Railway Science (Physics-Light and Optics) (Part-IV)

Total Questions: 50

31. Why is red colour preferred for danger signal lights? [RRB JE 24/05/2019 (Afternoon)]

Correct Answer: (d) Because red colour light has longest wavelength and least scattered by fog and smoke
Solution:

Wavelength - The distance between the two successive crests or troughs of the light wave. The frequency and wavelength are indirectly proportional to each other.

32. On what factor does the colour of scattered light depend upon? [RRB JE 25/05/2019 (Afternoon)]

Correct Answer: (d) The size of the scattering particles
Solution:

Scattering of Light - When light rays get deviated from its straight path on striking an obstacle like dust or gas molecules, water vapors etc. Examples - Red colour of sun at sunrise and sunset, Blue colour of sky, White color of sky at noon. It depends on Wavelength of Light, Nature of Particles, Angle of Incident Light, Polarization of Light.

33. What is the distance of the principal focus F from the pole P of the spherical mirror called? [RRB JE 26/05/2019 (Morning)]

Correct Answer: (d) Focal length (f)
Solution:

Focal length (f). The focal length is taken as positive (+) for a convex lens and convex mirror. It is taken as negative (-) for a concave lens and a concave mirror. Object distance (u) - The distance between the object and the pole of the mirror. Image distance (v) - The distance between the image and the pole of the mirror.

34. Which set of sign conventions is followed while dealing with reflection of light by spherical mirrors? [RRB JE 26/05/2019 (Afternoon)]

Correct Answer: (b) New Cartesian sign convention
Solution:

New Cartesian sign convention. In this convention, the pole (P) of the mirror is taken as the origin. The principal axis of the mirror is taken as the x-axis of the coordinate system. The sign conventions are: The object is always placed to the left of the mirror. All distances parallel to the principal axis are measured from the pole of the mirror. All the distances measured to the right of the origin (along + x-axis) are taken as positive while those measured to the left of the origin (along – x-axis) are taken as negative. Distances measured perpendicular to and above the principal axis (along + y-axis) are taken as positive and below the principal axis (along –y-axis) are taken as negative.

35. Why does dispersion occur when white light which is a mixture of seven colours passes through a glass prism? [RRB JE 26/05/2019 (Evening)]

Correct Answer: (c) 7 colours of white light travel at different speeds through a glass prism.
Solution:

Dispersion of light - The phenomenon of splitting a beam of white light into its seven constituent colours when passed through a transparent medium. It was discovered by Sir Issac Newton. Examples of Dispersion of light in daily life: Formation of a Rainbow, Compact disks, Petroleum spilled on water, Soap bubbles, Prism, Plastic rulers.

36. How is the refractive index of a material related to the speed of light in air? [RRB JE 27/05/2019 (Morning)]

Correct Answer: (d) Refractive index = Speed of light in air / Speed of light in material
Solution:

Refractive index (index of refraction) is measure of the bending of a ray of light when passing from one medium into another. Refractive index of some substance - Water (1.33) Kerosene (1.44), Crown glass (1.52), Canada Balsam (1.53), Rock salt (1.54), Diamond (2.42).

37. Sort the following materials in ascending order of their respective refractive indices - Water, Glass, Air. [RRB JE 27/05/2019 (Afternoon)]

Correct Answer: (c) Air, Water, Glass
Solution:

Air, Water, Glass. When a ray of light that travels obliquely from one transparent medium into another will change its direction in the second medium. The extent of the change in direction that takes place in a given pair of media is expressed in terms of the refractive index, the “constant”. It turns out that light propagates with different speeds in different media. The refractive index of medium 2 with respect to medium 1 is given by μ₂₁ =
𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚 1 /𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚 2 = v₁/v₂,
v₁ =  Speed of light in medium 1,
v₂ = Speed of light in medium 2.

38. The structure of the eye which serves to refract and focus light rays upon retina is called- [RRB JE 27/05/2019 (Afternoon)]

Correct Answer: (b) Lens
Solution:

Lens. Cornea - It is the clear, transparent, anterior portion of the external coat of the eyeball. The rays of light enter this layer. Cornea accounts for two-thirds of the total optical power of the eye. Pupil - It is an aperture of variable size in the center of Iris, which regulates the amount of light entering the eyeball. Iris - It is the coloured membrane behind the cornea and in point of lens with an aperture of variable size called pupil. It has a circular and long muscle fibre. Iris is attached to the ciliary body. Retina - It is an essential part of the eye that enables vision.

39. A ray of light passing through the optical centre of a concave lens emerge after refraction will be [RRB JE 27/05/2019 (Evening)]

Correct Answer: (a) Without any deviation
Solution:

Without any deviation. Convex lens - In which the rays reflected through the surface are dispersed in a parallel path using reflection of light. It has a positive focal length. The image formed is real and inverted. In hypermetropia a convex lens is used. Concave lens - Diverges a straight light beam from the source to a diminished, upright, virtual image. The image formed in a concave lens is virtual, upright and small in size. It has a negative focal length. A concave lens’s magnification is less than one. In myopia concave lens is used.

40. To obtain the same size image at 2𝐹₂ after refraction by a convex lens 2 where should the object be placed? [RRB JE 28/05/2019 (Afternoon)]

Correct Answer: (a) At 2𝐹₁
Solution:

At 2𝐹₁ . Convex lens (Converging lens): A lens having two spherical surfaces, bulging outwards is called a double convex lens (or simply convex lens). It is thicker in the middle as compared to the edges.