Assignments Class 12 Physics Ray Optics and Optical Instruments

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Ray Optics and Optical Instruments Assignments Class 12 Physics

Question: An object approaches a convergent lens from the left of the lens with a uniform speed 5 m s^–1 and stops at the focus. The image
(a) moves away from the lens with an uniform speed 5 m s^–1.
(b) moves away from the lens with an uniform acceleration.
(c) moves away from the lens with a nonuniform acceleration.
(d) moves towards the lens with a non-uniform acceleration. 

Question: A ray of light is incident on a thick slab of glass of thickness t as shown in the figure. The emergent ray is parallel to the incident ray but displaced sideways by a distance d. If the angles are small then d is,

Question: A tank is filled with water to a height of 12.5 cm. The apparent depth of a needle lying at the bottom of the tank is measured by a microscope to be 9.4 cm. If water is replaced by a liquid of refractive index 1.63 upto the same height, by what distance would the microscope have to be moved to focus on the needle again?
(a) 1.00 cm
(b) 2.37 cm
(c) 1.73 cm
(d) 3.93 cm

Question: The far point of a near sighted person is 6.0 m from her eyes, and she wears contacts that enable her to see distant objects clearly. A tree is 18.0 m away and 2.0 m high. How high is the image formed by the contacts?
(a) 1.0 m
(b) 1.5 m
(c) 0.75 m
(d) 0.50 m 

Question: A mark placed on the surface of a sphere is viewed through glass from a position directly opposite. If the diameter of the sphere is 10 cm and refractive index of glass is 1.5. The position of the image will be
(a) –20 cm
(b) 30 cm
(c) 40 cm
(d) – 10 cm 

Question: A converging lens is used to form an image on a screen. When the upper half of the lens is covered by an opaque screen,
(a) half the image will disappear
(b) complete image will disappear
(c) intensity of image will decreases
(d) intensity of image will increases.

Question: The final image in an astronomical telescope with respect to an object is
(a) virtual and erect
(b) real and erect
(c) real and inverted
(d) virtual and inverted 

Question: An object is placed at a distance of 1.5 m from a screen and a convex lens is interposed between them. The magnification produced is 4. The focal length of the lens is
(a) 1 m
(b) 0.5 m
(c) 0.24 m
(d) 2 m 

Question:Two lenses of focal lengths 20 cm and –40 cm are held in contact. The image of an object at infinity will be formed by the combination at
(a) 10 cm
(b) 20 cm
(c) 40 cm
(d) infinity 

Question: The image of the needle placed 45 cm from a lens is formed on a screen placed 90 cm on the other side of the lens. The displacement of the image, if the needle is moved by 5.0 cm away from the lens is
(a) 10 cm, towards the lens
(b) 15 cm, away from the lens
(c) 15 cm, towards the lens
(d) 10 cm, away from the lens 

Question: A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will
(a) become zero
(b) become infinite
(c) become small, but non-zero
(d) remain unchanged 

Question: A real image of a distant object is formed by a planoconvex lens on its principal axis.
Spherical aberration is
(a) absent.
(b) smaller, if the curved surfaces of the lens face the object.
(c) smaller, if the plane surface of the lens faces the object.
(d) same, whichever side of the lens faces the object.

Question: A ray incident at a point at an angle of incidence of 60° enters a glass sphere of refractive index 3 and is reflected and refracted at the farther surface of the sphere. The angle between the reflected and refracted rays at this surface is
(a) 50°
(b) 60°
(c) 90°
(d) 40° 

Question: A concave mirror of focal length f1 is placed at a distance d from a convex lens of focal length f2. A beam of light coming from infinity and falling on this convex lens – concave mirror combination returns to infinity. The distance d must equal
(a) f₁ + f₂
(b) –f₁ + f₂
(c) 2f₁ + f₂
(d) –2f₁ + f₂ 

Question: For a glass prism (μ = √3) the angle of minimum deviation is equal to the angle of the prism. The angle of the prism is
(a) 45°
(b) 30°
(c) 90° 
(d) 60°

Question:  The optical density of turpentine is higher than that of water while its mass density is lower. Figure shows a layer of turpentine floating over water in a container. For which one of the four rays incident on turpentine in figure, the path shown is correct?

(a) 1
(b) 2
(c) 3
(d) 4   

Question:  A small angle prism (μ = 1.62) gives a deviation of 4.8°. The angle of prism is
(a) 5°
(b) 6.36°
(c) 3°
(d) 7.74° 

Question:  A vessel of depth x is half filled with oil of refractive index μ₁ and the other half is filled with water of refractive index μ₂. The apparent depth of the vessel when viewed from above is

Question:  There are certain material developed in laboratories which have a negative refractive index. A ray incident from air (medium 1) into such a medium (medium 2) shall follow a path given by

Question:  The power of a biconvex lens is 10 dioptre and the radius of curvature of each surface is 10 cm. Then the refractive index of the material of the lens is
(a)3/2
(b)4/3
(c)9/8
(d)5/3   

Question:  You are given four sources of light each one providing a light of a single colour – red, blue, green and yellow. Suppose the angle of refraction for a beam of yellow light corresponding to a particular angle of incidence at the interface of two media is 90°. Which of the following statements is correct, if the source of yellow light is replaced with that of other lights without changing the angle of incidence?
(a) The beam of red light would undergo total internal reflection.
(b) The beam of red light would bend towards normal while it gets refracted through the second medium.
(c) The beam of blue light would undergo total internal reflection.
(d) The beam of green light would bend away from the normal as it gets refracted through the second medium.   

