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Chapter 9: Ray Optics
Class XII – Physics Maximum Marks: 45 Time Allowed: 1 Hour 30 Minutes
General Instructions:
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This question paper consists of 4 Sections A, B, C and D.
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All questions are compulsory.
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Each Section have provided makes along the section.
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Use of calculator is not permitted.
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Draw neat and labeled diagrams wherever required.
Section A (1 Marks Each)
An object of height 1.5 cm is situated at distance of 15 cm from a concave mirror. The concave mirror forms its real image of height 3.0 cm. The focal length of concave mirror will
(1) –10 cm (2) –20 cm (3) 20 cm (4) 30 cm
The minimum distance between the object and its real image for concave mirror is
(1) f (2) 2f (3) 4f (4) Zero
The angle of glass is 4.5° and its refractive index is 1.52. The angle of minimum deviation will be
(1) 1.5° (2) 2.3° (3) 4.5° (4) 2°
A ray of light passes through equilateral Prism angle. The angle of deviation is
(1) 60° (2) 30° (3) 45° (4) 120°
The angle of minimum deviation measured with a prism is 30° and the angel of prism is 60°. The refractive index of prism material is
(1) (2) 2 (3) (4)
Two convex lens of focal length 20 cm and 25 cm are placed in contact with each other then power of this combination is :
(1) + 1D (2) +9D (3) –1D (4) –9D
Two lenses of power +2.50 D and –3.75D are combined to form a compound lens. Its focal length in cm will be
(1) 40 (2) –40 (3) –80 (4) 160
Lenses of powers 3D and –5D are combined to from a compound lens. An object is placed at a distance of 50 cm from this lens. Calculate the position of its image
(1) –10 cm (2) +10cm (3) –25 cm (4) +25 cm
A convex lens of Focal length of 40 cm is in contact with a concave lens of focal length 25 cm. The power of the combination is
(1) –1.5 D (2) –6.5 D (3) +6.5 D (4) +6.67 D
The magnifying power of a telescope is 9. When it is adjusted for parallel rays, the distance between the objective and eyepiece is 20 cm. The focal length of lenses are
(1) 18 cm, 2 cm (2) 11 cm, 9 cm (3) 10 cm, 10 cm (4) 15 cm,5 cm
Section B (2 Marks Each)
An equi-convex lens has refractive index 1.5. Write its focal length in terms of radius of curvature R.
A converging lens is kept co-axially in contact with a diverging lens – both the lenses being of equal focal lengths. What is the focal length of the combination?
Write the relationship between angle of incidence ‘i’, angle of prism ‘A’ and angle of minimum deviation for a triangular prism.
A beam of light converges at a point P. Now a convex lens is placed in the path of the convergent beam at 15 cm from P. At what point does a beam converge if the convex lens has a focal length 10 cm?
Calculate the speed of light in a medium whose critical angle is 45°. Does critical angle for a given pair of media depend on wave length of incident light? Give reason.
Section C (3 Marks Each)
Draw a labelled ray diagram of a reflecting telescope. Mention its two advantages over the refracting telescope.
Derive the mirror formula. What is the corresponding formula for a thin lens?
(i) Name the phenomenon on which the working of an optical fibre is based. (ii) What are the necessary conditions for this phenomenon to occur? (iii) Draw a labelled diagram of an optical fibre and show how light propagates through the optical fibre using this phenomenon.
(a) A giant refracting telescope has an objective lens of focal length 15 m. If an eye piece of focal length 1.0 cm is used, what is the angular magnification of the telescope? (b) If this telescope is used to view the moon, what is the diameter of the image the moon formed by the objective lens? The diameter of the moon is 3.48 × 106 m and the radius of lunar orbit is 3.8 × 108
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A compound microscope uses an objective lens of focal length 4 cm and eyepiece lens of focal length 10 cm. An object is placed at 6 cm from the objective lens. Calculate the magnifying power of the compound microscope. Also calculate the length of the microscope.
A convex lens of focal length 20 cm and a concave lens of focal length 15 cm are kept 30 cm apart with their principal axes coincident. When an object is placed 30 cm in front of the convex lens, calculate the position of the final image formed by the combination. Would this result change if the object were placed 30 cm in front of the concave lens? Give reason.
Section D (5 Marks Each)
(a) Draw a ray diagram for formation of image of a point object by a thin double convex lens having radii of curvature R1 and R2. Hence, derive lens maker’s formula for a double convex lens. State the assumptions made and sign convention used (b) Draw a ray diagram to show the image formation by a concave mirror when the object is kept between its focus and the pole. Using this diagram, derive the magnification formula for the image formed.
(a) Draw the labelled ray diagram for the formation of image by a compound microscope. Derive an expression for its total magnification (or magnifying power), when the final image is formed at the near point. Why both objective and eyepiece of a compound microscope must have short focal lengths? (b) Draw a ray diagram showing the image formation by a compound microscope. Hence obtain expression for total magnification when the image is formed at infinity.
