Light (Reflection and Refraction) — Previous Year Questions (Class 10 Science)
This chapter tests laws of reflection, mirrors, refraction, lenses, and ray diagrams. CBSE questions mix numerical problems (mirror/lens formulas) with con
TL;DR: This chapter tests laws of reflection, mirrors, refraction, lenses, and ray diagrams. CBSE questions mix numerical problems (mirror/lens formulas) wit…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
This chapter tests laws of reflection, mirrors, refraction, lenses, and ray diagrams. CBSE questions mix numerical problems (mirror/lens formulas) with con
Light (Reflection and Refraction) — Previous Year Questions with Solutions
Q (2023, 2 marks): State the laws of reflection and draw a diagram showing the angle of incidence and angle of reflection.
Answer: Law of Reflection (two parts):
1. The incident ray, reflected ray, and normal all lie in the same plane.
2. The angle of incidence equals the angle of reflection (i = r).
Diagram description:
[Incoming ray from upper left, hitting mirror surface at point O, with normal drawn perpendicular to mirror]
Angle of incidence i: between incident ray and normal
Angle of reflection r: between reflected ray and normal
i = r
Key points:
- Angles are measured from the normal, NOT from the mirror surface.
- The law holds for all smooth reflecting surfaces.
Final Answer: i = r; incident ray, reflected ray, and normal are coplanar.
Q (2022, 3 marks): An object is placed 15 cm in front of a concave mirror of focal length 10 cm. Find the position and nature of the image.
Answer: Concave mirror: f = 10 cm, u = 15 cm (object distance, taken as positive)
Using mirror formula: 1/f = 1/u + 1/v
1/10 = 1/15 + 1/v
1/v = 1/10 - 1/15 = (3 - 2)/30 = 1/30
v = 30 cm
Since v is positive, image is real, inverted, and formed 30 cm in front of the mirror.
Magnification m = -v/u = -30/15 = -2
Image is magnified (2 times larger than object) and inverted.
Final Answer: Image is 30 cm in front of mirror, real, inverted, magnified 2 times.
Q (2021, 2 marks): A ray of light travels from air into glass (refractive index 1.5). The angle of incidence is 30°. Calculate the angle of refraction.
Answer: Using Snell's Law: n1 sin(i) = n2 sin(r)
n1 (air) = 1, i = 30°, n2 (glass) = 1.5, r = ?
1 × sin(30°) = 1.5 × sin(r)
1 × 0.5 = 1.5 × sin(r)
sin(r) = 0.5 / 1.5 = 1/3
r = arcsin(1/3) ≈ 19.47°
Final Answer: Angle of refraction ≈ 19.5° or arcsin(1/3)
Q (2023, 5 marks): Define critical angle and derive the condition for total internal reflection.
Answer: Critical Angle (C): The angle of incidence in the denser medium for which the angle of refraction is 90°.
Derivation:
Using Snell's law: n1 sin(C) = n2 sin(90°)
If light travels from denser to rarer medium (e.g., glass to air):
n_glass × sin(C) = n_air × 1
n × sin(C) = 1 × 1
sin(C) = 1/n
C = arcsin(1/n)
Condition for Total Internal Reflection:
When angle of incidence i > C, the refracted ray cannot exist in the rarer medium, and all light is reflected back into the denser medium.
For glass (n = 1.5): C = arcsin(1/1.5) ≈ 41.8°
When i > 41.8°, total internal reflection occurs.
Final Answer: C = arcsin(1/n); when i > C, total internal reflection occurs. Example: glass (n=1.5) has C ≈ 41.8°.
Q (2022, 2 marks): A convex lens of focal length 20 cm forms an image at 60 cm from the lens. Find the object distance.
Answer: Convex lens: f = 20 cm, v = 60 cm (image distance, real image is positive)
Using lens formula: 1/f = 1/u + 1/v
1/20 = 1/u + 1/60
1/u = 1/20 - 1/60 = (3 - 1)/60 = 2/60 = 1/30
u = 30 cm
Final Answer: Object distance = 30 cm
Q (2021, 3 marks): Explain why a glass slab does not deviate the direction of light, only causes lateral displacement.
Answer: When light enters a glass slab:
1. At the first surface: Light refracts towards the normal (angle of refraction < angle of incidence) due to glass being denser.
2. Inside the glass: Light travels in a straight line at the refracted angle.
3. At the second surface: Light refracts away from the normal (since it's going from denser to rarer). The angle of refraction equals the original angle of incidence.
Because the two surfaces are parallel:
- The deviation at the first surface is exactly compensated by the deviation at the second surface.
- The emergent ray is parallel to the incident ray (same direction).
- However, the emergent ray is displaced laterally (shifted sideways) due to the path inside the glass.
Final Answer: Parallel surfaces cause equal but opposite deviations, resulting in no net change in direction. The ray emerges parallel but laterally displaced.
Frequently Asked Questions
What is the difference between real and virtual images?
Real images: formed by actual convergence of light rays, inverted, can be projected on a screen (v is positive in mirror/lens formula). Virtual images: formed by divergence of light rays or by their backward extension, upright, cannot be projected (v is negative).
How do you distinguish between a concave and convex lens?
Concave lens: thinner at center, thicker at edges, diverging, always forms virtual/upright/diminished images. Convex lens: thicker at center, thinner at edges, converging, can form real or virtual images depending on object position.
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