Question:  An astronomical refractive telescope has an objective of focal length 20 m and an eyepiece of focal length 2 cm. Then
(a) the magnification is 1000.
(b) the length of the telescope tube is 20.02 m
(c) the image formed is inverted
(d) all of these.   

Question:  A compound microscope consists of an objective lens with focal length 1.0 cm and eye piece of focal length 2.0 cm and a tube length 20 cm the magnification will be
(a) 100
(b) 200
(c) 250
(d) 300 

Very Short Answer Type Questions 

Question:  When red light passing through a convex lens is replaced by light of blue colour, how will the focal length of the lens change?
Answer: Focal length of the lens decreases when red light is replaced by blue light.

Question:  When a monochromatic light travels from one medium to another its wavelength changes but frequency remains the same. Explain.
Answer: Frequency being a characteristic of source of light, does not change with change of medium.
Refractive index m of medium is defined as, μ =  c/v(speed of light in vacuum)/(speed of light in mediu)
As, v = uλ
∴ u∝1/λ(∴ v is same in differentmedia)
Hence, wavelength of light is different in different media. 

Question:  How does the angle of minimum deviation of a glass prism vary, if the incident violet light is replaced by red light?
Answer: 

Question:  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?
Answer: When the refractive index of the biconvex lens is equal to the refractive index of the liquid in which lens is immersed then the biconvex lens behaves as a plane glass sheet. In this case,
1/f= or f→∞

Question:  Two thin lenses of power –4 D and 2 D are placed in contact coaxially. Find the focal length of the combination.
Answer: Net power P = P₁ + P₂ +–4 + 2 = –2 D
Focal length f= 1 /p=1/-2 m=-0.5 m =-50 cm

Short Answer Type Questions

Question:  A concave lens of refractive index 1.5 is immersed in a medium of refractive index 1.65.
What is the nature of the lens?
Answer:

Question:  A ray of light falls on a transparent sphere with centre C as shown in the figure. The ray emerges from the sphere parallel to the line AB.
Find the angle of refraction at A if refractive index of the material of the sphere is √3 . 11 image
Answer:

Question:  Calculate the focal length of a biconvex lens if the radii of its surfaces are 60 cm and 15 cm, and index of refraction of the lens glass = 1.5.
Answer:

Question:  When a ray passes a glass slab of thickness t at an angle i with an angle of refraction r, what is the lateral shift of the emergent ray?
Answer:

Question:  Draw the image, when an object is kept beyond the focal point of a convex lens.
Answer: When the object is outside the focal point of a converging lens, the image is real and inverted.

Question:  A thin prism of angle A = 6°° produces a deviation d = 3°°. Find the refractive index of the material of prism.
Answer:

Question:  Draw the image, when an object is placed between the focus and pole of a convex lens.
Answer:  The object is between the pole and focal point of a converging lens. The image is virtual, enlarged and erect. 

Question:  A point object O is kept in air as shown. The radius of the curved part is 20 cm and medium on the other side is of refractive index 1.33. Find the image distance.
Answer:

Question:  A parallel beam of light is incident on a thin lens as shown. The radius of curvature of both surfaces is R. Determine the focal length of this system.
Answer:

Question:  A coin 2 cm in diameter is embedded in a solid glass ball of radius 30 cm. The index of refraction of the ball is 1.5, and the coin is 20 cm from the surface. Find the position and height of the image of the coin.
Answer: 

Question: A bird in air is diving vertically over a tank with speed 9 cm/s. Base of the tank is silvered.

Answer:

Question: What is the difference in the construction of an astronomical telescope and a compound microscope? The focal lengths of the objective and eyepiece of a compound microscope are 1.25 cm and 5.0 cm, respectively. Find the position of the object relative to the objective in order to obtain an angular magnification of 30 when the final image is formed at the near point.
Answer: 

Long Answer Type Questions 

Question:  (a) Plot a graph for angle of deviation as a function of angle of incidence for a triangular prism.
(b) Deduce the expression for the refractive index of glass in terms of angle of prism and angle of minimum deviation.
Answer: (a) If graph is plotted between angle of incidence i and angle of deviation d, it is found that the angle of deviation d first decreases with increase in angle of incidence i and then becomes minimum ‘dm’ when i = e and then increases with increase in angle of incidence i. Figure shows the path of a
ray of light suffering refraction through a prism of refracting angle ‘A’.

(b) At minimum deviation, the inside beam travels parallel to base of the prism.

Question:  (a) A point object is placed in front of a double convex lens (of refractive index n = n₂/n₁ with respect to air) with its spherical faces of radii of curvature R₁ and R₂. Show the path of rays due to refraction at first and subsequently at the second surface to obtain the formation of the real image of the object. Hence obtain the lens-maker’s formula for a thin lens.
(b) A double convex lens having both faces of the same radius of curvature has refractive index 1.55. Find out the radius of curvature of the lens required to get the focal length of 30 cm.
Answer:  (a) Refraction at convex spherical surface :
When object is in rarer medium and image formed is real.

Question:  (a) Define magnifying power of a telescope.
Write its expression.
(b) A small telescope has an objective lens of focal length 150 cm and an eye piece of focal length 5 cm. If this telescope is used to view a 100 m high tower 3 km away, find the height of the final image when it is formed 25 cm away from the eye piece.
Answer: (a) Magnifying power of refracting telescope (M) is defined as the ratio of the angle subtended by the image (b) at the eye to the angle subtended by the distant object at the unaided eye (∝).