

(Q.41) What are optical fibres? Give their one use. ( 2 Marks )

(Q.42) What is the total focal length of a convex lens of focal length 30 cm in contact with a concave lens of focal length 20 cm? Is the system a converging or a diverging lens? Ignore thickness of the lenses. (2Marks)

(Q.43) A small telescope has an objective lens of focal length 144 cm and an eye-piece of focal length 6.0 cm. What is the magnifying power of the telescope? What is the separation between the objective and the eye-piece? ( 2 Marks )

(Q.44) Which rays are conveniently chosen to form image of an object by a lens? ( 2 Marks )

(Q.45) A convex lens made of a material of refractive index n1 is kept in a medium of refractive indexn2 A parallel beam of light is incident on the lens. Complete the path of rays of light emerging from the convex lens if (i) n1 > n2, (ii) n1 = n2, (iii) n1< n2. ( 3 Marks )

(Q.46) Define critical angle with reference to total internal reflection. Calculate the critical angle for glass-air surface, if a ray of light which is incident in air on the glass surface is deviated through 15o , when angle of incidence is (H) 45o

(Q.47) A double concave lens of glass of refractive index 1.6 has radii of curvature of 40 cm and 60 cm. Calculate its focal length in air. ( 3 Marks )

(Q.48) Where should an object be placed from a converging lens of focal length 20 cm, so as to obtain a real image of magnification 2? ( 3 Marks )

(Q.49) A double convex lens made of glass of refractive index 1.5 has both radii of curvature of magnitude 20 cm. An object 2 cm high is placed at 10 cm from the lens. Find the position, nature and size of the image. (5Marks )

(Q.50) Draw a ray diagram to show the formation of image of an object placed between the optical centre and focus of the convex lens. Write the characteristics of image formed. Using this diagram, derive the relation between object distance, image distance and focal length of the convex lens. Write the assumptions and convention of signs used. (H)

 

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Submitted By Mr. Ajay Sharma
Email Id : aks.672@rediffmail.com