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Ray Optics and Optical Instruments MCQs

Class 12 Physics — questions with answers and worked explanations.

20 free questions Class 12 Physics Answers + explanations No sign-up

These questions are drawn from the Pariksha Sutra question bank for Ray Optics and Optical Instruments, part of the Class 12 Physics syllabus. Each one shows the correct answer and, where a method helps, the working behind it.

Read the question, decide your answer before looking, then check the explanation — that is what turns practice into marks. If a question catches you out, the explanation is the part worth re-reading.

Questions

Question 1
The focal length of a concave mirror of radius of curvature 20 cm is:
  1. 5 cm
  2. 20 cm
  3. 40 cm
  4. 10 cm
Answer: 10 cm
Question 2
The refractive index of a medium in which light travels at 2x10^8 m/s is:
  1. 2.5
  2. 1.5
  3. 2.0
  4. 1.0
Answer: 1.5
Question 3
The critical angle for a medium of refractive index 1.5 (with air) is about:
  1. 30 degrees
  2. 48 degrees
  3. 42 degrees
  4. 62 degrees
Answer: 42 degrees
Question 4
A convex lens forms a real, inverted, same-size image when the object is placed at:
  1. Between F and 2F
  2. Beyond 2F
  3. Focus
  4. 2F
Answer: 2F
Question 5
An equiconvex lens has each surface of radius 20 cm and n = 1.5. Its focal length is:
  1. 40 cm
  2. 10 cm
  3. 20 cm
  4. 15 cm
Answer: 20 cm
Question 6
Two lenses of focal lengths +20 cm and +30 cm are in contact. The combined focal length is:
  1. 10 cm
  2. 12 cm
  3. 15 cm
  4. 50 cm
Answer: 12 cm
Question 7
The sign convention (Cartesian) takes distances measured against the incident light as:
  1. Zero
  2. Undefined
  3. Negative
  4. Positive
Answer: Negative
Question 8
An object placed in a denser medium appears, to an observer in air, to be:
  1. Laterally shifted only
  2. At the same depth
  3. Raised towards surface
  4. Deeper than actual
Answer: Raised towards surface
Question 9
The formula for refraction at a single spherical surface is:
  1. n1/v - n2/u = (n1-n2)/R
  2. n2/v - n1/u = (n2-n1)/R
  3. 1/v - 1/u = 1/f
  4. n2/v + n1/u = (n2-n1)/R
Answer: n2/v - n1/u = (n2-n1)/R
Question 10
The lens maker's formula for a thin lens is:
  1. f = (n-1)(R1 - R2)
  2. 1/f = n(1/R1 - 1/R2)
  3. 1/f = (n-1)(1/R1 + 1/R2)
  4. 1/f = (n-1)(1/R1 - 1/R2)
Answer: 1/f = (n-1)(1/R1 - 1/R2)
Question 11
A concave lens of focal length 25 cm has power:
  1. +2.5 D
  2. +4 D
  3. -2.5 D
  4. -4 D
Answer: -4 D
Question 12
Dispersion of white light by a prism occurs because refractive index depends on:
  1. Wavelength
  2. Intensity
  3. Temperature only
  4. Prism angle
Answer: Wavelength
Question 13
An object is placed at the centre of curvature of a concave mirror. The image formed is:
  1. Real, inverted, diminished at F
  2. Virtual, erect, magnified
  3. Real, inverted, same size at C
  4. Virtual, erect, diminished
Answer: Real, inverted, same size at C
Question 14
When light passes from air into water, its:
  1. Wavelength and speed decrease, frequency constant
  2. Speed increases
  3. Frequency changes, wavelength constant
  4. Frequency decreases
Answer: Wavelength and speed decrease, frequency constant
Question 15
Total internal reflection occurs when light travels from:
  1. Rarer to denser medium at any angle
  2. Denser to rarer at normal incidence
  3. Denser to rarer medium at angle greater than critical angle
  4. Rarer to denser above critical angle
Answer: Denser to rarer medium at angle greater than critical angle
Question 16
A concave lens always forms an image that is:
  1. Real and magnified
  2. Virtual, erect and diminished
  3. Virtual and magnified
  4. Real and inverted
Answer: Virtual, erect and diminished
Question 17
A convex lens made of glass (n=1.5) is immersed in water (n=1.33). Its focal length:
  1. Decreases
  2. Becomes negative
  3. Increases
  4. Becomes zero
Answer: Increases
Question 18
A lens of power +5 D is combined with a lens of power -5 D. The combination behaves like a:
  1. Plane glass plate
  2. Prism
  3. Converging lens
  4. Diverging lens
Answer: Plane glass plate
Question 19
A convex mirror always forms an image that is:
  1. Real and inverted
  2. Real and magnified
  3. Virtual, erect and diminished
  4. Virtual, erect and magnified
Answer: Virtual, erect and diminished
Question 20
According to Snell's law, n1 sin(i) = n2 sin(r). If i = 45 degrees, n1 = 1, n2 = 1.414, then r is:
  1. 60 degrees
  2. 20 degrees
  3. 30 degrees
  4. 45 degrees
Answer: 30 degrees

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