CBSE Class 12 Physics Ray Optics and Optical Instruments MCQs

Question : The light reflected by a plane mirror may form a real image

(a) if the rays incident on the mirror are diverging
(b) if the rays incident on the mirror are converging
(c) if the object is placed very close to the mirror
(d) under no circumstances

Answer: B

Question : In image formation from spherical mirrors, only paraxial rays are considered because they

(a) are easy to handle geometrically
(b) contain most of the intensity of the incident light
(c) form nearly a point image of a point source
(d) show minimum dispersion effect

Answer: C

Question : For reflection through spherical surfaces, the normal at the point of incidence is

(a) perpendicular to the principle axis and passes through the centre of curvature
(b) perpendicular to the focal plane and passes through the pole.
(c) perpendicular to the tangent plane at pole and passes through the focus.
(d) perpendicular to the tangent plane at the point of incidence and passes through the centre of curvature.

Answer: D

Question : A person is six feet tall. How tall must a plane mirror be if he is able to see his entire length?

(a) 3 ft
(b) 4.5 ft
(c) 7.5 ft
(d) 6 ft

Answer: A

Question : An object is placed 40 cm from a concave mirror of focal length 20 cm. The image formed is

(a) real, inverted and same in size
(b) real, inverted and smaller
(c) virtual, erect and larger
(d) virtual, erect and smaller

Answer: A

Question : Which of the following is incorrect statement?

(a) the magnification produced by a convex mirror is always less than one
(b) a virtual, erect, same-sized image can be obtained using a plane mirror
(c) a virtual, erect, magnifield image can be formed using a concave mirror
(d) a real, inverted, same-sized image can be formed using a convex mirror.

Answer: D

Question : Total internal reflection can take place only if

(a) light goes from optically rarer medium (smaller refractive index) to optically denser medium
(b) light goes from optically denser medium to rarer medium
(c) the refractive indices of the two media are close to different
(d) the refractive indices of the two media are widely different

Answer: B

Question : When the angle of incidence of a light ray is greater than the critical angle it gets

(a) critically refracted         
(b) totally reflected
(c) total internally reflected
(d) totally refracted

Answer: C

Question : Critical angle of light passing from glass to water is minimum for

(a) red colour     
(b) green colour
(c) yellow colour
(d) violet colour

Answer: D

Question : A glass slab of thickness 4 cm contains the same number of waves as 5 cm of water when both are traversed by the same monochromatic light. If the refractive index of water is 4/3, what is that of glass?

(a) 5/3
(b) 5/4
(c) 16/15
(d) 1.5

Answer: A

Question : A point object is placed at the centre of a glass sphere of radius 6 cm and refractive index 1.5. The distance of the virtual image from the surface of the sphere is

(a) 2 cm
(b) 4 cm
(c) 6 cm
(d) 12 cm

Answer: C

Question : A planoconvex lens of focal length 16 cm, is to be made of glass of refractive index 1.5. The radius of curvature of the curved surface should be

(a) 8 cm
(b) 12 cm
(c) 16 cm
(d) 24 cm

Answer: A 

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, then the power of the other lens will be

(a) 1.66 D
(b) 4.00 D
(c) – 100 D
(d) – 3.75 D

Answer: D

Question: A plano-convex lens is made of material of refractive index 1.6. The radius of curvature of the curved surface is 60 cm. The focal length of the lens is

(a) 50 cm
(b) 100 cm
(c) 200 cm
(d) 400 cm

Answer: B

Question: A lens made of glass whose index of refraction is 1.60 has a focal length of + 20 cm in air. Its focal length in water, whose refractive index is 1.33, will be

(a) three times longer than in air
(b) two times longer than in air
(c) same as in air
(d) None of these

Answer: A 

Question:  A convex lens is in contact with concave lens. The magnitude of the ratio of their powers is 2/3. Their equivalent focal length is 30 cm. What are their individual focal lengths (in cm)?

(a) –15, 10
(b) –10, 15
(c) 75, 50
(d) –75, 50

Answer=A

Question:   A bi-convex lens made of glass (refractive index 1.5) is put in a liquid of refractive index 1.7. Its focal length will

(a) decrease and change sign
(b) increase and change sign
(c) decrease and remain of the same sign
(d) increase and remain of the same sign

Answer=B

Question:  The rays of different colours fail to converge at a point after going through a converging lens. This defect is called

(a) spherical aberration (b) distortion
(c) coma                        (d) chromatic aberration

Answer=D

Question:  What causes chromatic aberration?

(a) Marginal rays
(b) Central rays
(c) Difference in radii of curvature of its surfaces
(d) Variation of focal length of lens with colour

Answer=D

Question:  The angular dispersion produced by a prism

(a) increases if the average refractive index increases
(b) increases if the average refractive index decreases
(c) remains constant whether the average refractive index increases or decreases
(d) has no relation with average refractive index.

Answer=A  

Question: If a glass prism is dipped in water, its dispersive power

(a) increases
(b) decreases
(c) does not change
(d) may increase or decrease depending on whether the

Answer: B

Question: The image formed by an objective of a compound microscope is
(a) real and diminished
(b) real and enlarged
(c) virtual and enlarged
(d) virtual and diminished

Answer: B