Assertion and Reason Questions Class 10 Science: Light

Assertion and reason questions in Class 10 Science give you two statements and ask how they are connected. In this post you will learn what these questions test, a four-step method to answer them, and ten practice questions from the chapter Light – Reflection and Refraction, each with the answer and the reasoning.

What this question type tests

CBSE's curriculum documents for 2026-27 say that in Classes 9 and 10, about 50% of exam questions are competency-focused, the rest are multiple-choice, short-answer and long-answer questions, and that question types include assertion-reason. In Class 10 Science, the paper weights are 50% knowledge and understanding, 30% application, and 20% formulate, analyse, evaluate and create.

An assertion-reason question checks two things at once:

• Do you know whether a statement about science is correct?
• Do you understand why it is correct?

The Assertion (A) is a claim. The Reason (R) is a second statement that may or may not explain the claim.

The four possible answers

When you judge the two statements, only four outcomes are possible:

• (a) A and R are both true, and R correctly explains A.
• (b) A and R are both true, but R does not explain A.
• (c) A is true, but R is false.
• (d) A is false, but R is true.

The letters and exact wording of the options can differ, so always read the options printed in your paper before choosing. In this post, (a) to (d) mean the outcomes above.

A four-step method

1. Judge A alone. Cover R with your hand. Is A true or false?
2. Judge R alone. Now cover A. Is R true or false?
3. If both are true, test the link. Read them as "A, because R". Does that sentence make sense as a real explanation?
4. Pick the outcome. Match your findings to (a), (b), (c) or (d).

A frequent slip is choosing (a) just because both statements sound right. Step 3 is where marks are won.

Questions 1 to 5: mirrors and refraction

Q1. A: Concave mirrors are used in torches and vehicle headlights. R: When a light source is placed at the focus of a concave mirror, the mirror reflects the light as a powerful parallel beam.

Answer: (a). Both are true, and R is exactly why concave mirrors are used in headlights.

Q2. A: Convex mirrors are used as rear-view mirrors in vehicles. R: A convex mirror always forms a real image.

Answer: (c). A is true. R is false: a convex mirror always forms a virtual, erect and diminished image. The real reasons are its erect image and wider field of view.

Q3. A: A pencil partly dipped in water appears bent at the surface. R: Light changes speed when it passes obliquely from one transparent medium to another.

Answer: (a). Both are true. The change in speed causes refraction, and refraction makes the pencil appear bent.

Q4. A: Kerosene is optically denser than water. R: Kerosene has a higher mass density than water.

Answer: (c). A is true, because kerosene has a higher refractive index than water. R is false: kerosene is less dense in terms of mass and floats on water. Optical density is not the same as mass density.

Q5. A: The emergent ray from a rectangular glass slab is parallel to the incident ray. R: The two refracting surfaces of a rectangular glass slab are parallel to each other.

Answer: (a). Light refracts twice, at two parallel surfaces, so the emergent ray comes out parallel to the incident ray, shifted sideways.

Questions 6 to 10: lenses and numbers

Q6. A: A lens of power −0.5 D is a converging lens. R: Concave lenses have negative power.

Answer: (d). A is false: a negative power means a concave lens, which is diverging. R is true. (For P = −0.5 D, f = 1/P = 1/(−0.5) = −2 m.)

Q7. A: For a convex lens with f = +10 cm and an object at u = −15 cm, the image forms at v = +30 cm. R: A ray passing through the optical centre of a lens does not deviate.

Answer: (b). Check A with the lens formula: 1/v = 1/f + 1/u = 1/10 − 1/15 = 3/30 − 2/30 = 1/30, so v = +30 cm. A is true. R is also true, but R is a ray-diagram rule. It does not explain the calculated value of v.

Q8. A: A spherical mirror with a radius of curvature of 20 cm has a focal length of 10 cm. R: For spherical mirrors of small aperture, R = 2f.

Answer: (a). Using R = 2f: 20 = 2f, so f = 10 cm. R directly explains A.

Q9. A: Dentists use concave mirrors to see teeth. R: A concave mirror has a real principal focus.

Answer: (b). Both are true. But dentists use concave mirrors because an object placed between the pole and the focus gives a virtual, erect and enlarged image. The real focus does not explain that.

Q10. A: Light bends away from the normal when it passes from air into glass. R: Light slows down when it enters a denser medium.

Answer: (d). A is false: going from air into glass, light slows down and bends towards the normal. R is true.

Common mistakes to avoid

• Picking (a) whenever both statements are true. Always test whether R is the real cause.
• Mixing up mirror and lens formulas. Mirror: 1/v + 1/u = 1/f. Lens: 1/v − 1/u = 1/f.
• Forgetting signs. A concave mirror and a concave lens both have a negative focal length.
• Confusing optical density with mass density.

When you are ready to go deeper into mirrors, lenses and ray diagrams, study this chapter with Joy.

Last updated: 7 October 2026. Source: CBSE.