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

Total Questions: 50

41. Which mirror is used in a solar furnace? [RRB JE 29/05/2019 (Morning)]

Correct Answer: (a) Concave mirror
Solution:

Concave (converging) mirror - It is the only type of mirror that reflects light toward a single focal point which will help in effective heating. Spherical mirror - That has a consistent curve and a constant radius of curvature. The images formed by a spherical mirror can either be real or virtual. Plane mirror - A mirror with a flat reflective surface.

42. An incident ray makes an angle of 30° with the surface of a plane mirror. What is the angle of reflection? [RRB JE 29/05/2019 (Evening)]

Correct Answer: (a) 30°
Solution:

30° . The first Law of reflection states that the incident ray, the reflected ray, and the normal to the surface of the mirror, all lie in the same plane. The second law of reflection states that when a ray of light reflects off a surface, the angle of incidence (θᵢ) is equal to the angle of reflection (θᵣ).

43. How will the emergent ray be after refraction of light through a glass slab? [RRB JE 30/05/2019 (Morning)]

Correct Answer: (a) The emergent ray is at 90 degrees to the incident rays.
Solution:

The bending of the ray of light on the air - glass interface is equal and opposite to the bending of the ray of light on the air - glass interface. Thus, Angle of incidence = Angle of Emergence. Laws of Refraction of light - Law 1 : The incident ray, refracted ray and normal ray all lie on the same plane. Law 2 (Snell’s Law) : = Refractive index 𝑠𝑖𝑛 𝑖 / 𝑠𝑖𝑛 𝑟 (constant) of medium 2 with respect to medium 1. Incident ray (i) - The light ray which travels into a medium. Refracted ray (r) - The light ray which bends after refracting. Normal ray - The light ray which is perpendicular to the surface at Point of Intersection.

44. A spherical mirror and a thin spherical lens both have a focal length of -10 cm. The mirror and the lens are likely to be- [RRB JE 30/05/2019 (Morning)]

Correct Answer: (b) Both concave
Solution:

Both concave. Focal length:- It is the distance from the lens to the focal point. It is always positive for concave lenses (diverging) or convex mirrors (converging) and always negative for converging or concave mirror.

45. To get a virtual, erect and enlarged image behind a concave mirror, where should the object be placed? [RRB JE 30/05/2019 (Afternoon)]

Correct Answer: (a) Between pole P and focus F
Solution:

Between pole P and focus F. Image formation by a concave mirror for different positions of the object: At infinity {Image (small) - At the focus F, Real and inverted}. Beyond C {Image (small) - Between F and C, Real and inverted}. At C {Image (same) - At C, Real and inverted}. Between C and F {Image (large) - Beyond C, Real and inverted}. At F {Image (large) - At infinity, Real and inverted}. C - Centre of curvature, F - Focus, P - Pole.

46. Which of the following statements is FALSE? [RRB JE 31/05/2019 (Morning)]

1. The object is always placed to the left of the spherical mirror.
2. All distances parallel to principal axis are measured from pole of the mirror. 3. All distances measured to the right of the origin are taken as negative.
4. Distances measured perpendicular to and above the principal axis are taken as positive.

Correct Answer: (a) 3
Solution:

3. Spherical mirrors are of two types : concave mirror and convex mirror. Focal length of concave mirror is always negative, focal length of convex mirror is always positive. If a real image is formed, then it will form in front of the mirror, hence it will be negative. If a virtual image is formed, then it will be formed behind the mirror, hence it will be taken as positive.

47. To obtain a virtual enlarged erect image on the same side of a convex lens after refraction, where should the object be placed? [RRB JE 31/05/2019 (Afternoon)]

Correct Answer: (b) Between focus F₁ and optic centre O
Solution:

Between focus F₁ and optic centre O. Image Formation by convex lens - When Object is at infinity (At focus 𝐹₂ , Highly diminished, point-sized, Real  and inverted), object beyond 2𝐹₁ (Between 𝐹₂ and 2𝐹₂ , Diminished, Real  and inverted), Object is at 2𝐹₁ (At 2𝐹₂ ,  Same size, Real and inverted), Object is placed between 𝐹₂ and 2𝐹₁ (Beyond 2𝐹₂ ,  enlarged, Real and inverted), Object is at 𝐹₁ (At infinity, highly enlarged, Real and  Inverted), Object is between 𝐹₁ and optical centre (On the same side of the lens as the object, enlarged, Virtual and erect)
Lens Formula: 1/f = 1/v - 1/u.

48. If the object is placed between infinity and optical centre O of the concave lens, how will the image be formed after refraction? [RRB JE 31/05/2019 (Evening)]

Correct Answer: (b) Virtual and erect
Solution:

Virtual and erect. Image formed by a concave lens for various positions of the object: Object at infinity - Image form at the focus, highly diminished, Virtual and erect. Object between Infinity and optical centre - Image form between focus and optical centre, Diminished, virtual and erect. When an object is at the center of curvature, the real image is formed at the other center of curvature; Size of the image is the same as compared to that of the object. When an object is placed at the focus, a real image is formed at infinity; Size of the image is much larger than that of the object.

49. Which colour among the seven colours of white light gets least deviated after dispersion of light through a glass prism? [RRB JE 01/06/2019 (Morning)]

Correct Answer: (d) Red
Solution:

Red. Dispersion of light is the phenomenon of splitting a beam of white light into its constituent 7 colours on passing through a prism (Violet, indigo, blue, green, yellow, orange and red). The wavelength of red colour is the longest and the violet colour is the shortest. The speed of light depends on the refractive index of the medium.

50. If the image formed by a spherical mirror is virtual, then the magnification value is- [RRB JE 01/06/2019 (Morning)]

Correct Answer: (c) Positive
Solution:

Positive. The concave lens and the convex mirror give a virtual image. Their magnification is positive. The convex lens and the concave mirror can produce real and virtual images based on the object position, therefore, their magnification can be positive or negative. Formula of magnification of mirror
(m) = -v/u = hᵢ/hₒ
Where, v = image distance, u = object distance, hᵢ= height of image, hₒ = height of object.