The mirror formula under the Cartesian sign convention is 1/f = 1/v + 1/u and light in vacuum moves at about 3 × 10⁸ m/s, so this Class 12 sheet opens with the mirror equation and the speed of light. The critical-angle relation sin C = 1/n appears in the answers.
Key Takeaways
The mirror formula is 1/f = 1/v + 1/u (Q1).
Light in vacuum moves at about 3 × 10⁸ m/s (Q2).
Q3 records sin C = 1/n as the critical-angle relation.
Section A: Multiple Choice Questions (1 Mark Each)
Choose the correct option for each question.
1.The mirror formula, using the Cartesian sign convention, is:
(a) 1/f = 1/v + 1/u(b) f = uv/(u + v)(c) 1/f = 1/v − 1/u(d) f = u + v
3.For total internal reflection from a medium of refractive index n, the critical angle is given by:
(a) sin C = n(b) sin C = 1/n²(c) sin C = 1/n(d) sin C = n²
4.The power of a convex lens of focal length 0.5 m is:
(a) 0.5 D(b) 5 D(c) 2 D(d) 0.2 D
5.Total internal reflection can occur when light travels from a:
(a) rarer medium to a denser medium(b) denser medium to a rarer medium(c) medium to itself(d) vacuum to any medium
6.Using the Cartesian sign convention, the linear magnification of a lens is:
(a) v/u(b) u/v(c) f/v(d) v/f
Section B: Short Answer Type Questions (2 Marks Each)
Show all steps clearly.
7.State the mirror formula and describe the Cartesian sign convention used in optics.
8.State the laws of reflection of light.
9.State Snell's law and define the refractive index of a medium.
10.What is total internal reflection? State the conditions necessary for it to occur.
11.Define the power of a lens and write its SI unit.
12.Write the lens maker's formula and define each symbol used in it.
13.What is the focal length of a plane mirror? What is its power?
14.Define the critical angle for a pair of media and give its relation with the refractive index of the denser medium.
Section C: Numericals & Word Problems (3 Marks Each)
Apply the concepts to solve the problems. Show all working.
15.A concave mirror has a focal length of 20 cm. An object is placed 30 cm from it. Using the mirror formula 1/f = 1/v + 1/u with the Cartesian sign convention, find the position of the image.
16.Light travels from air into water of refractive index 4/3 with an angle of incidence of 30°. Using Snell's law n₁ sinθ₁ = n₂ sinθ₂, find the angle of refraction.
17.The refractive index of glass is 1.5. Using sin C = 1/n, find the critical angle for the glass-air pair.
18.A convex lens of focal length 20 cm forms an image of an object placed 30 cm from it. Using 1/f = 1/v − 1/u with the Cartesian sign convention, find the image position and its magnification.
19.Using P = 1/f with f in metres, find the power of a convex lens of focal length 25 cm.
20.An object of height 5 cm is placed 15 cm from a convex lens of focal length 10 cm. Using 1/f = 1/v − 1/u and m = v/u, find the image position, its height and its nature.
Answer Key
1. a) 1/f = 1/v + 1/u
2. b) 3 × 10⁸ m/s
3. c) sin C = 1/n
4. c) 2 D
5. b) denser medium to a rarer medium
6. a) v/u
7. Refer to solution guide.
8. Refer to solution guide.
9. Refer to solution guide.
10. Refer to solution guide.
11. Refer to solution guide.
12. Refer to solution guide.
13. Refer to solution guide.
14. Refer to solution guide.
15. Refer to solution guide.
16. Refer to solution guide.
17. Refer to solution guide.
18. Refer to solution guide.
19. Refer to solution guide.
20. Refer to solution guide.
End of Worksheet
People Also Ask
What is the mirror formula?
1/f = 1/v + 1/u, option (a) in this worksheet (Q1).
What is the speed of light in vacuum?
About 3 × 10⁸ m/s, option (b) in this worksheet (Q2).
What gives the critical angle?
sin C = 1/n, option (c) in this worksheet (Q3).
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