Physics (Part-II) (UPSC)

Total Questions: 60

1. Suppose a rocketship is receding from the earth at a speed of 2/10 th 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? [1995]

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

Correct Answer: (d) Yellow-orange

Solution:The observer on the earth will detect a wavelength given by

The wavelength of blue light varies from 450 nm to 475 nm. Therefore, the observed light will have wavelength ranging from 562.5 nm to 593.75 nm which will be yellow orange.

2. An air bubble in water will act like a: [1995]

(a) convex mirror
(b) convex lens
(c) concave mirror
(d) concave lens

Correct Answer: (d) concave lens

Solution:Air bubble in water would act as a concave lens, because the index of refraction of air is less than that of water.

3. When a mirror is rotated by an angle 0, the reflected ray will rotate by: [1996]

(a) θ°
(b) θ/2
(c) θ
(d) 2θ

Correct Answer: (d) 2θ

Solution:Consider a plane-mirror and a fixed incident ray of light. Before the mirror has rotated, the angle of incidence is 9 as is the angle of reflection. If the mirror is rotated through an angle & the normal is rotated by an angle fand thus the angle of incidence increases to 0+. Therefore, the angle of reflection must also increase by to 0+.

The difference between the final angle of reflection and the initial angle of reflection is 20. Thus for a fixed incident ray, the angle of the reflection is twice the angle through which the mirror has rotated.

4. Total internal reflection can take place when light travels from: [1996]

(a) diamond to glass
(b) water to glass
(c) air to water
(d) air to glass

Correct Answer: (a) diamond to glass

Solution:Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all light is reflected. The critical angle is the angle of incidence above which the total internal reflection occurs. Total internal reflection takes place when light travels from optically denser medium to optically rarer medium. Refractive index of diamond is very high (2.417) whereas refractive index of glass is only in between (1.5 to 1.6), whereas in rest all options there is no possibility of total internal reflection because refractive index of water (1.33) and air (1.0008) is less than refractive index of glass.

5. Match List I (Quantity) with List II (Units) and select the correct answer using the codes given below the lists: [1999]

List-I	List-II
A. High speed	1. Mach
B. Wavelength	2. Angstrom
C. Pressure	3. Pascal
D. Energy	4. Joule

Codes:

	A	B	C	D
(a)	1	2	4	3
(b)	1	2	4	3
(c)	1	2	3	4
(d)	2	1	4	3

Correct Answer: (c)

Solution:Mach number (Ma or M) is the speed of an object moving through air, or any other fluid substance. It is commonly used to represent the speed of an object when it is travelling close to or above the speed of sound. Pressure is the force per unit area applied in a direction perpendicular to the surface of an object.

The SI unit for pressure is the pascal (Pa), equal to one newton per square meter (N/m²). In physics, the wavelength of a sinusoidal wave is the spatial period of the wave the distance over which the wave's shape repeats. The unit for wavelength is the angstrom.

In physics, energy is a quantity that is the ability to do work. In the International System of Units (SI), energy is measured in joules, but in some fields other units such as kilowatt-hours and kilocalories are also used.

6. Consider the following statements: [1999]

1. If a person looks at a coin which is in a bucket of water, the coin will appear to be closer than it really is.

2. If a person under water looks at a coin above the water surface, the coin will appear to be at a higher level than it really is.

Which of the above statements is/are correct?

(a) 1 and 2
(b) 1 alone
(c) 2 alone
(d) neither 1 nor 2

Correct Answer: (a) 1 and 2

Solution:Refraction is the change in direction of a wave due to a change in its speed. This is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°. Refraction of light is the most commonly observed phenomenon. In both cases the phenomenon of refraction is applicable because there will be a change in the direction and speed as light enters from one media to other. In case 1 light for observer outside water the light ray is travelling from denser medium (water) to rarer medium (air). Thus, image of coin will appear closer due to refraction. Whereas in case 2 for observer under water refraction also takes place but in opposite way as light travels from rarer medium (air) to denser medium (water) Thus, the image of coin will appear at a higher level than actual position of coin

7. A noise level of 100 decibel would correspond to: [2000]

(a) Just audible sound
(b) Ordinary conversation
(c) Sound from a noisy street
(d) Noise from a machine shop

Correct Answer: (d) Noise from a machine shop

Solution:The decibel is widely known as a measure of sound pressure level, but is also used for a wide variety of other measurements in science and engineering. The decibel is commonly used in acoustics to quantify sound levels relative to a 0 dB reference which has been defined as a sound pressure level of .0002 microbar. The noise level of 100 decibel would corresponds to noise from a machine shop.

8. The following items consist of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer these items using the code given below [2000]

Assertion (A): In a motion picture, usually 24 frames are projected every second over the whole length of the film.

Reason (R): An image formed on the retina of eye persists for about 0.1 s after the removal of stimulus.

(a) Both A and R are true and R is the correct explanation of A
(b) Both 4 and R are true but R is not a correct explanation of A
(c) A is true but R is false
(d) 4 is false but R is true

Correct Answer: (a) Both A and R are true and R is the correct explanation of A

Solution:Both A and R are true and R is the correct explanation of A. Standard motion pictures typically project at 24 frames per second (fps), chosen because it is the minimum rate at which motion appears smooth to the human eye. This is due to "persistence of vision," where the eye retains an image for about 0.1 seconds. At 24 fps, each frame appears for roughly 0.0417 seconds, ensuring smooth transitions between frames. This frame rate creates the illusion of continuous motion, leveraging the brief retention of images by the retina, making 24 fps the optimal choice for motion pictures.

9. The following items consist of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer these items using the code given below [2000]

Assertion (A): Small glass beads fixed on traffic signals glow brightly when light falls upon them.

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 4 and R are true but R is not a correct explanation of A
(c) A is true but R is false
(d) 4 is false but R is true

Correct Answer: (a) Both A and R are true and R is the correct explanation of A

Solution:Both of the statements are correct and statement 2 is also correct explanation for statement 1. Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary, no light can pass through and all of the light is reflected. Thus because of this phenomena traffic light beads glow when light falls on them.

10. When a CD (compact disc used in audio and video systems) is seen in sunlight, rainbow like colours are seen. This can be explained on the basis of the phenomenon of: [2000]

The following item consist of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer this item using the code given below:

(a) reflection and diffraction
(b) reflection and transmission
(c) diffraction and transmission
(d) refraction, diffraction and transmission

Correct Answer: (a) reflection and diffraction

Solution:Reflection and Diffraction :

When sunlight hits a CD, the rainbow-like colors observed are due to the interaction of light with the CD's surface and structure. Reflection occurs as light bounces off the CD's surface, but it alone doesn't explain the dispersion of colors. Diffraction, the bending and spreading of light waves around obstacles, is crucial here. CDs have microscopic grooves that diffract light, dispersing it into a spectrum similar to a prism. Transmission, the passage of light through the CD, is minimal in creating the rainbow colors. Refraction, the bending of light through different mediums, is also not the main factor. Therefore, the primary mechanisms responsible for the rainbow colors on a CD are reflection and diffraction. The grooves on the CD's surface cause diffraction, and the microscopic structure reflects light, leading to the observed spectrum.

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