Optics (Physics) Part-III

Total Questions: 56

1. The number of images of an object placed between two parallel mirrors is: [U.P.P.C.S.(Mains) 2013, U.P.P.C.S. (Pre) 1994]

Correct Answer: (d) Infinite
Solution:

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

Correct Answer: (d) Infinite
Solution:

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

Correct Answer: (d) 2θ
Solution:

If a plane mirror is rotated through a certain angle, then the reflected ray rotates through twice of that angle. So if a mirror is rotated by an angle θ, the reflected ray will rotate by 2θ.

4. 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.

Code:

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).

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

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.

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

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).

7. 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:

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.

8. Dispersion of light is possible by : [66ᵗʰ B.P.S.C. (Pre) (Re. Exam) 2020]

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.

9. Which one of the following colours is seen in the middle of a Rainbow? [U.P.P.C.S. (Mains) 2012]

Correct Answer: (b) Green
Solution:

The correct order of colors of the rainbow is Red, Orange, Yellow, Green, Blue, Indigo and Violet. Thus it is clear that 'Green' Color is seen in the middle of a rainbow (VIBGYOR).

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

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.