Optics (Physics) Part-III

Total Questions: 56

11. In which direction the rainbow is seen at 12 noon? [43ʳᵈ B.P.S.C. (Pre) 1999]

Correct Answer: (d) It cannot be seen
Solution:

A rainbow is located opposite to the sun; this explains why rainbow is not seen at 12 noon with the sun overhead.

12. The reason for a swimming pool to appear less deep than the actual depth is: [U.P.R.O./A.R.O. (Pre) 2023]

Correct Answer: (c) Refraction
Solution:

A swimming pool appears less deep than its actual depth because light rays from the bottom of the pool are bent away from the normal as they pass from water into the air (denser to rarer medium).

This phenomenon is called the refraction of light. Due to this the image of the bottom of the pool seems nearer to us and the pool appears less deep.

13. Assertion (A): A stick is dipped in water in a slanting position. If observed sideways, the stick appears short and bent at the surface of the water. [I.A.S. (Pre) 2001]

Reason (R): A light coming from the stick undergoes scattering from water molecules giving the stick a short and bent appearance.

Code:

Correct Answer: (c) (A) is true but (R) is false
Solution:

When a stick is dipped in water in a slanting position, it appears short and bent at the surface of water. It is caused by the phenomenon of refraction of light. When you immerse a stick in water, you are actually putting and watching it from rarer medium to denser medium.

When the rays of light pass from denser to rarer medium they move away from the normal and reach the person's eye. Hence, stick appears to be bent and short when immersed in water.

14. Consider the following statements: [I.A.S. (Pre) 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?

Correct Answer: (a) 1 and 2
Solution:

Statement (1) is true. The coin will appear to be closer than it really is due to the refraction of light. In this case, the ray that is coming to the eyes is traveling from denser medium (water) to the rarer medium (air). The statement (2) is also true.

If a person underwater looks at a coin above the water surface the coin will appear to be at a higher level than it really is also due to the refraction of light. In this case, the ray is travelling from the rarer medium to denser medium.

15. An air bubble in water will act like a: [U.P. Lower Sub. (Pre) 2013, U.P.U.D.A./L.D.A. (Spl.) (Pre) 2010, U.P.P.C.S. (Spl.) (Pre) 2008, I.A.S. (Pre) 1995]

Correct Answer: (d) Concave lens
Solution:

An air bubble in water, whose surface is convex will act like a concave lens (divergent lens). The refractive index of water is more than that of air. Thus the nature of lens of submerged water bubble is changed.

16. An air bubble inside water behaves as a: [U.P.P.C.S. (Pre) 2002]

Correct Answer: (c) Divergent lens
Solution:

An air bubble in water, whose surface is convex will act like a concave lens (divergent lens). The refractive index of water is more than that of air. Thus the nature of lens of submerged water bubble is changed.

17. Two glass lenses of same focal length, one being convex and the other concave, are kept in contact, the combination will act as: [U.P.P.S.C. (GIC) 2017]

Correct Answer: (c) plane glass sheet
Solution:

If two glass lenses of same focal length, one being convex and the other concave are kept in contact then resultant focal length of the combination will be infinity and it will act as a plane glass sheet.

18. The minimum height of a plane mirror to see the full size image of a person is equal to- [U.P.P.C.S. (Pre) 2011-14, U.P.P.C.S. (Mains) 2011, U.P.U.D.A./L.D.A. (Spl.) (Mains) 2010]

Correct Answer: (b) Half the height of the person
Solution:

The minimum height of a plane mirror to enable a person to see full image is 1/2 of his height. According to the principle of reflection, the angle of incidence is equal to angle of reflection. When the ray of light travels from bottom to top of a person and reaches his eyes then he is able to see the clear image.

The ray from the foot strikes the mirror at its bottom and reaches his eye after reflecting from the mirror. Thus, the height of the mirror must equal to half the height of the person.

19. The minimum height of mirror required to see whole stand up image of a man of height 1.50 metre shall be [U.P.P.S.C. (R.I.) 2014]

Correct Answer: (a) 0.75 metre
Solution:

In order to see full-length image, the minimum height of the mirror required is half of a person's height.

Therefore, minimum height of mirror required = 1.50/2 metre

= 0.75 metre.

20. The radius of curvature of the plane mirror is: [U.P. Lower Sub. (Pre) 2015]

Correct Answer: (c) Infinity
Solution:

Thus, the radius of curvature of a plane mirror is considered as infinity.