# Worksheets Chapter 9 Ray Optics and Optical Instruments Class 12 Physics

Students should refer to Worksheets Class 12 Physics Ray Optics and Optical Instruments Chapter 9 provided below with important questions and answers. These important questions with solutions for Chapter 9 Ray Optics and Optical Instruments have been prepared by expert teachers for Class 12 Physics based on the expected pattern of questions in the Class 12 exams. We have provided Worksheets for Class 12 Physics for all chapters on our website. You should carefully learn all the important examinations questions provided below as they will help you to get better marks in your class tests and exams.

## Ray Optics and Optical Instruments Worksheets Class 12 Physics

Question: A screen is placed 90 cm away from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20 cm. Find the focal length of lens.
(a) 42.8 cm
(b) 21.4 cm
(c) 10.7 cm
(d) 5.5 cm

B

Question: A convergent beam of light passes through a diverging lens of focal length 0.2 m and comes to focus 0.3 m behind the lens. The position of the point at which the beam would converge in the absence of the lens is
(a) 0.12 m
(b) 0.6 m
(c) 0.3 m
(d) 0.15 m

A

Question: From a point source, a light falls on a spherical glass surface (m = 1.5 and radius of curvature = 10 cm). The distance between point source and glass surface is 50 cm. The position of image is
(a) 25 cm
(b) 50 cm
(c) 100 cm
(d) 150 cm

B

Question:The focal length of the lenses of an astronomical telescope are 50 cm and 5 cm. The length of the telescope when the image is formed at the least distance of distinct vision is
(a) 45 cm
(b) 55 cm
(c)275/6 cm
(d) 325/6 cm

D

Question: From a point source, a light falls on a spherical glass surface (m = 1.5 and radius of curvature = 10 cm). The distance between point source and glass surface is 50 cm. The position of image is
(a) 25 cm
(b) 50 cm
(c) 100 cm
(d) 150 cm

B

Question: Mirage is a phenomenon due to
(a) refraction of light
(b) total internal reflection of light
(c) diffraction of light
(d) none of these.

B

Question: A biconvex lens has a focal length 2/3 times the radius of curvature of either surface. The refractive index of the lens material is
(a) 1.75
(b) 1.33
(c) 1.5
(d) 1.0

A

Question: Two lenses of power +10 D and –5 D are placed in contact. Where should an object be held from the lens, so as to obtain a virtual image of magnification 2?
(a) 5 cm
(b) –5 m
(c) 10 cm
(d) –10 cm

D

Question: A convex lens of focal length 0.2 m and made of glass (amg = 1.5) is immersed in water (amw = 1.33). Find the change in the focal length of the lens.
(a) 5.8 m
(b) 0.58 cm
(c) 0.58 m
(d) 5.8 cm

C

Question: A small telescope has an objective lens of focal length 144 cm and an eyepiece of focal length 6.0 cm. What is the separation between the objective and the eyepiece?
(a) 0.75 m
(b) 1.38 m
(c) 1.0 m
(d) 1.5 m

D

Question: Two identical glass [μg = 3/2] equiconvex lenses of focal length f are kept in contact. The space between the two lenses is filled with water [μw = 4/3] . The focal length of the combination is
(a) f
(b) f/2
(c) 4f/3
(d) 3f/4

D

Question:  A point luminous object (O) is at a distance h from front face of a glass slab of width d and of refractive index m. On the back face of the slab is a reflecting plane mirror. An observer sees the image of object in mirror [figure]. Distance of image from front face as seen by observer will be

A

Question:  Light travels in two media A and B with speeds 1.8 × 108 m s–1 and 2.4 × 108 m s–1 respectively. Then the critical angle between them is
(a) sin−1[2/3]
(b) tan− 1[3/4]
(c) tan−1[2/3]
(d) sin−1 [3/4]

D

Question:  A ray of light  passes through an equilateral prism (refractive index 1.5) such that angle of incidence is equal to angle of emergence and the latter is equal to 3/4th of the angle of prism. The angle of deviation is
(a) 60°
(b) 30°
(c) 45°
(d) 120°

B

Question:  A point source of light is placed at a depth of h below the surface of water of refractive index m. A floating opaque disc is placed on the surface of water so that light from the source is not visible from the surface. The minimum diameter of the disc is
(a) 2h/ (μ2 −1 ) 1/2
(b) 2h(μ2 – 1)1/2
(c) h/2(μ2 − 1)1/2
(d) h(μ2 – 1)1/2

A

Question:  A concave lens is placed in contact with a convex lens of focal length 25 cm. The combination produces a real image at a distance of 80 cm, If an object is at a distance of 40 cm, the focal length of concave lens is
(a) – 400 cm
(b) – 200 cm
(c) + 400 cm
(d) + 200 cm

A

Question:  Critical angle for light going from medium (i) to (ii) is θ The speed of light in medium (i) is v, then the speed of light in medium (ii) is
(a) v(1 – cos θ)
(b) v/sin θ
(c) v/cos θ
(d)v/(1-sinθ)

