📘 Light – Reflection and Refraction (Class 10 Science)

🔹 Part 4: Questions 76–100

Q76. Which of the following lenses has negative focal length?

A) Convex lens
B) Concave lens
C) Biconvex lens
D) Plano-convex lens

Correct Answer: B) Concave lens

Explanation:

• A, C, D → Converging lenses → Positive focal length.
• ✅ B) Concave lens → Diverging → Negative focal length.

Q77. The phenomenon responsible for a coin appearing raised in water is:

A) Reflection
B) Refraction
C) Dispersion
D) Scattering

Correct Answer: B) Refraction

Explanation:

• ✅ B) Refraction makes coin appear at higher position.
• A, C, D → Other phenomena.

Q78. The refractive index of a medium is 2. Speed of light in that medium is:

A) 3×10⁸ m/s
B) 2×10⁸ m/s
C) 1.5×10⁸ m/s
D) 1.0×10⁸ m/s

Correct Answer: C) 1.5×10⁸ m/s

Explanation:

• n = c/v → v = c/n = 3×10⁸ / 2 = 1.5×10⁸ m/s.
• ✅ C.

Q79. A convex lens is also called:

A) Diverging lens
B) Converging lens
C) Neutral lens
D) Reflecting lens

Correct Answer: B) Converging lens

Explanation:

• ✅ B) Convex → Converges rays to a point.
• A → Concave lens.
• C, D → Wrong.

Q80. The nature of the image formed by plane mirror is:

A) Real, inverted, diminished
B) Real, erect, same size
C) Virtual, erect, same size
D) Virtual, inverted, magnified

Correct Answer: C) Virtual, erect, same size

Explanation:

• ✅ C) Plane mirrors always form virtual, erect, same-sized images.

Q81. If a convex lens forms a real image twice the size of the object, the object must be placed:

A) At focus
B) Between F and 2F
C) Beyond 2F
D) At infinity

Correct Answer: B) Between F and 2F

Explanation:

• ✅ B) Between F and 2F → Enlarged real image beyond 2F.

Q82. When object is placed at infinity in front of convex mirror, image is formed:

A) At infinity
B) At focus
C) At pole
D) Between pole and focus

Correct Answer: D) Between pole and focus

Explanation:

• ✅ D) Convex → Always diminished, erect, between P and F.

Q83. Which of the following correctly defines focal length?

A) Distance between pole and centre of curvature
B) Distance between optical centre and principal focus
C) Distance between object and image
D) Distance between object and pole

Correct Answer: B) Distance between optical centre and principal focus

Explanation:

• ✅ B) Correct definition of focal length.
• A, C, D → Incorrect.

Q84. Which lens is used in simple microscope?

A) Concave lens
B) Convex lens
C) Plano-concave lens
D) Cylindrical lens

Correct Answer: B) Convex lens

Explanation:

• ✅ B) Convex lens → Virtual, magnified image for simple microscope.

Q85. If magnification produced by mirror is –2, the image formed is:

A) Real, inverted, twice size
B) Virtual, erect, twice size
C) Real, erect, half size
D) Virtual, inverted, half size

Correct Answer: A) Real, inverted, twice size

Explanation:

• ✅ A) Negative sign → Real, inverted. |m| = 2 → Twice size.

Q86. A concave mirror of focal length 20 cm forms image of an object placed 30 cm away. Image distance is:

A) 20 cm
B) 30 cm
C) 60 cm
D) 40 cm

Correct Answer: C) 60 cm

Explanation:

• Using mirror formula: 1/f = 1/v + 1/u. f = –20, u = –30. → v = –60 cm.
• ✅ C) Image at 60 cm (real, inverted).

Q87. Which mirror has always a positive focal length?

A) Concave mirror
B) Convex mirror
C) Plane mirror
D) None

Correct Answer: A) Concave mirror

Explanation:

• ✅ A) Concave → Converging → Positive focal length.
• B) Convex → Diverging → Negative.
• C) Plane → Infinite focal length.

Q88. Which lens has always negative power?

A) Convex lens
B) Concave lens
C) Plano-convex lens
D) Biconvex lens

Correct Answer: B) Concave lens

Explanation:

• ✅ B) Concave lens → Diverging → Power negative.

Q89. The image formed by concave lens is always:

A) Real and inverted
B) Virtual, erect, diminished
C) Same size
D) Enlarged and real

Correct Answer: B) Virtual, erect, diminished

Explanation:

• ✅ B) Concave lens always produces diminished, virtual images.

Q90. The speed of light is maximum in:

A) Glass
B) Diamond
C) Water
D) Air

Correct Answer: D) Air

Explanation:

• ✅ D) Refractive index of air ≈1, speed almost maximum.

Q91. A ray passing through focus of convex lens after refraction emerges:

A) Parallel to principal axis
B) Through optical centre
C) Diverging
D) Towards pole

Correct Answer: A) Parallel to principal axis

Explanation:

• ✅ A) Lens rule: Ray through F emerges parallel.

Q92. Which phenomenon is used in prism to produce a spectrum?

A) Reflection
B) Refraction and dispersion
C) Scattering
D) Interference

Correct Answer: B) Refraction and dispersion

Explanation:

• ✅ B) Splitting of white light into 7 colours → Dispersion via prism.

Q93. A concave mirror forms a virtual image only when object is placed:

A) At centre of curvature
B) At focus
C) Beyond centre of curvature
D) Between pole and focus

Correct Answer: D) Between pole and focus

Explanation:

• ✅ D) Only in this position → Virtual, erect, enlarged image.

Q94. When refractive index increases, speed of light in medium:

A) Increases
B) Decreases
C) Remains constant
D) Becomes zero

Correct Answer: B) Decreases

Explanation:

• ✅ B) Higher n → Light slows down more.

Q95. The power of a lens of focal length –50 cm is:

A) –2 D
B) +2 D
C) –0.5 D
D) +0.5 D

Correct Answer: A) –2 D

Explanation:

• P = 100/f (cm). f = –50 → P = –2D (concave lens).

Q96. Which of the following always has an infinite focal length?

A) Concave lens
B) Convex mirror
C) Plane mirror
D) Convex lens

Correct Answer: C) Plane mirror

Explanation:

• ✅ C) Plane mirror → R = ∞ → f = ∞.

Q97. The phenomenon due to which stars appear to twinkle is:

A) Reflection
B) Refraction of light
C) Scattering
D) Diffraction

Correct Answer: B) Refraction of light

Explanation:

• ✅ B) Atmospheric refraction causes apparent shifting and twinkling.

Q98. Which mirror is used in searchlights?

A) Convex mirror
B) Concave mirror
C) Plane mirror
D) Cylindrical mirror

Correct Answer: B) Concave mirror

Explanation:

• ✅ B) Concave → Produces parallel strong beam.

Q99. The absolute refractive index of a medium is ratio of:

A) Speed of light in medium / speed in air
B) Speed of light in vacuum / speed in medium
C) Speed of light in water / speed in medium
D) None

Correct Answer: B) Speed of light in vacuum / speed in medium

Explanation:

• ✅ B) Definition of refractive index n = c/v.

Q100. The device based on multiple reflection of light is:

A) Microscope
B) Telescope
C) Kaleidoscope
D) Magnifying glass

Correct Answer: C) Kaleidoscope

Explanation:

• ✅ C) Kaleidoscope → Uses reflection in mirrors.
• A, B, D → Refraction-based.

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