Optics part (2)

Total Questions: 45

21. The number of images of an object placed between the two parallel mirrors is : [U.P.P.C.S.(MAINS) 2013 U.P.P.C.S.(pre) 1994]

(a) two
(b) one
(c) six
(d) infinite

Correct Answer: (d) infinite

Solution:When an object is placed in between two parallel plane mirrors, then the number of images formed will be infinite. Each image will be the result of an image. When two plane mirrors are inclined at an angle of θ , then the number of images formed by the object between them is
{(360° /θ)-1} (if the value of 360°/θ is even)
or
(360°/θ ) ( if the value of 360°/θ is odd).

22. How many images will be formed if the point light source is placed between two parallel plane mirrors ? [U.P.P.C.S.(pre) 2015]

(a) two
(b) four
(c) eight
(d) infinite

Correct Answer: (d) infinite

Solution:When an object is placed in between two parallel plane mirrors, then the number of images formed will be infinite. Each image will be the result of an image. When two plane mirrors are inclined at an angle of θ, then the number of images formed by the object between them is
{(360° /θ)-1} (if the value of 360°/θ is even)
or
(360°/θ ) ( if the value of 360°/θ is odd).

23. When a mirror is rotated by an angle θ the reflected ray will rotate by : [I.A.S. (PRE) 1996]

(a) 0
(b) θ/2
(c) θ
(d) 2θ

Correct Answer: (d) 2θ

Solution:If a plane mirror is rotated through a certain angle then the reflected ray rotate through twice of that angle so the mirror is rotated by an angle θ the reflected ray will rotate by 2θ.

24. Assertion (A): Small glass beads fixed on traffic signals glow brightly when light falls upon them. [I.A.S. (Pre) 2000]

Reason (R): Light is totally reflected when the angle of incidence exceeds a certain critical value and light travelling in a denser medium is reflected from a rarer medium.

(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 a 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:When light travelling through a denser medium to rarer medium, then the light is totally reflected when the angle of incidence exceeds a certain critical value, which is called total internal reflection. Here small glass beads are denser medium, so due to the phenomenon of total internal reflection, traffic signals glow brightly. Thus, both (A) and (R) are true and (R) is the correct explanation of (A).

25. Light is made of seven colours. What is the method of separating the colours? [47th B.P.S.C. (Pre) 2005]

(a) The colours can be separated by a prism
(b) The colours can be separated by a filter
(c) The colours can be separated by plants
(d) The colours cannot be separated

Correct Answer: (a) The colours can be separated by a prism

Solution:Colours could be separated by using a prism. The white colour is made up of all seven colours. When it passes through a prism, due to different wavelengths, different colours of light is refracted by a different amount.

26. The correct order of colours in a rainbow is: [R.A.S./R.T.S.(Pre) 1999]

(a) Blue, green, violet
(b) Violet, green, blue
c) Blue, yellow, green
(d) Blue, green, yellow

Correct Answer: (d) Blue, green, yellow

Solution:A rainbow is a meteorological phenomenon that is caused by reflection, refraction, and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. The correct order of colours in a rainbow is Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR).

27. Rainbow is produced when sunlight falls on drops of rain. which of the following physical phenomena are responsible for this? [I.A.S. (Pre) 2013]

1. Dispersion
2. Refraction
3. Internal reflection
Select the correct answer using the code given below:

(a) 1 and 2
(b) 2 and 3
(c) 1 and 3
(d) All of these

Correct Answer: (d) All of these

Solution:The appearance of a rainbow involves all of the above factors that are refraction, dispersion and internal reflection. When sunlight hits a raindrop then depending on the angle, some of the light is reflected from the surface of the drop and some enter the drop through the process known as refraction. When light enters a new medium, its speed changes and different coloured components of light are affected differently by this. This leads to the separation of light into its seven coloured spectrum.

28. Dispersion of light is possible by : [66th B.P.S.C. (PRE)(RE.EXAM) 2020]

(a) prism
(b) convex lens
(c) concave lens
(d) simple mirror
(e) none of the above /more than one of the above

Correct Answer: (a) prism

Solution:Dispersion of light is possible by prism upon passage through the prism the white light is separated into its different component colours which is known as dispersion.

29. Which one of the following colours is seen in the middle of a rainbow ? [U.P.P.C.S.(MAINS) 2012]

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

Correct Answer: (b) green

Solution:The correct order of the rainbow is (red orange yellow green blue indigo and violet.Thus it is clear that green colour is seen in the middle of a rainbow (VIBGYOR).

30. The terminal colour bands of the rainbow are: [U.P. Lower Sub. (Mains) 2015]

(a) Blue and Orange
(b) Green and Red
(c) Yellow and Green
(d) Violet and Red

Correct Answer: (d) Violet and Red

Solution:A rainbow is a meteorological phenomenon that is caused by reflection, refraction, and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. The apparent discreteness of main colours is Red, Orange, Yellow, Green, Blue, Indigo and Violet. Hence, option (d) is the correct answer.

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