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Ray Optics and Optical Instruments Class 12 MCQs Questions with Answers
Question 1.
A ray of light is incident on a plane mirror at an angle of incidence 30°. The ray after reflection is deviated through
(a) 30°
(b) 90°
(c) 60°
(d) 120°
Answer
Answer: (d) 120°
Question 2.
A spherical mirror forms a real image of a point object placed in front of it. The distance of the image and object from the mirror is 30 cm and 20 cm respectively. The focal length and nature of the mirror is:
(a) 120 cm; convex
(b) 60 cm convex
(c) 60 mm, concave
(d) 120 mm; concave.
Answer
Answer: (d) 120 mm; concave.
Question 3.
If the critical angle for total internal reflection from a medium to vacuum is 30°, the velocity of light in the medium is :
(a) 3 × 108 ms-1
(b) 6 × 108 ms-1
(c) 1.5 × 108 ms-1
(d) 15 × 108 ms-1
Answer
Answer: (c) 1.5 × 108 ms-1
Question 4.
A diver in a swimming pool wants to signal his distress to a person standing on the edge of the pool by flashing his water proof flash light:
(a) He has to direct the beam horizontally.
(b) He has to direct the beam at an angle to the vertical which is slightly less than the critical angle of incidence for total internal reflection.
(c) He has to direct the beam at angle to the vertical which is slightly more than the critical angle of incidence for total internal reflection.
(d) He must direct the beam vertically upward.
Answer
Answer: (c) He has to direct the beam at angle to the vertical which is slightly more than the critical angle of incidence for total internal reflection.
Question 5.
A fish is a little away below the surface of a lake. If the critical angle is 49°, the fish could see things above the water surface within an angular range of θ°, where
(a) θ = 49°
(b) θ – 98°
(c) θ = 90°
(b) 24.5°
Answer
Answer: (b) θ – 98°
Question 6.
A thin prism P, with angle 4° and made from glass of refractive index 1.54, is combined with another thin prism P2 made from glass of refractive index 1.72 to produce dispersion without deviation. The angle of prism P2 is:
(a) 5.33°
(b) 4°
(c) 2.6°
(d) 3°
Answer
Answer: (d) 3°
Question 7.
When light rays are incident on a prism at an angle of 45°, the minimum deviation is obtained. If the refractive index of the material of prism is \(\sqrt{29}\) the angle of prism will be
(a) 30°
(b) 40°
(c) 50°
(d) 60°
Answer
Answer: (d) 60°
Question 8.
If the refractive index for water is 4/3 and the velocity of light in vacuum is 3 x 1010 cm s-1, the time taken by light in travelling a distance of 500 m in water is
(a) 2.22 × 10-1 s
(b) 2.22 × 10-6 s
(c) 2.22 × 10-8 s
(d) 2.22 × 10-10 s.
Answer
Answer: (b) 2.22 × 10-6 s
Question 9.
The equivalent path in Q. 8. is in:
(a) 66.64 m
(b) 666.64 m
(c) 666.0 m
(d) 1.000 m
Answer
Answer: (b) 666.64 m
Question 10.
When a glass slab is placed on a cross made on a sheet, the cross appears raised by 1 cm. The thickness of the glass is 3 cm. The critical angle for glass is
(a) sin-1 (0.33)
(b) sin-1 (0.5)
(c) sin-1 (0.66)
(d) sin-1 (\(\frac {√3}{2}\))
Answer
Answer: (c) sin-1 (0.66)
Question 11.
A beam of monochromatic blue light of wavelength 4,200 A° in air travels in water (µ = \(\frac {4}{3}\)). Its wavelength in water will be
(a) 2800 Å
(b) 5600 Å
(c) 3150 Å
(d) 4000 Å
Answer
Answer: (c) 3150 Å
Question 12.
A double convex thin lens, made of glass (µ = 1.5) has both radii of curvature of magnitude 20 cm. Incident light rays parallel to principal axis of the lens will converge at a distance L such that:
(a) L = 20 cm
(b) L = 10 cm
(c) L = \(\frac {20}{3}\)
(d) L = 40 cm
Answer
Answer: (a) L = 20 cm
Question 13.
A lens has a power of 10 D when placed in air. When it is immersed in water, (µ = 1.333), the change in power is (R.I. of lens material is 1.5):
(a) 2.55 D
(b) + 7.45 D
(c) – 7.5 D
(d) 5.0 D
Answer
Answer: (c) – 7.5 D
Question 14.
A person can see objects only at a distance greater than 40 cm. He is avised to use lens of power:
(a) – 2.5 D
(b) + 2.5 D
(c) -6.25 D
(d) +1.5 D
Answer
Answer: (d) +1.5 D
Question 15.
A person cannot see clearly at distance less than 2m from the eye. The power of the lens needed by him to see clearly at a distance of 0.25 m will be
(a) +1 D
(b) -1 D
(c) +3.5 D
(d) -3.5 D
Answer
Answer: (c) +3.5 D
Question 16.
For a telescope, larger the diameter of the objective lens,
(a) smaller is the resolving power
(b) greater is the resolving power
(c) greater is the magnifying power
(d) smaller is the magnifying power.
Answer
Answer: (b) greater is the resolving power
Question 17.
If a glass rod is immersed in a liquid of the same refractive index, then it will:
(a) look bent
(b) look longer
(c) disapper
(d) none of these.
Answer
Answer: (c) disapper
Question 18.
Which of these statements is correct about rainbow?
(a) In primary rainbow, red colour is on the outside and violet colour is on the inside.
