- A convex lens of focal length 20 cm forms an image at 40 cm from the lens. What is the object distance?
30 cm
40 cm
60 cm
20 cm
Answer and explanation
Using lens formula: , solve for u.
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5 questions · Page 1 of 1
30 cm
40 cm
60 cm
20 cm
Using lens formula: , solve for u.
Convex lens
Concave lens
Cylindrical lens
Plano-convex lens
Concave lenses are used to correct myopia because they diverge light rays before reaching the eye.
Where:

0° - No reflection occurs
45° - When incident angle is 45°k
90° - Perpendicular to surface
180° - Complete reversal
Law of Reflection: The angle of reflection equals the angle of incidence.
| 📐 Incident Angle | 🔄 Reflected Angle | 🪞 Surface Type | ✅ Verification |
|---|---|---|---|
| 30° | 30° | Plane Mirror | θᵣ = θᵢ |
| 45° | 45° | Plane Mirror | θᵣ = θᵢ |
| 60° | 60° | Plane Mirror | θᵣ = θᵢ |
💡 Note: This fundamental law forms the basis for understanding mirrors, periscopes, and many optical instruments used in physics.
Where:

0° - No reflection occurs
45° - When incident angle is 45°k
90° - Perpendicular to surface
180° - Complete reversal
Law of Reflection: The angle of reflection equals the angle of incidence.
| 📐 Incident Angle | 🔄 Reflected Angle | 🪞 Surface Type | ✅ Verification |
|---|---|---|---|
| 30° | 30° | Plane Mirror | θᵣ = θᵢ |
| 45° | 45° | Plane Mirror | θᵣ = θᵢ |
| 60° | 60° | Plane Mirror | θᵣ = θᵢ |
💡 Note: This fundamental law forms the basis for understanding mirrors, periscopes, and many optical instruments used in physics.
0°
45°
90°
180°
The angle of reflection equals the angle of incidence.