Optics part (1)

Total Questions: 78

61. Choose the correct statement . [64th B.P.S.C. (PRE) 2018]

(a) Wavelength of red light is less than violet light
(b) Wavelength of red light is more than violet light
(c) Wavelength of violet light is more than green light
(d) Wavelength of violet light is more than yellow light
(e) None of the above / More than one of the above

Correct Answer: (b) Wavelength of red light is more than violet light

Solution:In a visible light spectrum, VIBGYOR (Violet, Indigo, Blue Green, Yellow, Orange and Red) is the order of the increasing wavelength of the various coloured lights where violet coloured light has the minimum and red coloured light has the maximum wavelength.

Similarly, the same (VIBGYOR) the order of decreasing frequency where violet light has the maximum and red light has the minimum frequency.

62. Which one of the colours has the longest wavelength ? [M.P.P.C.S.(PRE) 1997]

(a) green
(b) yellow
(c) blue
(d) red

Correct Answer: (d) red

Similarly, the same (VIBGYOR) the order of decreasing frequency where violet light has the maximum and red light has the minimum frequency.

63. The light with the shortest wavelength is [Uttarakhand P.C.S.(PRE) 2005]

(a) red
(b) yellow
(c) blue
(d) violet

Correct Answer: (d) violet

Solution:In a visible light spectrum, VIBGYOR (Violet, Indigo, Blue Green, Yellow, Orange and Red) is the order of the increasing wavelength of the various coloured lights where violet coloured light has the minimum and red coloured light has the maximum wavelength.

Similarly, the same (VIBGYOR) the order of decreasing frequency where violet light has the maximum and red light has the minimum frequency.

64. In the visible spectrum, the colour having the shortest wavelength is: [U.P.R.O./A.R.O.(MAINS) 2014]

(a) red
(b) yellow
(c) blue
(d) violet

Correct Answer: (d) violet

Similarly, the same (VIBGYOR) the order of decreasing frequency where violet light has the maximum and red light has the minimum frequency.

65. Which one among the following has the highest energy ? [I.A.S. (PRE) 2009]

(a) blue light
(b) green light
(c) red light
(d) yellow light

Correct Answer: (a) blue light

Solution:Energy ∝ 1/wavelength

The energy of any of the colour is inversely proportional its wavelength. According to VIBGYOR order violet, Indigo and blue have the minimum wavelengths, thus they have maximum energy level. So according to the options given in the question, blue has the maximum energy while red the minimum energy.

66. Which of the following colours of white light least deviated by the glass prism ? [U.P.R.O./A.R.O.(pre)(re.exam) 2016]

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

Correct Answer: (b) red colour

Solution:Speeds of different colours of white light are same in a vacuum but differs in different matters. Thus, the refractive index of any matter differs for different colours of light.

The speed of violet light is minimum and the speed of red light is maximum in the glass. So the refractive index of glass is maximum for violet light and minimum for the red light and the violet light is most deviated and red light is least deviated by the glass prism.

It also understood that angle of deviation is inversely proportional to wavelength. Red have longer wavelength and least deviation.

67. Assertion (A): In the visible spectrum of light, red light is more energetic than green light.

Reason (R) : The wavelength of red light is more than that of green light.

(a) Both A and R are individually true and R is the correct explanation of A
(b) Both A and R are individually true and R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true

Correct Answer: (d) A is false but R is true

Solution:(1) The wavelength of red light (700nm) is greater than the wavelength of green light (530 nm). Thus reason (R) is correct.
(ii) The energy of different colors are inversely proportional to its wavelength Thus, the energy of red light (1.77ev) is less than the energy of green light (2.34 ev) so, statement (A) is wrong.

68. The Sun is visible to us a few minutes before the actual sunrise because of: [R.A.S./R.T.S (PRE) 1992]

(a) Scattering of light
(b) diffraction of light
(c) total internal reflection
(d) refraction of light

Correct Answer: (d) refraction of light

Solution:Due to the atmospheric refraction of light the sun is visible to us a few minutes before sunrise and a few minutes after sunset.

69. The Sun and the Moon appear elliptical near the horizon because of: [U.P.P.C.S.(spl) (pre) 2008]

(a) refraction
(b) optical illusion
(c) interference phenomenon
(d) their actual shape

Correct Answer: (a) refraction

Solution:The sun and the moon appear elliptical near the horizon because of refraction.

70. Twinkling of stars in clear sky during night time can be explained with: [67th B.P.S.C. (PRE) 2022]

(a) refraction of light
(b) reflection of light
(c) polarization of light
(d) interference of light
(e) None of the above/More than one of the above

Correct Answer: (a) refraction of light

Solution:The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the Earth's atmosphere, undergoes refraction continuously before it reaches the Earth.

The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position.

The star appears slightly higher (above) than its actual position when viewed near the horizon. Since the stars are very distant, they approximate point-sized sources of light.

As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering eye flickers the star sometimes appear brighter and sometimes fainter, which is twinkling effect.