Optics part (1)

Total Questions: 78

71. Red light is used in traffic signals because : [U.P.P.C.S.(MAINS) 2013]

(a) it has the longest wavelength
(b) it is beautiful
(c) it is visible to people even with the bad eyesight
(d) none of the above reasons

Correct Answer: (a) it has the longest wavelength

Solution:The wavelength of red colour is highest while of violet colour is lowest. Dispersal is inverse to wavelength so red light can be seen from far away more clear. That's why the red colour is used in traffic signals.

72. Assertion (A): The danger signal is made up of red colour. [U.P.P.C.S.(pre) 2007]

Reason (R): Red colour is scattered the least.

Select the correct answer using the codes given below:

(a) Both (A) and (R) are true and (R) is the correct explanation of (A).
(b) Both (A) and (R) are true but (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: (a) Both (A) and (R) are true and (R) is the correct explanation of (A).

Solution:The primary reason why the red colour is used to danger signals is that the red light has the highest wavelength and scattered least by the air molecules so that it can be seen from the far away.

The appearance of the red colour of the sun at the sunset is due to the fact that neighbouring colour are scattered away in the path and the light reaching the observer is predominantly red.That is why the sun is seen red to us.

73. Red light signal is used as an danger signal because - [U.P.P.C.S.(MAINS) 2008,2011,2005, U.P.P.C.S.(pre) 1995]

(a) red light is scattered least
(b) this is comfortable
(c) it produce least chemical reaction
(d) it is least absorbed in air

Correct Answer: (a) red light is scattered least

74. The sky appears blue is due to - [U.P.U.D.A./L.D.A.(PRE) 2013]

(a) blue colour scattered the most
(b) the red colour is scattered the most
(c) blue light is minimum absorbed by atmosphere
(d) red light is ultimately absorbed by atmosphere

Correct Answer: (a) blue colour scattered the most

Solution:The blue light of the sky is due to the scattering of sunlight from the atmosphere. when we look at the sky it is the scattered light enters in the eyes. Among the shorter wavelength the blue colour is present in a larger proportion in sunlight.

Light of shorter wavelength is more scattered by the air molecules is because of their small size. Blue light is strongly scattered by the air the molecules and reach the observer.

75. Sky is blue because - [Uttarakhand P.C.S.(PRE) 2005, U.P Lower Sub.(PRE) 1998, 39th B.P.S.C. (PRE) 1994]

(a) blue colour in the sunlight is more than the other colour
(b) short waves are scattered more than long waves by atmosphere
(c) blue colour is more absorbing to eyes
(d) atmosphere absorbs long wavelength more than short wavelength

Correct Answer: (b) short waves are scattered more than long waves by atmosphere

Solution:The blue light of the sky is due to the scattering of sunlight from the atmosphere. when we look at the sky it is the scattered light enters in the eyes.

Among the shorter wavelength the blue colour is present in a larger proportion in sunlight. Light of shorter wavelength is scattered by the air molecules is because of their small size. Blue light is strongly scattered by the air the molecules and reach the observer.

76. Blue colour of the sky is due to - [U.P.P.C.S.(pre)(re.exam) 2015]

(a) scattering of the light
(b) dispersion of the light
(c) interference of the light
(d) emission of more blue light as compared to others colours by the sun

Correct Answer: (a) scattering of the light

Light of shorter wavelength is scattered by the air molecules is because of their small size. Blue light is strongly scattered by the air the molecules and reach the observer.

77. The sky appears blue due to - [U.P.P.C.S.(MAINS) 2011]

(a) diffraction of light
(b) reflection of the light
(c) refraction of the light
(d) scattering of the light

Correct Answer: (d) scattering of the light

Light of shorter wavelength is scattered by the air molecules is because of their small size. Blue light is strongly scattered by the air the molecules and reach the observer.

78. Consider the following statements : [Uttarakhand U.D.A./L.D.A. (Pre) 2003]

1. The Sky appears blue because of scattering.
2. The main cause of seven colors of the rainbow is scattering of light.
3. At the time of sunrise, the sun appears red, due to scattering of light.
Choose the correct statement/s from the given options:

(a) Only 1 is true
(b) 1 and 3 is true
(c) Only 2 is true
(d) 2 and 3 is true

Correct Answer: (b) 1 and 3 is true

Solution:The intensity of scattered light is inversely proportional to its wavelength and the wavelength of violet and blue is low, thus they scattered more. This is the main cause for the blue colour of the sky.

As we know that sunlight consists of seven colours, of these, red has the maximum wavelength. During sunrise and sunset, the rays have to travel a larger part of the atmosphere because they are very close to the horizon.

Therefore light other than red is mostly scattered away. Most of the red light which is least scattered enters our eyes. Hence, the sun appears red.

The rainbow is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. Thus, statements 1 and 3 are correct.