B

Question:  Double convex lenses are to be manufactured from a glass of refractive index 1.55, with both faces of same radius of curvature. What is the radius of curvature required if the focal length is to be 20 cm?
(a) 11 cm
(b) 22 cm
(c) 7 cm
(d) 6 cm

B

Assertion & Reasoning Based MCQs
two statements are given-one labelled Assertion (A) and the other labelled Reason (R).
Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is NOT the correct explanation of A
(c) A is true but R is false
(d) A is false and R is also false

Question:  Assertion (A) : Convergent lens property of converging remains same in all media.
Reason (R) : Property of lens whether the ray is diverging or converging is independent of the surrounding medium.

D

Question:  Assertion (A) : Higher is the refractive index of a medium or denser the medium, lesser is the velocity of light in that medium.
Reason (R) : Refractive index is inversely proportional to velocity.

A

Question:  Assertion (A) : If optical density of a substance is more than that of water, then the mass density of substance can be less than water.
Reason (R) : Optical density and mass density are not related.

A

Question:  Assertion (A) : Endoscopy involves use of optical fibres to study internal organs.
Reason (R) : Optical fibres are based on phenomena of total internal reflection.

A

Question:  Assertion (A) : A double convex lens (μ = 1.5) has focal length 10 cm. When the lens is immersed in water (m = 4/3) its focal length becomes 40 cm.
Reason (R) :

A

Question:  Assertion (A) : Microscope magnifies the image.
Reason (R) : Angular magnification for image is more than object in microscope.

A

Question:  A compound microscope is used because a realistic simple microscope does not have magnification.
Answer: A compound microscope is used because a realistic simple microscope does not have large magnification.

Question:  What is the angle of incidence for maximum deviation through a prism?
Answer: The deviation is maximum when angle is 90°.

Question:  A biconvex lens made of a transparent material of refractive index 1.25 is immersed in water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason.
Answer: The lens will act as a diverging lens as the refractive index of water is greater than that of lens.

Question:  For the same value of angle of incidence, the angles of refraction in three media A, B and C are 15°, 25° and 35° respectively. In which media would the velocity of light be minimum?

Question:  An optical instrument uses an objective lens of power 100 D and an eyepiece of power 40 D. The final image is formed at infinity when the tube length of the instrument is kept at 20 cm.
Identify the optical instrument.

Question:  Define power of a lens. Write its units.
Deduce the relation 1/f =1/f1+1/f2 for two thin lenses kept in contact coaxially.
Answer: Power of lens : It is the reciprocal of focal length of a lens.
P=1/f(f is in metre) Unit of power of lens : Dioptre

Question:  Which two of the following lenses L1, L2, and L3 will you select as objective and eyepiece for constructing best possible (i) telescope (ii) microscope? Give reason to support your answer.

Answer:  An astronomical telescope should have an objective of larger aperture and longer focal length while an eyepiece of small aperture and small focal length. Therefore, we will use L2 as an objective and L3 as an eyepiece.
For constructing microscope, L3 should be used as objective and L1 as eyepiece because both the lenses of microscope should have short focal lengths and the focal length of objective should be smaller than the eyepiece.

Question:  (a) State the necessary conditions for producing total internal reflection of light.
(b) Draw ray diagrams to show how specially designed prisms make use of total internal reflection to obtain inverted image of the object by deviating rays (i) through 90° and (ii) through 180°.
Answer: (a) Essential conditions for total internal reflection:
(i) Light should travel from a denser medium to a rarer medium.
(ii) Angle of incidence in denser medium should be greater than the critical angle for the pair of media in contact.
(b) (i) To deviate a ray of light through 90° :

A totally reflecting prism is used to deviate the path of the ray of light through 90°, when it is inconvenient to view the direct light. In Michelson’s method to find velocity of light, the direct light from the octagonal mirror is avoided from direct viewing by making use of totally reflecting prism.
(ii) To deviate a ray of light through 180° : When the ray of light comes to meet the hypotenuse face BC at right angles to it, it is refracted out of prism as such along the path RS.
The path of the ray of light has been turned through 180° due to two total internal reflections.

Question:  Find the equivalent focal length of the combination of lens.

Question:  Three rays of light red (R), green (G) and blue (B) fall normally on one of the sides of an isosceles right angled prism as shown. The refractive index of prism for these rays is 1.39, 1.44 and 1.47 respectively. Find which of these rays get internally reflected and which get only refracted from AC. Trace the paths of rays.

