Optics part (2)

Total Questions: 45

1. The red colour of the sun at sunset and sunrise is due to - [U.P.P.C.S.(MAINS),2013 U.P.P.C.S.(pre) 1992]

(a) scattering of the light
(b) refraction of the light
(c) total internal reflection of the light
(d) dispersion of the light

Correct Answer: (a) scattering of the light

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.

2. Read the following statements and choose the correct options [Chhattisgarh P.CS. (Pre) 2022]

(1) For passengers flying at very high altitudes, sky appears dark.

(2) Scattering is not prominent at such heights

(a) (1) and (2) both are true and (2) is correct reasoning of (1)
(b) (1) and (2) both are true but (2) is not correct reasoning of (1)
(c) (1) is correct and (2) is incorrect
(d) (1) is incorrect and (2) is correct

Correct Answer: (a) (1) and (2) both are true and (2) is correct reasoning of (1)

Solution:The scattering of the light takes place because of the particles present in the atmosphere. At a higher altitude, the atmospheric medium is very rarer and thus, scattering of light is very low or not prominent. Hence, the sky appears dark to the passengers flying at high altitudes. So, statements (1) and (2) both are true, and (2) is correct reasoning of (1).

3. An astronaut sees the colour of the sky as- [U.P.P.C.S. (MAINS) 2005]

(a) blue
(b) white
(c) black
(d) red

Correct Answer: (c) black

Solution:An astronaut sees the colour of the sky is black it is because of the fact that in the space there is no atmosphere and there are no air particles against which the sunlight can strike get reflected or scattered . Hence astronaut see everything black.

4. When we see the sky from a plane in high altitude then it seems - [U.P.P.C.S.(R.I) 2014]

(a) blue
(b) black
(c) white
(d) red

Correct Answer: (b) black

5. Sea seams blue due to - [40th B.P.S.C. (Pre) 1995]

(a) excess deepness
(b) reflection of the sky and scattering of the light by the water drops.
(c) blue colour of water
(d) upper layer of water

Correct Answer: (b) reflection of the sky and scattering of the light by the water drops.

Solution:The sea looks blue due to reflection of sky and scattering of light by the drops of water. Light scattered as it passes through air or water since blue light has a short wavelength it scatters most easily .

6. The green grass appears green due to [U.P.P.C.S.(MAINS) 2017]

(a) it reflects green colour light to our eyes
(b) it absorbs green colour light
(c) it reflects all the other colour lights except that of green colour
(d) none of above

Correct Answer: (a) it reflects green colour light to our eyes

Solution:The colour we see on the surface of an object is not actually the colour in that surface it a surface looks green it means that it absorbs all the colour of the spectrum except green it looks black to our eyes in case no colour is reflected if an objects is white it reflects all light.

7. When a soap film on the water is seen in the daytime. it shows beautiful colours. This phenomenon is due to [67th B.P.S.C. (Pre) 2022]

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

Correct Answer: (d) interference of light

Solution:The colours that we see when observing a soap film are caused by interference of light reflecting off of two nearby surfaces the outer surface and the inner surface. Interference patterns are created when two reflected waves line up in phase or out of phase. White light is a mixture of all the wavelengths of the visible spectrum and the interference enhances some wavelengths and diminishes others, creating visible colours. The specific wavelengths that are enhanced or diminished are determined by the thickness of the film and its index of refraction.

8. Suppose a rocketship is proceeding from the earth at a speed of 2/10th the velocity of light. A light in the rocketship appears blue to the passengers on the ship. What colour would it appear to an observer on the Earth? [I.A.S. (PRE) 1995]

(a) Blue
(b) Orange
(c) Yellow
(d) Yellow-Orange

Correct Answer: (c) Yellow

Solution:To determine the colour that would appear to an observer on the Earth, we use the following formula.

v = 1{(λ-λο)/λ)}C

Where λ = Prime wavelength

λο = Virtual wavelength

v = Velocity of rocket

C = Velocity of light

Because the rocket is proceeding from the Earth, thus positive sign is taken.

2/10 × C = {(λ-λο)/λ)}C

⇒2/10 = [(λ-λο)/λ)]

⇒2/10 = 1 - λο/λ

⇒λο/λ=1 -2/10

⇒λο/λ=8/10

⇒λ=10/8λο=1.25λο

Because the wavelength of blue colour lies between 450 nm to 475 nm. Thus, the wavelength of the light would appear to an observer on the earth has a range between 562.5 nm to 593.75 nm, which is yellow light.

9. What will be the nature of image formed by a convex mirror when the position of object is between infinity and pole p of the mirror ? [Chhattisgarh P.C.S.(PRE) 2020]

(a) virtual and erect
(b) real and erect
(c) real and inverted
(d) virtual and inverted

Correct Answer: (a) virtual and erect

Solution:

when the position of the objects is between infinity and pole p of the convex mirror then the image formed between the pole and the focus (f) behind the mirror. The image thus formed is diminished virtual and erect .

10. Large number of thin strips of black paints are made on the surface of a convex lens of focal length 20 cm to catch the image of s white horse. the image will be [68th B.P.S.C.(PRE) 2022]

(a) a horse of less brightness
(b) a zebra of black strips
(c) a horse of black strips
(d) more than one of the above

Correct Answer: (a) a horse of less brightness

Solution:When light passes through a convex lens, it converges or comes together at a focal point. In this case, the convex lens has a focal length of 20 cm. When the lens has a large number of thin black strips on its surface, these strips will block or absorb some of the incident light. As a result, less light will reach the focal point and form the image. Hence, thin black strips on the surface of a convex lens will only reduce the intensity of the light but will not cast any shadow on the image, as it is quite thin. Thus, option (a) is the correct answer

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