(b) In primary rainbow, violet colour is on the outside and red colour is on the inside.
(c) In secondary rainbow, light wave suffers one total internal reflection before coming out.
(d) secondary rainbow is brighter than primary rainbow,
Answer
Answer: (a) In primary rainbow, red colour is on the outside and violet colour is on the inside.
Question 19.
The objective of an astronomical telescope has a large aperture to:
(a) reduce spherical aberration.
(b) increase span of observation.
(c) have high resolution.
(d) have low dispersion.
Answer
Answer: (c) have high resolution.
Question 20.
Which of the following is used in optical fibres?
(a) total internal reflection
(b) scatterring
(c) reflection
(d) interference.
Answer
Answer: (a) total internal reflection
Question 21.
Rainbow is formed due to the combination of:
(a) absorption and refraction
(b) refraction and scattering
(c) Dispersion and focussing.
(d) Dispersion and total internal reflection.
Answer
Answer: (d) Dispersion and total internal reflection.
Question 22.
Maximum deviation takes place when there is
(a) grazing incidence
(b) 30° incidence
(c) 0° incidence
(d) 60° incidence
Answer
Answer: (a) grazing incidence.
Fill in the Blanks
Question 1.
A concave lens made of a material of refractive index 1.5 is immersed in a medium of refractive index 1.5. A parallel beam of light is incident on the lens. The beam will …………………
Answer
Answer: remain parallel.
Question 2.
The focal length of a lens of focal power 100 D is …………………
Answer
Answer: 1 cm.
Question 3.
The image of an object formed by a concave lens is …………………
Answer
Answer: always virtual.
Question 4.
Two lenses of power +16 D and – 6 D are placed in contact. Then the power and focal length of the combination are given by ………………… and ………………… respectively.
Answer
Answer: + 10 D, 10 cm.
Question 5.
A bird flying high in the air appears to be ………………… as the refraction takes place from rarer to denser medium.
Answer
Answer: higher than in reality.
Question 6.
The brightness of the image produced by a lens which is half painted black reduces to ………………… and the size of the image …………………
Answer
Answer: half, remains the same.
Question 7.
A lens forms a sharp image on a screen. On inserting a parallel sided slab of refractive index p between the screen and lens, it is found necessary to move the screen through a distance ‘d’ in order for the image to be again sharply focused. The thickness of the slab is …………………
Answer
Answer: d(1 – \(\frac {1}{µ}\))
Question 8.
The dispersive power of for a given prism is independent of ………………… but depends …………………
Answer
Answer: angle of prism, refractive index for the extreme and mean colours.
Question 9.
When a ray of white light passes through a glass prism, red light is divided through a ………………… angle than violet light because the refractive index for violet light is ………………… than that of red light.
Answer
Answer: Smaller, greater.
Question 10.
In glass violet light travels ………………… than red light.
Answer
Answer: Slower.
Question 11.
………………… is the main source of light.
Answer
Answer: Sun.
Question 12.
With the rise in temperature, the density and hence the refractive index of the medium …………………
Answer
Answer: decreases.
Question 13.
With an object placed in a denser medium and observed by an observer in the rarer medium, the image of the object shifts ………………… and when the object placed in rarer medium is observed in denser medium, the image shifts …………………
Answer
Answer: nearer the refracting surface, away from the refracting surface.
Question 14.
The sparkle of diamond, the shine of air bubbles in a glass, paper weight, the mirage in hot deserts and the looking in cold countries is due to …………………
Answer
Answer: the total internal reflection of light.
Question 15.
In the position of minimum deviation, the deviation is equally shared at the two plane surfaces enclosing the angle of prism and the refracting ray passes through the prism symmetrically with respect to the refracting edge. If the base angles of the prism are equal, then the refracting ray ………………… in the position of minimum deviation.
Answer
Answer: runs parallel to the base.
Question 16.
Compound microscope consists of an objective of ………………… focal length and ………………… aperture and an eyepiece of comparatively ………………… focal length and ………………… aperture.
Answer
Answer: small, small, larger, larger.
Question 17.
In reflecting astronomical telescope, parabolic mirrors are used because they are free from ………………… and in an astronomical telescope f0 ………………… fe where f0 and fe are the focal lengths of objective and eye piece respectively.
Answer
Answer: spherical aberration, greater than.
Question 18.
The length of the astronomical telescope for normal adjustment is …………………
Answer
Answer: f0 + fe.
Question 19.
Focal length of a concave mirror when it is dipped in water does …………………
Answer
Answer: not changes.
Question 20.
The magnifying power of the erecting lens in a terrestrial telescope is …………………
Answer
Answer: One.
Question 21.
Red light is used for danger signals because it is ………………… and scattered …………………
Answer
Answer: having high wave lengths, least.
Question 22.
If f be the focal length of a convex lens used as a simple microscope and D is the least distance of distinct vision, then its magnifying power is ………..
Answer
Answer: 1 + \(\frac {D}{f}\)
Question 23.
Green colour when seen in sodium light appears ………………… and when red and blue colours are mixed ………………… colour will be formed.
Answer
Answer: black, magenta.
Question 24.
Light travels from denser to rarer medium and when it gets incident on the interface at angle 0, the reflected and refracted rays are perpendicular to each other. The critical angle is …………………
Answer
Answer: sin-1 (cot θ).
Question 25.
The distance between the nearest point and the farthest point is known as …………………
Answer
Answer: range of vision.
Question 26.
An eye which does not show this range of vision is called …………………
Answer
Answer: defective or ametropic.
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