Question: A small bulb (assumed to be a point source) is placed at the bottom of a tank containing water to a depth of 80 cm. Find out the area of the surface of water through which light from the bulb can emerge. Take the value of the refractive index of water to be 4/3.
Answer: The light rays starting from  bulb can pass through the surface if angle of incidence at surface is less than or equal to critical angle (C)
for water air interface. If h is the depth of bulb from the surface, the light will emerge only through a

Question: (a) Obtain relation between the critical angle of incidence and the refractive index of the medium.
(b) Explain briefly how the phenomenon of total internal reflection is used in fibre optics.
Answer: (a) Relation between refractive index and critical angle When i = C, r = 90°

(b) Optical fibre is made up of very fine quality glass or quartz of refractive index about 1.7.
A light beam incident on one end of an optical fibre at appropriate angle refracts into the fibre and undergoes repeated total internal reflection.
This is because the angle of incidence is greater than critical angle. The beam of light is received at other end of fibre with nearly no loss in intensity. To send a complete image, the image of different portion is send through separate fibres and thus a complete image can be transmitted through an optical fibre.

Question: (a) A small telescope has an objective lens of focal length 140 cm and an eyepiece of focal length 5.0 cm. Find the magnifying power of the telescope for viewing distant objects when
(i) the telescope is in normal adjustment,
(ii) the final image is formed at the least distance of distinct vision.
(b) Also find the separation between the objective lens and the eyepiece in normal adjustment.

Question: An isosceles prism has one of the refracting surface silvered. A ray of light is incident normally on the refracting face AB. After two reflections the ray emerges from the base of the prism perpendicular to it. Find the angle of the prism.
Answer: The incident ray passes without deviation from face AB. It suffers reflections at P and Q. From figure, incident ray and normal at Q are parallel; therefore

Question:  A sphere of radius R is made of material of refractive index m2. Where would an object be placed so that a real image is formed at equidistant from the sphere?

Answer:  Let the object be placed at a distance x from the pole P1 of the sphere. If a real image is to be formed at equidistant from the sphere, then the ray must pass symmetrically through the sphere, as shown in the figure. Applying the equation at the first surface, we get

Note that the real image is formed only when the refractive index of the sphere is more than that of the surrounding
Question:  Draw a ray diagram to show the image formation by a combination of two thin convex lenses in contact. Obtain the expression for the power of this combination.

Consider two lenses placed close to each other. The focal lengths of lens A and B is f1 and f2 respectively.
For lens A,

Case Based MCQs
Refraction Through Lens
A convex or converging lens is thicker at the centre than at the edges. It converges a parallel  beam of light on refraction through it. It has a real focus. Convex lens is of three types :
(i) Double convex lens (ii) Plano-convex lens (iii) Concavo-convex lens. Concave lens is thinner at the centre than at the edges. It diverges a parallel beam of light on refraction through it.
It has a virtual focus.

Question:  Two thin lenses are in contact and the focal length of the combination is 80 cm. If the focal length of one lens is 20 cm, the focal length of the other would be.
(a) –26.7 cm
(b) 60 cm
(c) 80 cm
(d) 20 cm

A

Question:  A point object O is placed at a distance of 0.3 m from a convex lens (focal length 0.2 m) cut into two halves each of which is displaced by 0.0005 m as shown in figure. 41 image
What will be the location of the image?
(a) 30 cm right of lens
(b) 60 cm right of lens
(c) 70 cm left of lens
(d) 40 cm left of lens

B

Question:  Lens used in magnifying glass is
(a) Concave lens
(b) Convex lens
(c) Both (a) and (b)
(d) Neither concave lens nor convex lens

B

Question:  A spherical air bubble is embedded in a piece of glass. For a ray of light passing through the bubble, it behaves like a
(a) converging lens
(b) diverging lens
(c) plano-converging lens
(d) plano-diverging lens

B

Question:  The magnification of an image by a convex lens is positive only when the object is placed
(a) at its focus F
(b) between F and 2F
(c) at 2F
(d) between F and optical centre

D

Compound Microscope
A compound microscope is an optical instrument used for observing highly magnified images of tiny objects. Magnifying power of a compound microscope is defined as the ratio of the angle subtended at the eye by the final image to the angle subtended at the eye by the object, when both the final image and the object are situated at the least distance of distinct vision from the eye. It can be given that : m = me × mo, where me is magnification produced by eye lens and mo is magnification produced by objective lens.
Consider a compound microscope that consists of an objective lens of focal length 2.0 cm and an eyepiece of focal length 6.25 cm separated
by a distance of 15 cm.

Question:  How far from the objective should an object be placed in order to obtain the condition described in Q.No. 46?
(a) 4.5 cm
(b) 2.5 cm
(c) 1.5 cm
(d) 3.0 cm

B

Question:  The object distance for eye-piece, so that final image is formed at the least distance of distinct vision, will be
(a) 3.45 cm
(b) –5 cm
(c) –1.29 cm
(d) 2.59 cm

B

Question:  The magnifying power of a compound microscope increases when
(a) the focal length of objective lens is increased and that of eye lens is decreased.
(b) the focal length of eye lens is increased and that of objective lens is decreased.
(c) focal lengths of both objective and eye-piece are increased.
(d) focal lengths of both objective and eye-piece are decreased.